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Private Pilot License Written Exam – Meteorology

Meteorology is the second section of my study notes for the private pilot license written exam. There are 15 sections in the Sharper Edge Exam Guide:

  • Structure of the Atmosphere
  • Drivers of Temperature Change and Weather
  • Atmospheric Stability
  • International Standard Atmosphere
  • Basic Definitions and Terminology
  • Atmospheric Pressure and Wind Patterns
  • Clouds
  • Local Weather Phenomena
  • Fog
  • Air Masses
  • Fronts
  • Thunderstorms
  • Altimetry
  • Forecasts and Reports
  • Airplane Icing

1) Structure of the Atmosphere

(Sharper Edge question 92).
80% of the earth’s atmosphere (by mass) lies below 18,000′. If you look at a picture the number would be much, much less, but when you introduce mass you introduce weight. It makes sense that most of the weight in the atmosphere is lower down. We also know that the mass in the atmosphere, and how it reacts to heating and cooling, is what weather really is. 80% by mass is below 18,000. We do most of our flying (and all small aircraft flight) in the 80% below 18,000′.


2) Drivers of Temperature Change and Weather

(Sharper Edge question 87). The sun heats the earth. The earth heats the atmosphere. The earth is heated by short wave radiation (short things are close to earth). The atmosphere is heated by long wave radiation that is radiated from the earth (long things touch the atmosphere). The atmosphere absorbs very little short wave radiation.


3) Atmospheric Stability

(Sharper Edge questions 8,9,13,97).
There are three lapse rates to think about: the wet adiabatic lapse rate, the dry adiabatic lapse rate, and the environmental lapse rate. The first two are standard rates: 3 degrees per 1,000′ for dry ALR and 1.1.-2.8 degrees for wet ALR. The environmental rate is what it is on any given day in any given locale. (That said, the “standard” environmental lapse rate is 1.98 degrees per 1,000′)

  • DALR = 3 degrees/1,000′
  • SALR = 1.1-2.8 degrees/1,000′, but standard SALR is 1.5 degrees/1,000′
  • ELR = 1.98 degrees/1,000′


The SALR varies between 1.1 and 2.8 degrees/1,000 because as the air rises and gives off heat the moisture content falls. It gives off heat because when the moisture condenses it gives off latent heat. That heat warms the same air that is cooling as it rises. The DALR is constant because it hasn’t got moisture giving off latent heat.

Higher up it gives off less heat. Lower down it gives off more. Think of the moisture as heating fuel. More fuel = less net heat loss/ less fuel (higher up) means more net heat loss.

Where the actual environmental lapse rate is in relation to the dry and wet ALRs determines whether its a stable atmosphere, a conditionally/potentially unstable atmosphere, or an unstable atmosphere.

When the environmental lapse rate is less than the wet or dry ALRs, (that is, air cools at less than 1.1 degrees per thousand feet) you’ll get a stable atmosphere. Figure it out: we’re saying that the air is not changing very much as we go up in altitude. Little change = stability.

If the environmental lapse rate is negative it means that the air is losing a negative number of degrees as it rises. Losing a negative is a double negative, which means it’s a positive. The air is getting warmer as it rises. That’s another name for an inversion. The air temperature 1,000 feet above you is warmer than the air where you are.

Stable air does not cause turbulence, but an inversion can lead to wind shear. Turbulence cause smoke to spread and dissipate. If smoke from a building rises, and then spreads out there is an inversion. The air from below is rising, cooling, and falling back to earth, leaving the smoke suspended around the height of the inversion. The same thing happens over the city with smog.

The relationship between the dry ALR (DALR) and wet or saturated ALR (SALR) is as follows: dry air has less moisture, wet air has more, but the moisture is vapor, not liquid. The vapor contains latent heat energy. As the vapor cools it begins to condense, and when it condenses it gives off the latent heat. That latent heat slows down the cooling of the air. More moisture, more latent heat, slower ALR. Therefore, the SALR is lower than the DALR, because as the moisture in the moist air condenses it gives off heat.


4) International Standard Atmosphere

(Sharper Edge questions 42,70).
The International Standard Atmosphere (ISA) is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth’s atmosphere behaves. You will not always encounter these conditions in the real world, but many aviation standards and flying rules are based on this, altimetry being a major one.

  • Standard temperature at sea level: 15 degrees Celsius;
  • Environmental lapse rate:2 degrees;
  • Standard pressure at sea level: 29.92 Hg/1013 hPa/1013 millibars/101.3 kPa/ 14.69 psi;
  • Atmospheric pressure drops approx. 1″ of mercury per 1,000′;
  • The height of the tropopause is 36,090′

Click here for a pop quiz on ISA


5) Basic Definitions and Terminology

(Sharper Edge questions 1,11,29,42,70,83).
When the dewpoint and temperature are close, clouds will form.

  • Dew point is the temperature at which moisture in the air condenses.
  • It is also the temperature at which the air becomes “saturated” – the temperature at which it can no longer hold any additional moisture;
  • At that point the relative humidity of the air is 100% – no more room for moisture, we’re 100% full;
  • Temperature falls as altitude increases (most of the time).
  • So, if the ground temperature is close to the dewpoint we can assume that as we gain altitude the two temperatures will become even closer, and then coincide.
  • At that point clouds will form.

