Moisture, Clouds, & Precipitation

Chapter 18

200

Water in the Atmosphere ch. 18.1

• When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere.

Water’s Changes of State

Humidity• What is humidity?

– general term for amount of water vapor in the air• There are two “types” of humidity:

– absolute (specific) humidity

– relative humidity

(1 kg)

(g)

• What is water vapor capacity?– amount of water air

can “hold” at a given temperature• How does temperature

affect the amount of water vapor needed to saturate the air?

– Warm air can hold more water than cold air.

Humidity

Humidity• What is specific (absolute) humidity?

– the actual amount of water vapor in the air (for a given time & place)

• How do we express it?– specific humidity = mass of H2O vapor (g)

1 kg of air

(1 kg)

(g)

What is the absolute humidity of this 1 kg

parcel of air?

15 g H20

Humidity• What is relative humidity (RH)?

– ratio of how close the air is to maximum capacity (How “full” of water vapor the air is.)• How do we express it?

– relative humidity = specific humidity X 100

maximum capacity

RH = ?100%

• What is the relative humidity of each container?– 50%

• How can they be the same????– b/c both are ½ full to capacity

Humidity

50%50%

• Relative humidity can be changed in two ways.– adding or removing water vapor– changing the air temperature

Humidity

Relative Humidity & Temperature• How does temp. affect relative humidity?

– As air temp goes down, RH goes up.• Why?

– b/c cold air can hold less water than warm

– As temp goes up, RH goes down.• Why?

– b/c warm air can hold more water than cold

Humidity• When air has 100% relative humidity it is

also called…– saturated

• & air is “full” of water vapor (has reached capacity)

Humidity• What is the special name for the temp.

at which saturation (100% RH) occurs?– dew point (temperature)

• What happens at the dew point temp?– condensation (& precipitation)

» gas liquid

Saturation & Temperature• How does temp. affect saturation?

– It takes more water to saturate warm air & less to saturate cold air.

• Why?– b/c warm air can hold more water & cold

air can hold less.

What information can we gather from these figures?

• Using a sling psychrometer:– 1. Wet the wick on the wet bulb.– 2. Spin the psychrometer for the specified amount

of time.– 3. Read temperatures on both the wet & dry bulbs.

• The wet bulb temp. should always be equal or cooler than the dry bulb temp. due to the evaporation of the water.

– 4. Record your data.

Measuring Relative Humidity

• Using the RH chart– 1. Calculate

difference between wet & dry bulb temperatures

– 2. Locate difference on the (top of) RH chart

– 3. Locate dry bulb temp. on the (left of) RH chart

– 4. Where they intersect gives the RH (in %)

Measuring Relative Humidity

Interactive Relative Humidity Chart

Example: What is the RH?• If the wet bulb temp. = 16°C & dry bulb temp.

= 26°C, what is the Relative Humidity?o34%

Interactive Relative Humidity Chart

• Same method as determining relative humidity, except use dew point chart

Determining Dew Point Temperature

Interactive Dew Point Chart

• If the wet bulb temp. = 16°C & dry bulb temp. = 26°C, what is the dew point?o9° Celsius

Example: What is the Dew Point?

Interactive Dew Point Chart

Practice Determining Dew Point & Relative Humidity Using Psychrometer Readings

Dry Bulb Temp.

Wet Bulb Temp.

Diff. Between

Dew point Temp. in °C

% Relative Humidity

24 C 17 °C

20 C 16 °C

7 12 °C 49%EX.

EX. 4 14 °C 66%

Cloud Formation ch. 18 sec. 2

• Where, in the atmosphere, can clouds form?– anywhere in troposphere

• Why?– b/c only layer w/ water vapor

• What are clouds made of?– water (liquid or ice)

– condensation nuclei

Cloud Formation• How do clouds form?

– warm ground heats air above it• air rises & cools until reaches dew point &

condenses (gas liquid) or undergoes deposition (gas solid)

– cloud forms (LIQUID or solid water NOT gas)

– What is the name for the atmospheric level where condensation occurs?• condensation level

condensationlevel

Internet Investigation ES1806 Which Way Does the Wind Blow?

• What is the orographic effect?– rainfall that results from the “lifting” of air over

mountains• different effects on windward & leeward sides of

mountain• windward moist air forced over mountain & rises…

the air expands and cools precipitation• leeward air is now dry & cool at top air sinks &

warms less rain/cloud cover = “rain shadow desert”

Processes that Lift Air

Orographic Lifting & the Orographic Effect

The Wind Blew Over the MountainTune: For He's a Jolly Good Fellow

Written By: Unknown/Copyright Unknown

The wind blew over the mountain,The wind blew over the mountain,The wind blew over the mountain,

And it was wet on the windward side

wet on the windward sidewet on the windward side

The leeward side of the mountain,The leeward side of the mountain,The leeward side of the mountain,

Was as dry as it could be

Was as dry as it could be,Was as dry as it could be,

The leeward side of the mountain,Was as dry as it could be!

• frontal wedging– masses of warm & cold air collide producing a front

• warmer, less dense air rises over cooler, more dense air

Processes that Lift Air

• convergence– air flows

horizontally, collides, & gets pushed upward

Processes that Lift Air

• localized convective lifting– air is warmed more than surrounding air,

becomes less dense, rises

Processes that Lift Air

Condensation• What causes condensation?

– as air is cooled, water vapor capacity decreases (& RH ↑)• When air reaches dew point

temperature, air becomes saturated (100% RH)

– condensation occurs (gas liquid)» forms dew, clouds, fog, precipitation

Condensation• Besides cooling air to the dew point, what

else is needed for condensation to occur?– material for water vapor to condense onto

• condensation nuclei– dust, sand, salt, aerosol particles

Cloud Types & Precipitation ch. 18 sec. 3

• Where, in the atmosphere, can clouds form?– anywhere in troposphere

• Why?– b/c only layer w/ water vapor

• What are clouds made of?– water (liquid or ice)

– condensation nuclei

Cloud Types• Names formed from

one or more of 5 words/word parts

• Classified according to:1. shape

• stratus or strato-– layers

• cumulus or cumulo-– upward puffs/heaps

2. altitude (height in atm)• low

– stratus, nimbostratus, cumulus, stratocumulus

• alto (middle)– altostratus, altocumulus

• cirrus or cirro- (high)– cirrus, cirrostratus,

cirrocumulus

3. dark, rain clouds• nimbus or nimbo-

– nimbostratus, cumulonimbus

» vertical development

ch. 18 sec. 3

Low Clouds

Middle Clouds

High Clouds

Dark, Rain Clouds

also considered a low-altitude cloud

shows vertical development

a.k.a. thunderheads

cirrocumulus

Precipitation• What is precipitation?

– any form of water that falls to Earth from a cloud• H2O droplets/ice crystals have

to be heavy enough to fall– examples:

» drizzle» rain» glaze/freezing rain» snow» sleet» hail

Formation of Hail Animation

Measuring Precipitation

• National Weather Service (NWS) measures in hundredths of an inch

• What instruments are used to measure precipitation?

• rain?–rain gauge

• snow?–measuring stick