David Young
Technical Subject Areas
1. Subject – Weather.
2. Introduction – The earth’s weather is continually changing. It affects just about
everything we do, especially flying. However, most people do not understand why
weather occurs. Knowing what causes weather will help a pilot understand how his
aircraft is affected by it.
3. Outline –
a. Discuss what causes weather.
b. Discuss wind.
i. Explain the high to low pressure tendency.
c. Discuss convection.
i. Explain the density of warm vs. cold air.
d. Discuss humidity.
e. Discuss clouds.
f. Discuss precipitation.
g. Discuss air stability.
h. Discuss thunderstorms.
i. Discuss warm and cold fronts.
4. Contents –
a. Simply stated, weather is caused by differential heating of the earth.
i. As the sun’s light strikes the earth, the earth is heated.
1. When anything is heated, the molecules that make it up are given
kinetic energy, or energy in the form of motion.
2. Think of warm air as “hyper” molecules and cool air as “calm”
molecules.
ii. However, the parts of the earth that receive direct sunrays will heat up
more than those that do not.
iii. This creates a temperature differential, which causes the air to move about
the earth.
1. This movement occurs because of the creation of high and low
pressure, which is a function of air density.
2. Cooler air is denser than warm air because the air molecules do not
move as much, which compacts the molecules against one another.
3. Because cooler air is less dense, it creates less pressure, or force,
on the air that surrounds it. This means that cooler air brings about
low pressure and warmer air brings about high pressure.
a. The more “calm” a molecule is, the less it moves around.
This lack of “hyper” movement causes less pressure on the
surrounding air (hence the low pressure for cooler air).
Young 2, Weather
b. The more “hyper” a molecule is, the more it moves around.
This movement causes more pressure on the surrounding
air (hence the higher pressure for warm air).
iv. In the case of low pressure, air moves inward, counterclockwise, and
upward.
v. In the case of high pressure, air moves outward, clockwise, and
downward.
b. Wind is the movement of air.
i. All fluids tend to go from high pressure to low pressure.
1. Vacuums operate on this principle. They create a lower pressure
than the room’s pressure. Compared to the vacuum, the room’s air
has a high pressure. As you know, the room’s air is sucked into
the vacuum (from high pressure to low pressure).
ii. Whenever you feel air moving over your body, it is because that air is
going towards an area of lower pressure.
iii. The bigger the difference in the high pressure compared to the low
pressure is, the more wind is created.
1. If you turn your vacuum on a low setting, it won’t suck in as much,
but if you put it on the high setting, it will suck a lot more air into
it. Same principle.
c. If you have ever been in an airliner with a display that tells you the outside
temperature at altitude, you will note that the temperature is MUCH colder than
the temperature on the ground.
i. This cold air does not stay where it’s at for the rest of time. Instead, as it
cools down, it increases its density, which causes the air to “fall” back
down towards the surface.
ii. As the air “falls” back towards the surface, it encounters warmer air
below.
1. This air heats up the originally cold air.
2. As air warms up, its density decreases.
iii. The decrease in the air’s density causes it to begin “floating” back to a
higher altitude, where it cools.
1. This creates a cycle.
iv. This cycle is called convection, and it plays a HUGE role in weather.
d. If you have ever walked outside and felt that you were practically swimming, it is
because of the air’s humidity, or the amount of water in the air.
i. Air has the ability to carry evaporated water.
ii. This water fits between the air molecules.
iii. The more water the air holds, the higher its humidity is.
Young 3, Weather
1. This is often referred to in terms of relative humidity, which is the
ratio of the amount of water in the air to the maximum amount of
water the air could hold.
a. As the air becomes denser or less dense (depends on
temperature), the molecules become more compact or more
spread out, respectively.
b. The more spread out the air molecules are, the more room
there is to fit water molecules in between them, and vice
versa for colder, denser air.
c. For each density value of air, there is a certain number of
water molecules that can fit between the air molecules.
2. The temperature at which air would have to reach before there is
no more room for water molecules is called the dew point.
iv. If the air is cooled down to the dew point (100% relative humidity), then
the water will be “squeezed” out of the air because the air can no longer
hold the water.