Temperature determines how much water a given volume of air can hold.

  • Higher temperatures allow for the air to hold more moisture.
  • Cool that same air under the same pressure and moisture will start condensing.
  • That’s why warm, moist rising air turns into clouds. The air cools as it rises and the moisture condenses.

To convert Celsius to Fahrenheit:

  • Multiply the Celsius temperature by 9;
  • Divide that number by 5;
  • Add 32.

Example:

  • 25C x 9=225;
  • 225/5=45;
  • 45+32=77F

To convert Fahrenheit to Celsius do it backwards:

  • Subtract 32 from the Fahrenheit temperature;
  • Multiply that number by 5;
  • Divide that number by 9

Example:

  • 77F-32=45;
  • 45 x 5 =225
  • 225/9=25C

Remember 5/9 and 9/5. Fahrenheit (bigger number) is always the 9, Celsius is always the 5 (Celsius numbers are smaller).

CAVOK means: Ceiling And Visibility OK, which means no cloud below 5,000′, no precipitation, no Cb’s or thunderstorm activity, no fog or drifting snow, and vis of 6 sm or more.

Ice can change straight to water vapor, without melting into liquid first. This is called “sublimation”.
Water vapor (or any gas) can change right to a solid without condensing into a liquid first. This is called “deposition”.

Click here for a pop quiz on Basic Definitions and Terminology


6) Atmospheric Pressure and Wind Patterns

(Sharper Edge questions 7,14,24,26,41,69,79,86,91,95,100).
Wind Terminology:

  • Gust-A rapid increase in wind speed that lasts for a short time (less than a minute) before returning to normal;
  • Squall-A rapid increase in wind speed that lasts for a longer time (more than a minute) before returning to normal;
  • Line Squall-a series of squalls that occur along a front, caused by thunderstorms;
  • Veering – when the wind changes direction toward higher numbers on the compass;
  • Backing – when the wind changes direction toward lower numbers on the compass (it’s going “back” toward zero, as in from 180 degrees to 90 degrees);
  • Downburst – a very strong, localized downdraft;
  • Micro downburst – a downburst of less than 2 nm in diameter;
  • Macro downburst – a downburst of more than 2 nm in diameter.

Winds tend to veer as we climb and back as we descend. Remember: “Back to earth”; this happens even more at night time.

Higher up the winds tend to parallel the isobars; closer to earth friction causes winds to cross isobars.

Highs flow to lows. This means high pressure areas move toward low pressure areas, seeking to equalize pressure. The pressure changes come from heating and cooling, which causes air to rise and sink. Displacement or replacement of this sinking or rising air causes a second movement of air – horizontal movement – which we call “wind”.

Isobars are lines on a map that join areas of constant pressure. If the isobar lines are spaced out the pressure differences are gradual; if they are crowded together over the same distance the pressure differences are great. Greater pressure differences (isobars close together) cause strong winds.

Low pressure zones:

  • The air circulates counter-clockwise (caused by Coriolis Force);
  • They are sometimes called “troughs”, especially when they elongate;
  • Surface low pressure zones are associated with ascending air;
  • The rising air picks up moisture, leading to clouds and precipitation.

Why does air in a low pressure area flow anti-clockwise? As the wind moves inward (from high in to the low) it is deflected right from the center of high pressure area, which becomes an anti-clockwise rotation. In the southern hemisphere the Coriolis Force bends to the left, so it makes lows spin clockwise.

High pressure zones:

  • The air circulates clockwise (remember “12 o’clock high” (also caused by Coriolis Force);
  • They are often called “ridges”, especially when they elongate;
  • High pressure systems are associated with descending (or subsiding) air;
  • They bring clear weather and gentle winds.

Why does air in a high pressure area spin clockwise? The air is under pressure and so flows out from the high’s center, but while doing this the earth is spinning counter clockwise. The Coriolis effect is stronger towards the poles and weaker toward the equator. As the air flows out of the high it gets deflected to the right, but it deflects more to the right on the side of the high closer to the pole and less to the right on the side toward the equator. That’s what gives the high it’s clockwise rotation – the differential between the Coriolis force from one side of the high to the other.

Buys Ballots Law:

  • Low pressure zones have air circulating anti-clockwise;
  • With your back to the wind draw a counter clockwise circle with your finger;
  • For the circular pattern to point in the direction that would hit your back the circle (the low pressure zone) has to be to your left;
  • Therefore, Buys Ballots Law states that with your back to the wind the low pressure area is to your left;
  • “Buys Ballots Law = Buys Ballot’s Left”.

Corialis Forces:

  • Winds deflect to the right in the northern hemisphere;
  • Winds deflect to the left in the southern hemisphere;
  • This is caused by the earth’s rotation;
  • The wind may want to flow in a straight line because of a pressure difference, but the spinning of the earth deflects it.

Click here for a pop quiz on Atmospheric Pressure and Wind Patterns.


7) Clouds

(Sharper Edge questions 10,19,20,30,37,50,78,89).
Sky Coverage:

This is a horizontal division of the sky as opposed to a vertical division of it.

