5.2WATER IN THE ATMOSPHERE

Changing Forms of WaterWeather is affected by the amount of water in the air.

◦ The heat energy that is absorbed or released by a substance during a phase change is called latent heat.

When liquid water evaporates, the water absorbs energy from the environment, which becomes potential energy between the molecules.

When water vapor changes back into a liquid through the process of condensation, the energy is released to the surrounding air.

HumidityAs water evaporates, it

becomes water vapor, or moisture in the air, which is invisible.

The amount of water vapor in the air is called humidity.◦ When water evaporates, the

humidity of the air is increasing.

◦ As the temperature of the air increases, the air’s ability to hold water vapor also increases.

◦ Recall, warm air holds MORE water vapor than cool air.

Absolute HumidityAbsolute Humidity is the mass of water

vapor contained in a given volume of air.

◦A measure of the actual amount of water vapor in the air. Calculated by using the following formula:

◦However, as air moves, its volume changes as a result of temperature and pressure changes.

Absolute Humidity =

mass of water vapor (grams)volume of air (cubic meters

m3)

________________________________

Relative HumidityRelative humidity (RH) is a ratio of the actual

water vapor content of the air to the amount of water vapor needed to reach saturation at a certain temperature.

◦ More common way to express the humidity; uses the following formula:

When air holds all of the water it can at a given temperature, it is said to be saturated, with a relative humidity of 100% and condensation will likely result.

Relative Humidity (RH) % =

________________________________actual water vapor content (g/kg)saturation water vapor

content (g/kg)

x 100

Relative HumidityRelative Humidity example:

Air at 20°C is saturated when it contains 14 g/kg of water vapor. What is the relative humidity of a volume of air that is 20°C and contains 10 g/kg of water vapor?

10 g/kg 14 g/kg =x

100

71 % Relative Humidity

Relative Humidity2 factors affecting the

relative humidity (RH) include:

◦ Amount of water vapor At constant temperature and

pressure, the more water vapor there is in the air, the higher the relative humidity.

◦ Temperature At constant water vapor

content in the air, increased temperature leads to lower relative humidity and decreased temperature leads to higher relative humidity.

Measuring HumidityA psychrometer is an

instrument used to measure relative humidity made of two thermometers; dry-bulb and wet-bulb (covered with a damp cloth).

◦ The difference between the readings indicates the amount of water vapor in the air. The larger the difference, the less

water vapor in the air, therefore lower humidity.

The smaller the difference, the more water vapor in the air, therefore higher humidity.

CondensationRecall, condensation is the

process by which a gas, such as water vapor, becomes a liquid.◦ Before this can occur

however, the air must be saturated RH of 100%!

◦ Air can become saturated: When water vapor is added

through evaporation. When air cools to the dew

point, which is the temperature a gas condenses into a liquid.

Cloud FormationA cloud is a collection small water droplets or

ice crystals in the air.◦ Form as warm air rises and cools, condenses onto

tiny particles of gas and dust in the atmosphere.

◦ Classified based on shape and altitude.

Cloud Formation For water vapor to condense and form a cloud, a solid

surface on which condensation can take place must be available.◦ The troposphere contains millions of particles of ice, salt, dust,

smoke, and other particles serving as solid surfaces.

◦ These suspended particles providing a surface necessary for water vapor to condense are called condensation nuclei.

Stratus CloudsStratus clouds are clouds

forming in layers and have a flat, uniform base beginning to form at low altitudes.

Cover large areas of the sky and often block out the sun.

◦ Form where a layer of warm, moist air lies above a layer of cool air. Overlying warm air cools to the dew

point, creating a cloud.

◦ The prefix -nimbo and the suffix –nimbus mean “rain”. Nimbostratus clouds are dark stratus

clouds usually producing heavy, continuous rainfall.

Cumulus CloudsPuffy, white clouds

tending to have flat bottoms are called cumulus clouds.

Form when warm, moist air rises and cools.◦Normally indicate fair

weather.

◦ If they get larger, they can produce thunderstorms cumulonimbus clouds.

Cirrus CloudsThin, feathery clouds

found at high altitudes are called cirrus clouds.◦ Made of ice crystals due to

low temperatures at higher altitudes in the troposphere.

◦ Form as a result of strong winds.

◦ If they get thicker, cirrus clouds indicate a change in weather is coming.

Clouds and AltitudeAs mentioned previously, clouds are classified

on their altitude as well as their form, or shape.◦The prefix cirro-, is used to describe clouds forming

at higher altitudes. Normally made of ice crystals.

◦The prefix alto-, is used to describe clouds forming a middle altitudes. Can be made of both water droplets and ice crystals.

◦There is no prefix for clouds forming at low altitudes. Made of water droplets.

FogFog is water vapor that has

condensed very near the surface of the Earth because air close to the ground has cooled.

◦ A type formed from the nightly cooling of Earth is known as radiation fog. Layer of air in contact with the

ground becomes chilled to below the dew point.

◦ Another type is advection fog, which forms when warm, moist air moves across a cold surface. Common along coasts.

PrecipitationWhen water from the air (condensed clouds) returns

to the Earth’s surface, it is known as precipitation.

◦ 5 major forms include: Rain (most common)

Begins as a drop smaller than a period at the end of a sentence.

Snow Forms when temperatures are so cold, water vapor changes directly to solid

and falls to the ground as a single crystal or snowflake.

Sleet Forms when rain falls through a layer of freezing air. The rain freezes in the air,

producing falling ice.

Freezing Rain Occurs when cold water droplets freeze when they make contact with Earth’s

surface.

Hail Balls or lumps of ice falling directly from clouds. Forms in cumulonimbus

clouds. Common during thunderstorms and tornadoes.