Water in the Water in the AtmosphereAtmosphere

I. Atmospheric MoistureI. Atmospheric MoistureWater exists on Earth in 3 forms:

LiquidSolid (ice)Gas

In our atmosphere, water exists mainly in its gaseous form: water vapor

What is the principal source of water vapor in Earth’s atmosphere?

The oceans!

A. HumidityA. Humidity

Humidity: the amount of water vapor in the atmosphere

Saturated: when the air is holding all the water vapor it can at a given temperature

As the air temperature increases, what happens to the amount of water vapor that volume of air can hold?

Warmer air can hold Warmer air can hold moremore water vapor than cold airwater vapor than cold air

1. 1. Relative Humidity:Relative Humidity: ratio of the ratio of the amount of water vapor in the air amount of water vapor in the air to the amount it to the amount it can can hold when hold when

saturated.saturated.

Psychrometer: instrument used to measure relative humidity.

Sling

Electronic

What does it mean to What does it mean to say the air is say the air is saturatedsaturated??

It cannot hold any more It cannot hold any more water!water!

What happens when What happens when the air becomes the air becomes

saturated?saturated?

Fill in the blank…Fill in the blank…

The higher the relative The higher the relative humidity, the humidity, the _______________ the _______________ the chance that water chance that water vapor will condense vapor will condense into rain or snow.into rain or snow.

When a certain volume When a certain volume of air is of air is saturatedsaturated, , what is its relative what is its relative

humidity?humidity?

100%100%

As outside As outside temperatures temperatures increaseincrease during the day, what during the day, what happens to relative happens to relative humidity?humidity?

Hygrometer

RH decreasesRH decreases (with (with increasing increasing

temperatures)temperatures)

If outside temperatures If outside temperatures stay the same or stay the same or decreasedecrease, what , what happens to relative happens to relative humidity?humidity?

RH increasesRH increases (greater (greater chance of chance of precipitation) with precipitation) with decreasing decreasing temperaturestemperatures

2. Specific Humidity2. Specific HumidityThe The actualactual amount of moisture amount of moisture

in the air.in the air.

High

Low

B. Dew PointB. Dew Point

The temperature to which The temperature to which air must be air must be cooledcooled to reach to reach saturationsaturation

At any temperature lower At any temperature lower than the dew point, water than the dew point, water vapor begins to vapor begins to condensecondense

What happens during condensation?

Water vapor changes to liquid water

Dew: Dew: air contacts a cool air contacts a cool surface and loses heat until surface and loses heat until

it reaches saturationit reaches saturation

Frost: Frost: if dew point falls if dew point falls below freezing, water below freezing, water

vapor changes directly to vapor changes directly to solid ice crystals, or frostsolid ice crystals, or frost

II. CloudsII. Clouds

Clouds are visible masses of liquid water droplets suspended in the atmosphere

A. Cloud FormationA. Cloud Formation Clouds form when water

vapor condenses into liquid water droplets in the air

In order for condensation to occur:

1. air must be saturated (cooled to dew point)

2. must have a solid surface to condense on (condensation nuclei)

Condensation Nuclei: small particles in the air created by:

–dust –volcanoes –factory smoke –forest fires –ocean salt

Several processes may bring about the cooling necessary for clouds to form:

1. Convective Cooling1. Convective Cooling

Most clouds form this way

Air temperatures decrease as air rises and expands

Adiabatic Temperature Adiabatic Temperature Changes:Changes:

temperature changes without the addition or removal of heat

temperature changes due to rising or sinking air

Warm air rises, expands and cools

What happens to cool air?

Cool air sinks, compresses and warms

2. Forceful Lifting2. Forceful Lifting

Air cools as it is forced over a topographical feature (like a mountain

range).

3. Temperature 3. Temperature ChangesChanges

Cold Air

Warm Air

Two masses of moist air with different temperatures mix

4. Advective Cooling4. Advective Cooling

Wind carries warm moist air over cold oceans or cold land

The cold water or land absorbs heat from the air and the air cools

B. Classifications of B. Classifications of CloudsClouds

1. Stratus Clouds1. Stratus Clouds

low level clouds sheet-like or layered cover a large areaNimbostratus = stratus cloud

with rainAltostratus = stratus formation

at higher altitude

2. Cumulus Clouds2. Cumulus Clouds

puffy, piled, popcorn, or heapedform when warm moist air rises

and coolsflat baseCumulonimbus: cloud of great

vertical development (“thunderhead”)

middle altitude clouds

3. Cirrus Clouds3. Cirrus Clouds

cirrus means “curly”wispy, stringyhigh altitude clouds made up of ice crystals due to

the low temperature and high altitude

seen prior to a snowfall or rainfall

III. PrecipitationIII. Precipitation

Any moisture that falls from the air to Earth’s surface

May be liquid or solidFour main types:

rainsnowsleethail

1. RAIN: forms when separate drops of water fall to the Earth from clouds

2. SNOW: forms when water vapor condenses directly into ice crystals

3. SLEET: a mixture of snow and rain; forms when rain passes through a cold layer of air and freezes into ice pellets

4. HAIL: balls or irregular lumps of ice (hailstones); usually form in cumulonimbus clouds

Big Hail!Big Hail!

Bad Hail!Bad Hail!

How does precipitation How does precipitation form?form?

Clouds produce precipitation when its droplets or ice crystals become large enough to fall as rain or snow

Coalescence:Coalescence:Droplets are carried by the

updrafts and downdrafts in a cloud

They collide and coalesce to form larger droplets.

When the droplets become too large to be sustained on the air currents… they begin to fall as rain or snow.