  • Few – More than 0 to 2/8s coverage
  • Scattered – 3/8 – 4/8 coverage
  • Broken – 5/8 to just short of 8/8s
  • Overcast 8/8s.

Note that it is not a smooth division. The first two categories are roughly equivalent. Few is something to 25% coverage. Scattered is 3/8s to 4/8s (or 1/2) – the range can be as little as 12.5%. The broken is larger than each of those – it’s almost 50%.

  • Clear is no clouds under 10,000′;
  • A “Few” clouds is 1/4 or less;
  • “Scattered” clouds are up to half sky coverage;
  • A “broken” sky has more than half covered;
  • “Overcast” is completely covered;
  • Express them in 8ths.

Clouds, however, occur in the air column, so we could have “few at 2,000′, scattered at 7,000′ and overcast at 10,000′ That leads to Cloud Classification:

Height classifications

  • High (Cirrus or Cirro) – base higher than 20,000′
  • Middle (Alto) – base between 6,500′ and 20,000′
  • Low (no label)- base below 6,500′
  • Clouds of vertical development – they cross all boundaries.

Clouds also form shapes:
Type classifications

  • Stratus = flat, or stratified
  • Cumulus = puffy

Combine the labels to describe the clouds.

  • Cirrus + stratus = cirrostratus (high, stratified cloud)
  • Cirrus + cumulus = cirrocumulus (high puffy cloud)
  • Alto + stratus = altostratus (mid level stratified cloud)
  • Alto + cumulus = altocumulus (mid-level puffy cloud)
  • No height label + cumulus = Cumulus cloud (low puffy cloud)
  • No height label + stratus = Stratus cloud (low stratified cloud)

Low level clouds can also be divided into “nimbostratus” (rain carrying status clouds) or “fractostratus” (broken stratified cloud).

Mid-level clouds can be “altocumulus castellanus” – mid-level puffy clouds which grow big tops.

Clouds of vertical development are cumulus clouds which start as low level clouds (cumulus), proceed through towering cumulus and end as cumulonimbus (moisture condenses and precipitates).

Calculating cloud base from outside temperature and dewpoint

  • Subtract dewpoint from temperature (5 degrees from 20 degrees = 15 degrees);
  • Divide the result by 3 (Transport Canada uses 3 degrees per thousand feet as ALR) 15/3=5 5,000′;
  • That is where the cloud base will form.

8) Local Weather Phenomena

(Sharper Edge questions 21,90).

Anabatic winds are caused by the sun heating upper parts of hills or mountains before heating up the valleys. The higher air heats up and rises, pulling up more air from the valley floor. They are uphill winds and are more of a morning thing.

Katabatic winds are downslope winds. The higher parts of the hill radiate the day’s heat away more quickly than the lower parts of the valley. The heat leaves, the air cools and condenses, and drops lower down into the valley because it’s heavier. This happens once the sun starts to leave or has set.

Katabatic winds are caused by radiation cooling!

In both cases you have to ask: what happens as a result? With anabatic winds the air rises off the hilltops, drawing in valley floor air, which in turn heats up and rises. Meantime the rising air cools and drops back down into the valley between the hillsides, creating a circular pattern.

At night the katabatic pattern is the reverse. The cooler air flows downslope, and the warmer air in the valley bottom rises straight up. This is not a circular pattern, however.

Land and Sea Breezes

A similar thing happens with land and sea breezes. Land absorbs heat 9 times faster than the sea, but it also radiates it away faster. Air in contact with warming land heats up and rises. As it leaves the area by rising it draws in air that was over the sea.

As the warm air rises it cools and sinks, often back over the ocean where air has been drawn inland. The circular pattern is established.

At night the land cools off faster than the sea. The air above it cools faster, and sinks, and pushes out to sea to replace air that is relatively warmer and rising as it absorbs heat from the ocean (which gives it off 9 times slower than the land does, but nonetheless gives off heat).

Chinooks
A chinook begins as a mass of air coming from the ocean. It contains moisture. As it reaches the mountains it rises, and as it rises it cools. At first it cools quickly, at the adiabatic lapse rate of 3 degrees per thousand feet, but as the moisture in the air condenses, forming clouds, it gives of heat. This slows the lapse rate to the saturated rate of 1.5 degrees/1,000′.

Clouds form and they drop rain, and ore often snow, on the windward side of the mountains.

The clouds reach the top of the mountains and drop over the other side. As they drop they warm, adiabatically. The moisture content is much reduced, so the lapse rate is the dry adiabatic lapse rate of 3 degrees/1,000′. If the air is dropping from a 10,000’mountain range the increase in temperature can be substantial.

The difference in moisture content between the windward side and leeward side results in all of the heat gain being at the higher, dry lapse rate. What was -20 at the mountain top could become +10 at the mountain base.

Mountain Waves

When stable air runs into mountains it rises (orographics). This kills the stability. When it passes over the mountains it drops, and as it drops it warms, expands, and gets turbulent. Air masses fall on themselves and bounce back, expanding and contracting.

