Review: Airspace cloud requirements
Objective: To teach the student about the formation of clouds
Materials: Pictures and handouts. Whiteboard.
New terminology: Condensation nuclei, adiabatic lapse rate, dewpoint, stability (of air), rising force, inversion, stratus (layer), nimbus (rain), cumulus (piling), alto, cirrus, thunderstorm
Attention/motivation: (2 minutes)
What do you think of the idea of flying around in clouds? I mean, it's just water vapor, right? According to Rod Machado's book, weather accounts for a quarter of airplane accidents and 40 percent of fatalities. This is a big big part of the knowledge test and a good thing to know. We want to stay out of clouds, (and we have to, legally), so it's good to know how they're likely to pop up. Because they can. THE WORLD ACCORDING TO BYRON, "A superior pilot is one who uses his superior judgment to avoid having to use his superior skills."
PART I: How clouds form: (20 minutes)
Weather is a result of unequal heating of the earth's surface. Everything else that I say about weather now and in the future is based on that.
1. Why do water drops form on a glass of ice water on a hot day? Why does dew form on grass? That is exactly how clouds form. Tiny droplets of water cool on "condensation nuclei" (meaning, mostly, dust).
2. These droplets form when the temperature is low enough for humidity in the air to condense.
3. Have you noticed a temperature change when we fly? Does it get cooler or hotter as we go up? The reason: the higher in the atmosphere, the lower the pressure, which means that parcels of air expand and cool down. It cools because the individual molecules are farther apart and therefore rub less against each other and against other things.
4. How much does the air cool? That depends on the "lapse rate." The dry adiabatic lapse rate is 3 degrees C per 1000 ft., but it depends on humidity, which lowers the lapse rate.
5. So, where do clouds form? At the "dewpoint": the temperature at which the air can no longer sustain the water vapor in it. Cloud bases will be found where moist, warm air has risen to where the air temperature is at the dewpoint. As a rule of thumb we figure that (Unsaturated lapse rate = 3 C per 1000 ft. and dewpoint changes .5 C per 1000 feet) as 2.5 C per 1000 ft, SO:
a. ((Temp. - Dewpoint)/2.5) * 1000 = cloud bases
6. So why aren't there always clouds?
a. Not enough water vapor in the air
b. Temperature inversion (air stability)
i. Surface inversion
ii. Inversion aloft as a result of pressure areas (San Francisco). Look for flattening out of smoke from smokestacks.
7. REVIEW and encapsulization: What do you need to form a cloud?
a. Moisture in the air
b. Instability (a rising force):
i. Heating from the ground
ii. Upslope winds
iii. Frontal activity (We'll get to that in a minute.)
8. Why does unstable air generally mean good visibility, and stable air generally mean poor visibility (other than flying in clouds, of course)?
PART II: Kinds of clouds
High, middle, low, vertical development
1. Low (below 6,500 AGL): Fog, stratus, stratocumulus, nimbostratus Fog types:
2. Middle (6,500 to FL200): altostratus, altocumulus. May contain turbulence and (possibly severe) icing
3. High (above FL200): cirrus, cirrostratus, cirrocumulus
4. Vertical development: cumulus, towering cumulus, cumulonimbus (thunderstorm)
1. cirriform: composed mostly of ice crystals
2. cumuliform: heaped, unstable air rising and cooling
3. stratiform: layered, formed by the cooling of a stable layer
4. nimbus: causing rain
5. castellanus: in lines, common base but separate vertical development
6. lenticularis: lens-shaped, formed by strong winds over mountains
PART III: Clouds and fronts (15 minutes)
Frontal activity means that air masses of one kind of pressure (low, high) are moving in on air masses of another kind of pressure. Guess that that might mean, in terms of the stability and cloud formation that we just talked about?
1. Show pictures of warm and cold fronts, and discuss.
2. Low pressure areas (warm) are generally unstable, and usually mean poor weather.
3. High pressure areas (cold), after the frontal passage, generally mean stability and hence good weather.
4. Now, given all of that, take a look at the weather map I've given you, and tell me what we might expect to see. (Go over symbols.)
Oral evaluation/quiz and discussion questions:
Q: What elements are required for cloud formation?
Q: Can clouds form suddenly? Under what conditions might we expect to see clouds pop up?
Q: Where might we expect to see cloud bases if the ATIS says, "Temperature 15, dewpoint 10"?
REVIEW: What do all the terms listed at the top
of the lesson plan mean?
Do you have any questions for me?