1. This squeezing out of water molecules is called condensation,
which when it falls to the earth is called precipitation.
e. The form that precipitation takes on (i.e. snow, rain, hail, ….) depends on the
temperature of the air where the water was “squeezed” out and the temperature of
the air the water will fall through on its way to the ground.
i. If the water molecules can fall all the way to the ground in above-freezing
temperatures, then you will most likely get rain.
ii. If the water molecules form and fall through below-freezing temperatures,
then you will most likely get snow.
iii. If the water molecules form in above-freezing temperatures, but fall
through below-freezing temperatures, then you will most likely get
freezing rain.
iv. If the temperature is below freezing and there exists condensation nuclei
(i.e. dust or salt that is floating in the atmosphere), the water could freeze
onto the condensation nuclei.
1. When this occurs millions of times, you get clouds.
2. Fog is basically a low-lying cloud. There are four types:
radiation, steam, advection, and upslope.
a. Radiation Fog – formed by the cooling of the land after
sunset. The cool ground produces condensation in the
nearby air.
i. Usually occurs at night.
b. Steam Fog – formed by cold air passing over much warmer
water or moist land.
Young 4, Weather
c. Advection Fog – formed when moist air passes over a cool
surface by wind and is cooled.
d. Upslope Fog – formed when winds blow air up a slope,
which cools and condenses as it rises.
3. The kind of cloud you get depends on the stability of the air.
f. The stability of the air depends on how quickly air can move upward or
downward.
i. If the air is unstable, then there tends to be a promotion in the upward
motion of the air.
ii. If the air is stable, then there tends to be a difficult in moving either
upward or downward.
1. Think of this as aircraft stability – when the air is stable, the air
tends to return to its state of equilibrium. However, when the air is
unstable, the air tends to move away from its state of equilibrium.
iii. Unstable air promotes convection.
iv. When you look up at the sky, it is easy to see whether or not the air is
stable or unstable if clouds exist.
1. Stable air will produce thin, stratus clouds (refer to Figure A).
a. These types of clouds are associated with a smooth flight.
2. Unstable air will produce poofy, cumulus clouds (refer to Figure
B).
a. These types of clouds are associated with a bumpy flight.
3. REALLY unstable air will produce towering cumulonimbus clouds
(refer to Figure C).
a. These types of clouds are associated with thunderstorms
and should be avoided at all costs.
g. Thunderstorms are systems that produce lightning.
i. To form, they need unstable air, moisture, and a lifting action (provided by
convection or orthographic lifting – the lifting of air by topographical
features on the earth).
ii. They often form when cold air catches up to warm air in front of it and
forcibly lifts the warm air up (because of the density difference).
1. This upward motion creates instability.
iii. Warm air near the surface fuels a thunderstorm.
iv. There are three stages of a thunderstorm: cumulus, mature, and
dissipating.
1. Cumulus – tall cumulus clouds will begin forming, and they will
grow continually taller because of the updrafts (upward
movements) of air (refer to Figure D).
Young 5, Weather
2. Mature – tall cumulonimbus clouds have formed, and precipitation
begins falling (refer to Figure E).
a. A distinct anvil shape will form at the top of the cloud, and
it tends to point in the direction the thunderstorm is moving
(refer to Figure C).
3. Dissipating – heavy downdrafts occur because the thunderstorm
has lost most of its convection that fueled the storm (refer to
Figure F).
a. Precipitation also decreases to a steadier rate.
h. Many times, air of the same temperature and humidity will move cross the world
in fronts.
i. These fronts include warm, cold, stationary, and occluded.
1. Warm Front – characterized by warm, humid air.
a. Provides stable air, steady precipitation, and stratus clouds.
2. Cold Front – characterized by cold, dry air.
a. Provides unstable air, showery precipitation, and
cumuliform clouds.
3. Stationary Front – characterized by a mass of air that has little to
no motion.
4. Occluded Front – characterized by a cold front “catching up” to a
warm front around a pivot (refer to Figure G).
a. Creates a change in air temperature, humidity, and stability.
b. Thunderstorms are sometimes created along an occluded
front.
i. There are plenty of ways to check the weather, and those ways are covered in
“Weather Services.”
5. Evaluation –
a. The student can explain what causes all weather.
b. The student can explain air density and the properties of cold and warm air.
c. The student can explain convection.
d. The student can explain wind.
e. The student can explain humidity.
f. The student can explain clouds.
g. The student can explain precipitation.
h. The student can explain air stability.
i. The student can explain thunderstorms.
j. The student can explain fronts.
6. References –
a. Aviation Weather, 2nd Edition
Young 6, Weather
Figure A
Figure B
Young 7, Weather
Figure C
Figure D
Young 8, Weather
Figure E
Figure F
Young 9, Weather
Figure G