If there is enough moisture in the air mass clouds can form. Rotor clouds will form inside large eddies, and lenticular clouds will form on the top of the oscillating waves. (lenticular clouds are Alto Cumulus Standing Lenticular, or ACSL in cloud reports).

The “wave” means that the air mass drops after the mountain, then pops up, then drops, then goes up, etc, forming a wave like oscillation. There are eddies below the “top”of the “wave”, so to speak. They are big.

The hazards of mountain waves are obvious. If you’ve got falling and rising air masses and they’re unstable you’re going to get downdrafts, wind shear and turbulence. The downdrafts can be extreme (remember the Chris Georgas story of being forced almost to the ground from the top of the mountain ridge approaching Squamish) and the oscillation also creates pressure differences, meaning the altimeter can read incorrectly.


9) Fog

(Sharper Edge questions 34,35,77,93).

Radiation fog
occurs when the earth radiates heat at night and cools. Air that then comes into contact with the cool ground cools. If there is no wind and the air contains moisture, dew forms, but if there is a small amount of wind (no more than 5 knots) the lower band of air will get mixed around and cool, and the moisture in it will condense into fog. Too little or too much wind and no radiation fog will form. Because cooling is more pronounced on clear nights it is more likely that radiation fog will occur on clear nights with light wind and a small temperature and dewpoint spread.

Advection fog forms when warmer moist air from a relatively warmer area (for example, the ocean) comes into contact with colder land or a cooler ocean current. Advection fog can form at higher windspeeds than radiation fog. Advection fog forms when a warmer moist air mass moves over a cooler surface and the moisture condenses. Air moving from the Pacific over the land on the West Coast of Canada will tend to form advection fog if it forms fog at all.

Frontal fog forms as a front passes. Imagine a warm front passing. Stratus and then nimbus clouds form aloft. Rain falling from high evaporates are it reaches the cold air below (remember, it’s a warm front passing over colder air). As the rain evaporates it cools even more. The air becomes saturated near the ground, which is cold from the cold air mass, and fog forms.

It can also happen with a cold front passing over a warm air mass, because as the rain falls and evaporates it cools and saturates the air as described above. Rain falling from a warm air mass into a cold air mass, regardless of which way the front is moving or what type of front, can cause frontal fog.

Arctic Sea Smoke
Like frontal fog, it develops through evaporation and then re-condensation. Cold air flows over relatively warm water. The water warms the air a bit, allowing it to hold more moisture, which it gets through evaporation. As the warmed air rises it mixes with the surrounding cool air, cools down, and the moisture condenses.

Upslope fog

Warm air with moisture gets moved up a slope and cools adiabatically.


10) Air Masses

(Sharper Edge question 75).
There are four types of air mass that dominate Canadian weather:

  • Continental Arctic (cA) comes from the Pole in winter;
  • Maritime Artic (mA) comes from the Pole in summer, and from the Bering Sea and North Pacific in winter;
  • Maritime Polar (mP) comes from the North Pacific and Bering Seas (i.e, the northwest) summer and winter, south of mA in winter;
  • Maritime Tropical (mT) comes from Pacific as well as Gulf of Mexico.

From north to south they go cA, mA, mP, mT.

Continental Arctic tends to be stable, and high pressure, bring cold, clear weather. The other three are moisture containing, and tend to be unstable. It’s a winter thing because the polar cap is frozen, meaning its relatively dry.

mA is arctic air from open water. It’s in the center of the North in summer, and west of Alaska in winter. Cold because of where it’s from, and unstable due to the moisture.

mP is warmer than mA, colder than mP, and also unstable.

mT from the Gulf of Mexico is based on a large scale frying pan of water (the Gulf) sending unstable air into the center of the continent. You always see that weather system working its way northeast from Louisiana bring rain/snow/etc.


11) Fronts

(Sharper Edge questions 6,15,31,33,36,38,39,51,52,53,67,76,80,84).

Fronts are the boundaries between two different air masses. There is marked difference in temperature between the two air masses. Stuff happens at the front as a result.

The name of the front comes from whichever air mass is colder (cA, mA, mP, mT).

The type of front is determined by which air mass is advancing (warm or cold).

If a cA air mass meets an mT air mass the name will be “Continental Arctic”, and the type will depend on which is advancing, so if the mT is advancing it will be a cA warm front, but if the cA is advancing it will be a cA cold front.

The weather that occurs when fronts pass can be predictable because of how the air masses behave.

Cold fronts are heavy air that bumps along the ground with a big bulging profile. It tends to push the air up abruptly in the warmer air mass it is pushing into. If that warmer air mass has moisture and is unstable the rapid pushing up of warm moist air creates…cumulus clouds. No surprise there. Unstable moist air rising quickly always creates cumulus clouds. No moisture? No clouds. Less moisture? Fewer clouds. If the warm air mass that the cold front is bumping into is moist and unstable you’ll get cumulonimbus and TCU.

Because the frontal grade is steeper in a cold front the transition area tends to be narrower. Faster moving front? Narrower still. Slow moving front? Wider transition area.

