Transcript CH 18

Chapter 18:1 Clouds & Precip.
Precipitation - any form of water that
falls from a cloud.
 Water vapor makes up only 0-4 % of the
volume of air.
 3 phases or states of matter(s,l,g)
 6 phase changes - (f&m, e&c, s&d)

Changing state requires that energy is
transferred in the form of heat.
 Latent Heat=“hidden heat” READ

Evaporation is a cooling process.
 Energy must be absorbed…(pool,
sweat, energy comes from skin)

Condensation energy must be released
 violent storms - energy from warm
tropical waters.

Humidity=amount of water
vapor in air.
Saturated - air can NOT hold any more
water vapor
 Warm air can hold more water vapor
than cold air.
 Relative Humidity - ratio of the actual
amount of water vapor to how much it
can hold at that temperature.

How to change relative humidity…
 1. Changed by adding or removing
water vapor
 2. Change the temperature.

Lowering air temperature causes an
increase in relative humidity.
 Raising air temperature causes a
decrease in relative humidity.

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Dew Point - temperature to which air
must be cooled to reach saturation or
condensation (fog, clouds).

2 instruments used to measure humidity
1. Hair Hygrometer – uses a hair!
2. Sling Psychrometer - 2 bulbs (one
dry one wet)
High dew point temperatures indicate
moist air.
 Low dew point temperatures indicate
dry air.
 When the dew point is at or near the
actual surface temperature expect
humid, cloudy, or foggy conditions.

18.2 CLOUDS
Saturation occurs when water vapor is
added to air or air is cooled.
 Compressed air warms.
 Expanded air cools.

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Adiabatic Temp. Changes—occur when
air is compressed or expanded.
4 Air Lifting Processes:
1. Orographic lifting = mountains
 2. Frontal Wedging = at fronts the warm
air rises over the cooler air.
 3. Convergence = two air masses meet
and the result is air rising.
 4. Localized Lifting = warmer pockets
of air over parking lots etc. rise

AIR STABILITY:
Stable air (similar temperature top &
bottom) does not rise too much thus few
or no clouds.
 Unstable air (warmer temperatures at
bottom compared to air at top) air rises
thus clouds & possibly thunderstorms.
 Radiosonde = weather balloon.
 Temperature Inversions = read it!
(flipped)
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18.3 Clouds are classified by
form & height.
Cirrus - (curl of hair) - high, thin, wispy,
made of ice. (front approaching)
 Cumulus - (pile/heap) - cauliflower,
puffy, smaller ones - fair weather, larger
& darker - thunderstorms.
 Stratus - (layered) - sheets, steady rain
to overcast conditions.
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Nimbus - rain clouds(dark gray)
HIGH - cirro - icy, smaller, thinner
 MIDDLE - alto - medium sized
 LOW - larger
 Vertical Developed Clouds - low to high
severe thunder/anvil/cumulonimbus
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Unusual Clouds
Lenticular=lens shaped over mountains
 Heimholtz Waves=uh looks like waves
 Contrails=exhaust of of jets
 Mammatus=light bulb looking clouds
 Anvil Head=cumulonimbus – vertically
developed thunder clouds
 Jomammatus=just kidding!

Fog-cloud with base near or at
the surface.
Fog forms from either cooling or
evaporation.
 How precipitation forms? (read about it)
 Pages 520-521
 Types of precipitation - snow, sleet, hail,
glaze or freezing rain, and rain
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How All Clouds Form
1. Warmer air rises.
2. As it gets higher it cools down.
3. As it cools down it reaches its dew
point (because cool air can’t hold as
much water vapor).
4. Air at its dew point is saturated.
5. Saturated air condenses into water
droplets (and they make up clouds).