Passage of a cold front means you start relatively warm, then the temperature drops suddenly, and usually keeps dropping. The dew point falls with cold front passage (colder air reaches saturation level earlier, and saturation level is what dew point means). As the front approaches pressure decreases (probably because the impending cold front is bumping a lot of air out of the way, reducing some pressure sort of like in a mountain wave), but once it passes the pressure increases because…cold air is denser.

Winds veer and increase. Veer means they blow toward higher numbers on the compass, meaning they’re spinning clockwise, meaning they’re higher pressure. Highs flow to lows, so cold fronts are often colder high pressure zones. When flying across a cold front you will have to adjust heading to the right in order to maintain track.

Clouds start as cirrostratus but become cumulus, and if there is enough moisture, cumulonimbus clouds. After the front passed cumulus clouds can remain.

If the warmer air mass has enough moisture you get precipitation before the front passes, often heavy. Once the front passes the precipitation stops.

Visibility follows the same pattern as precipitation: bad before, but clearing after.

Warm fronts pass more gradually. The air is lighter than cold air, so the gradient of the front is smoother, like a long sloping line.

As a result the clouds that form are more extensive. They start with cirrus clouds, then cirrostratus, gradually lowering to alto stratus and nimbostratus/ stratus. After the front passes there may be scattered stratus and some cumulonimbus clouds. Again, the clouds require moisture in the advancing front. The idea is that the moisture in the warm air gets pushed up over the cold air mass gradually, and as it rises it cools and clouds form.

If the air in the warm front contains moisture but is stable then stratiform (i.e., nimbostratus) clouds will form. If it contains moisture but is unstable then cumulus clouds will form.

With the passage of a warm front you go from relatively colder air to warmer air. The rise in temperature is sudden, and the air will remain warm after the front has passed.

The dew point rises with a warm front (warmer air can hold more moisture before it becomes saturated), but the change depends on the relative moisture contents of the air masses involved in the frontal activity. If the warm air mass is close to saturated, for example, and the cold air mass was not, then the dewpoint will come closer together.

Like a cold front, the wind veers, but less than with the passage of a cold front.

Highs still flow to lows, but the warm front is warmer air, and that tends to be higher pressure. So…with the passage of a warm front the pressure rises slowly and slightly, and then decreases.

You can get precipitation with the passage of the front, and depending on temperature it can be snow, sleet or drizzle. As the front passes the snow may turn to freezing rain and then rain.

Visibility improves with passage of the front, but can remain hazy. It’s not as clear as after the passage of a cold front.

Frontogenesis is (surprise surprise) the creation, formation or “genesis” of a front.
Frontolysis is the dissapation of a front.

Occlusions
An occlusion occurs when a cold front catches up with a warm front around a low pressure zone. The winds behind a cold front move faster than the winds in a warm front. Imagine a warm front moving over a cold air mass. The frontal slope is gradual. Then imagine either a second cold air mas or the original air mass circulating anti-clockwise around a low pressure zone and bumping into the back of the warm air mass. The frontal slope for the cold front is bulged. It pushes the warm air aloft, and eventually either connects with the first cold air mass at ground level (or connects with itself if its one large air mass circulating around a low). When the two cold air masses collide at ground level the warm air mass gets pushed up into what is called a “trowal”.
occlusion 3-stage

occlusion graphic
If there are two cold air masses and the second is relatively warmer than the first the frontal slope created below the trowal will be the gradual sort associated with a warm front (remember, “warm” and “cold” are relative). The warmest air mass of the three involved has to be the one caught in the middle, and it will be forced above both of the cold air masses.

warm front occlusion

Click here for a pop quiz on Fronts



12) Thunderstorms

(Sharper Edge questions 2,3,40,78,81,82,85,94,98).
Thunderstorms are pretty straightforward, but they can cause huge problems. They are made up of towering cumulus clouds (TCU), which are clouds of vertical development spanning low, middle and high cloud elevations. They combine strong updrafts and downdrafts and can be very hazardous to flying.

They require a source of lifting. This can be heat, or it can be orographic. They also require fuel, in the form of moisture. If they have enough of both they’ll create a thunderstorm.

What happens is that moist air rises. As it rises it cools adiabatically. The moisture condenses and clouds begin to form. However, the condensation releases heat, which slows the cooling. This makes the air stay warmer, causing it to rise further, condensing some more and giving off more heat. The air that rises draws in more moist air from below, and the cycle continues. If the cooled air aloft starts to sink it runs into more rising, condensing air that is giving off heat.

The air will continue to rise until it runs into stable air (for example, the troposphere) or until it runs out of fuel.

Stage one is the developing or cumulus stage. The moist air rises, cools, condenses and starts dropping either large raindrops or ice pellets. As they fall they drag air down with them. Updrafts reach 2500-3000 ft. per minute. That’s around 40 mph.

Stage two is the mature stage. This is when rain first reaches the earth’s surface. Downdrafts can reach 2500 ft per minute. The friction between the rising air and downdrafts create lightening and turbulence. As rain falls it evaporates in the heat released by the condensing and compressing of the air, which exerts a cooling effect. The updrafts can send cloud tops 5,000-6,000 feet into the stratosphere (40,000′ ASL). When the downdrafts hit the earth’s surface there will be large outward gust fronts, sometimes called “plow winds”.

The third stage is the dissipation stage. The entire area runs out of lifting energy and becomes a sustained downdraft. This removes the source of turbulence, lightening and hail.

There are four types of thunderstorms. Classification is based on the trigger.

  • Orographic thunderstorms are are where the source of lifting comes from terrain features;
  • Convective thunderstorms are where the source of lifting comes from convective heating associated with uneven heating of the earth’s surface. Convective heating is when heat transfers a warm area to a cool area vertically;
  • Nocturnal thunderstorms are where the source of lifting comes from convective heating of air from a warm body of water at night, land breeze action (advection heating) or the cooling of moist air aloft;
  • Frontal thunderstorms are where the source of lifting comes from the air in the warm air mass rising up over the air in the cold air mass.
  • Thunderstorm Hazards

    • Turbulence
    • Hail
    • Icing
    • Low Visibility
    • Low Ceiling
    • Lightening
    • Precipitation Static – scratchy interference
    • Low Level Wind Shear

    Low level windshear can be from updrafts or downdrafts from micro or macro downbursts (micro<2 minutes / macro>2 minutes). Flying into a downburst you experience headwinds, causing you to reduce power to control airspeed. Flying out you’ll experience a tailwind and a downdraft, requiring full power.

    Thunderstorm avoidance

    • Do not fly through thunderstorms
    • Do not fly under thunderstorms, regardless of their size
    • Circumnavigate at least 3 miles upwind and 10 miles downwind
    • Slow the airplane below it’s manoeuvring speed (Va)
    • Remember that in flight you can’t distinguish hail from rain
    • Hail can be thrown out from the thunderstorm; it doesn’t just fall below it
    • Gusts from thunderstorms can cause windshear up to 10 miles away
    • Thunderstorms cause windshear and downbursts, which are very hazardous to take offs and landings; do not attempt either in the vicinity of a thunderstorm.

    13) Altimetry

    (Sharper Edge questions 4,5,16,22,88).

    Air pressure is the weight of air. Air pressure at any point is the weight of all the air above that point. As you raise the point (gain altitude) you reduce the amount of air over the point by the amount left below it.

    Standard pressure is 29.92, at sea level,at 15 degrees centigrade. That seldom occurs. The altimeter must therefore be corrected prior to and sometimes during flight or else it will read incorrectly.

    In cold weather air is more dense, which will make the altimeter think it is lower than it really is. Hot weather does the opposite.

    High pressure zones do the same thing.

    In a hot location the altimeter can over read because a bigger height change is required to produce the same pressure drop (in other words, 29.92 to 29.82 at 30 degrees celsius requires more altitude change for the same change in pressure than 29.92 to 29.82 at 5 degrees C.).

    That means if you’re flying at a roughly consistent pressure from a hot lotocation to a cold location you can be lower than your altimeter reports. Hot cold, don’t be bold!
    Also, from high pressure to low pressure areas you run the same risk – going into a relatively low pressure environment means the altimeter “thinks” there’s less air in the air column above you (meaning the altimeter “thinks” you’re higher than you are). High to low, look out below!

    The altimeter setting at the aerodrome is the pressure at aerodrome level reduced to sea level at standard rate. On the ground the elevation in the altimeter should read what the actual aerodrome elevation is. The sub-scale in the altimeter becomes the baseline from which the altimeter readings are derived. If the sub-scale is set incorrectly the altimeter will read incorrectly.

    Pressure Altitude

    Pressure altitude is the altitude the altimeter should read when the sub-scale is set to 29.92. In other words, if the air pressure is higher than 29.92, but you put the sub-scale to 29.92, the altimeter will indicate an elevation that is lower than what the elevation actually is, and if the air pressure is lower than 29.92 the altimeter will indicate an elevation that is higher than it actually is.

    Pressure altitude is,in a sense, the altitude that the airplane “thinks” it is flying at. The airport I fly from, CYPK, is 11′ ASL. If there is a high pressure system overhead the air is denser (forget about temperature for a second), so the airplane will perform better. The airplane will perform as if it had a larger column of air over top of it, or as if it was at a lower elevation – even below sea level.

    To calculate pressure altitude you take the pressure from ATIS (let’s assume 30.12 for our purposes) and subtract standard pressure from it. That’s 29.92 – 30.12= -.2. We know that by convention that 1 inch of mercury equals 1,000′ of altitude. Therefore, .2 x 1,000′ = 200′. If the elevation of CYPK is 11′ ASL at 29.92, then the pressure altitude is 200′ different at 30.12.

    Which way? Well, 30.12 is higher pressure. It makes the airplane feel like its got more air column above it, meaning the airplane is lower. The 11′ ASL becomes 189′ below sea level.

    If the ATIS gave a pressure of 29.72 we’d do the math the same way: 29.92-29.72= .2, which, means .2 x 1,000′ = 200′. Lower air pressure is like a smaller column of air above the airplane, or like a higher elevation. The 11′ ASL becomes 211′ pressure altitude.

    You can either think about the air column and whether higher/lower pressure means adding or subtracting the difference, or simply remember to subtract the ATIS pressure from the standard pressure (that is, always 29.92 – ATIS pressure).

    Density Altitude
    Standard pressure implies something else: standard temperature. I said earlier that high pressure means denser air – that’s sort of true. Air density is decreased by low pressures, high temperatures and high humidity. That’s not exactly the same as saying high pressure air will be denser – it could be high pressure air that is hot and humid.

    Density altitude is calculated using the E6B or an electronic calculator or a chart in the airplane manual. However, you can also do it long hand.

    You take the standard pressure and subtract the ATIS pressure to get pressure altitude: 29.92-30.12 =-.2×1000′ = -200′
    You take the elevation of the airport and correct for pressure altitude: say 11′ ASL – 200′ = -189’ASL.
    Then you factor in temperature. Standard temperature is 15 degrees C at sea level. Environmental lapse rate is 2 degrees/1,000′. 11/1000= 0.011 CYPK is 11′ ASL or 11′ x environmental lapse rate colder than sea level = 15 degrees – .011 degrees = 14.989 (use a higher elevation if you want a bigger number!).

    Then you correct for actual temperature. Let’s say it’s 30 degrees out. That’s a difference of 15 degrees from standard. The formula is 100 x actual temperature (30) minus the difference between standard temperature (15)and the temperature corrected for altitude (14.989), or 100 x (30-14.989)= 1501.1′. In other words, density altitude at CYPK at 30 degrees and 30.12 ATIS pressure is 1,501.1′ ASL.

    Density altitude, in other words, is actual altitude corrected for difference between actual pressure and standard pressure and then for actual temperature and standard temperature.

    14) Forecasts and Reports

    (Sharper Edge questions 17,18,23,25,27,28,32,43,44,45,46,47,48,49,54,55,56,57,58,59,60,61,62,63,64,65,66,68,71,72,73,96,99).

    Weather forecasts and weather information either comes from human observation or from automated weather reporting stations. The automated ones come in two forms: AWOS and LWIS. AWOS means “Automated Weather Observation System” and LWIS means “Limited Weather Information System”. AWOS are always located at an aerodrome.

    AWOS measure cloud base to 10,000′ AGL, visibility to 9SM, temperature, dewpoint, wind direction and speed,altimeter setting, precipitation and icing.

    LWIS only reports wind speed and direction, temperature and altimeter setting.

    Take note: AWOS and ATIS report wind in degrees magnetic. METARS report in degrees true.
    METARs
    METARs are issued every hour, on the hour;
    Visibility is reported in miles;
    Wind is reported in degrees true;

    Here is an example:
    CLINTON/BLEIBLER RANCH/BC (show WxCam)

    METAR CYIN 110300Z AUTO 23009G15KT 9SM OVC100 08/M03 A2994 RMK
    SLP160=
    METAR CYIN 110200Z AUTO 18005KT 130V230 9SM BKN130 08/M03 A2992 RMK
    SLP159 DENSITY ALT 3700FT=
    METAR CYIN 110100Z AUTO 19009KT 9SM CLR 09/M03 A2993 RMK SLP158
    DENSITY ALT 3800FT=

    • “CYIN” is the aerodrome identifier;
    • “110300Z” means 11th day, 0300 Zulu;
    • “AUTO” means it’s an automatic reporting station;
    • “23009G15KT” means the wind is coming from 230 degrees true at 9 knots gusting to 15 knots;
    • “9SM” means visibility is 9 statute miles;
    • “OVC100″ means overcast at 10,000′ AGL;
    • “08/M03″ means the temperature is 8 degrees C, and the dewpoint is -3 degrees C;
    • “A2994″ means the altimeter setting is 29.94;
    • “RMK” means remarks, and can relay cloud cover,or in this case “SLP” (sea level pressure) in hectopascals (158=1015.8).

    The METAR for the same aerodrome but a different day:
    METAR CYIN 150100Z AUTO 25009G17KT 220V320 9SM CLR 13/M09 A2983 RMK
    SLP124 DENSITY ALT 4400FT=

    • “METAR” means it’s a METAR;
    • “150100Z” means 15th day, 100 ZULU;
    • “AUTO” means auto report;
    • “25009G17KT” means wind from 250 degrees true at 9 kts, gusting to 17kts;
    • “220V320″ is an addition to the last one, and means the wind is varying from 220 degrees true to 320 degrees true;
    • “9SM” means 9 stature miles visibility;
    • “CLR” means the sky is clear;
    • “13/M09″ is the temp/dewpoint spread;
    • “A2983″ is the altimeter setting;
    • “RMK” this tie includes sea level pressure (1012.4) as well as density altitude of 4,400′ ASL.
    • METARs use abbreviations. Two important ones are “RE” and “VC”, meaning “recent” and “vicinity”, respectively. This is important because a lot of the weather abbreviations are easy to figure out, but when joined with “RE” or “VC” may become confusing. Its easy to figure out that “TS” means “thunderstorms”, but “RETS” may llok like something completely new instead of “RE” & “TS” for “RETS” – recent thunderstorms.

    SPECI’s are special metars that are issued between the hours to report on changes worth noting.

    GFAs – Graphic Area Forecasts
    These forecasts consist of six charts issued 4 times per day forecasting weather up to 24,000′ AGL.

    The six charts are divided into 2 valid at the time of forecast, 2 for 6 hours later, and 2 for 12 hours later. The charts come out 30 minutes before the validity period starts.

    One chart is for weather and clouds and the second is for icing and turbulence.

    TAFs – Terminal Area Forecasts or Aerodrome Forecasts
    These forecasts are for he surface of the airport only. They come in the same format as METARs. Not all stations produce TAFs.

    TAF CYLW 160038Z 1601/1613 VRB03KT P6SM SCT070 BKN110 TEMPO 1601/1604
    P6SM -SHRA BKN070
    RMK FCST BASED ON AUTO OBS. NXT FCST BY 160700Z=

    • “TAF CYLW” means its a TAF from Kelowna;
    • “160038Z” means it was issued on the 16th day at 0038Z;
    • “1601/1613″ is something you don’t see on METARs – a validity period. This TAF is valid from day 16 0100Z to 1300 Z – 12 hours;
    • Forecast validity periods can be from 6 to 30 hours;
    • “VRB” means “variable, in this case, direction;
    • “03KT” means 3 kts. The wind is coming from variable directions at 3 kts;
    • “P6SM” means vis is plus 6 statute miles;
    • “SCT070″ means scattered clouds at 7000′ AGL;
    • “BRKN110″ means broken cloud cover at 11,000′ AGL;
    • “TEMPO 1601/1604″ means a temporary change between 100Z and 400Z – vis stays the same, with light showers and broken ceiling at 7000′ AGL;
    • The RMK (remarks) explain its from an automated reporting station and the next forecast will come at 700Z.


    AIRMETs

    AIRMETs tell you about short term weather events that are not in the current GFA or something that was forecast but did not occur. Here’s an example:

    AIRMETS GFACN34

    WACN34 CWUL 232351
    AIRMET R2 VALID 232350/240045 CWUL-
    CZQM MONCTON FIR CNCL AIRMET R1 232045/240045
    RMK GFACN34
    END/WACN26/CWAO

    • “WACN34″ means Airmet;
    • “CWUL 232351″ means this comes from the NFLD weather center on the 23rd day at 2351Z;
    • “AIRMET R2″ is the discrete name of this AIRMET – “R2″;
    • “VALID 232350/240045 CWUL-” means its valid from 2350 to 0045 the next day;
    • “CZQM MONCTON FIR CNCL AIRMET R1 232045/240045″ means its cancelling an AIRMET R2, which was issued earlier. This is a case of something forecast not occurring.

    SIGMETs
    SIGMETs are significant short term weather warnings for stuff that didn’t make it into a GFA or TAF. They are valid from when they come out until they are updated or cancelled.

    What do they deal with? The obvious: thunderstorms, hail, icing and turbulence,hurricanes, mountain waves, widespread ash or dust storms, volcanic activity and low level windshear.

    PIREPS
    PIREPS are pilot reports of observed weather. They report stuff actually observed by pilots flying through the weather they’re reporting on.


    15) Airplane Icing

    (Sharper Edge questions 12,74).
    Icing is important because it increases drag and weight. More drag and more weight means less lift. Unlike extra weight that you pack into the plane, the weight of ice is unknown, and the drag effect is unknown.

    More weight and drag and less lift means longer landing and take off rolls, reduced performance (including climb), perhaps reduced control and increased stall speed.

    Put all that together and it means: dangerous test flight ending in death!

    CARS prohibits operation of the aircraft when ice or snow adheres to any critical surface. Critical surface includes wings, prop, control surfaces, stabilizers, etc.

    A warm hangar will melt ice, but taking the plane back into the cold re-freezes it. Guess where? Flap and control hinges. Tough to fly without control due to moving control parts being frozen, so you have to dry the airplane after melting.

    What about in-flight icing? This is a real problem, obviously. If you’re flying and you notice ice build up you have to get un-iced. The ice will come from freezing rain or drizzle.

    When a warm front passes it rains, but the rain falls from the warm air mass into the cold air mass below, and that can lead to freezing. The ice will accumulate first on smaller surfaces (pitot tube, antennae, wing struts).

    You’d think that dropping altitude would unfreeze the ice, but if the lower air is colder, think again. The best thing to do is turn 180 degrees and go back to where you were alright, and then divert.

    If you’re still flying but iced up remember: higher stalling speed, longer rolls, and poorer performance. You need to increase power to compensate. Play it safe on approach. Smooth, stable, little or no flap.



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    My name is Rob Chipman and I’m a realtor and pilot based in Vancouver, BC. I AM NOT A FLIGHT INSTRUCTOR AND I AM NOT OFFERING FLIGHT INSTRUCTION! I am sharing my study notes and other things I’ve learned while getting my education as a pilot. You’re welcome to make use of this information, but do not treat it as expert advice.

    I really enjoy flying, real estate and the Chilcotin. My company is Coronet Realty Ltd., located at 3582 East Hastings Street, Vancouver, BC, V5K 2A7. I have a C-150L that I own with two other pilots, based out of Pitt Meadows. Do not hesitate to contact me by email if I can help you do anything, especially if its likely to be interesting or concerns selling remote property in British Columbia.

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