Chapter 18

Water in the Atmosphere

18.1 Humidity and condensation

While you read:

1. List and describe the three states of water.

Solid: ice, snow, and hail; temperatures of 0ºC or below.

Liquid: rain, cloud droplets; temperatures between 0ºC and 100ºC

Vapor: bubbles in boiling water; temperatures of 100ºC or higher

While you read

2. Write two sentences comparing and contrasting specific and relative humidity. Tell how each is measured.

Specific humidity measures the amount of water vapor actually present in the air while relative humidity measures how close the air is to saturation. Specific humidity is measured in grams of water vapor/kg of air while relative humidity is stated as a percentage.

After you read1. Contact with colder surface: causes

condensation on that surface.2. Radiation: ground loses heat and cools the air

above, leaving droplets of water in the air that take time to fall.

3. Mixing: warm air blows over cold ground, cooling the air and lowering it below the dew point.

4. Condensation: change from vapor to liquid when air cools beyond its saturation point.

5. Condensation nuclei, tiny particles around which condensation can accumulate.

Chapter 18.2

Clouds

While you read

• Cirrostratus: high altitude; horizontal; thin sheets that may suggest precipitation

• Cirrocumulus: high altitude; vertical; puffy clouds especially in winter

• Altostratus: middle altitude; horizontal; similar to cirrostratus

Cirrostratus

Altostratus

Nimbostratus

Cirrocumulus

Altocumulus

Stratocumulus

CumulonimbusCumulonimbus

• Altocumulus: middle altitude; vertical; similar to cirrocumulus

• Nimbostratus: low altitude; horizontal; dark gray layers of cloud that produce steady rain

• Cumulonimbus/cumulus: can span many altitudes; vertical; cumulonimbus produce heavy rain with thunder and lightning.

After you read

• Altostratus clouds form as air rises and cools in layers. These layers form when the surrounding air is stable and forces rising air to move horizontally rather than vertically.

18.3 PrecipitationWhile you read

1. Rain: water droplets that have become big enough to fall to the ground.

2. Sleet: forms when rain falls into a layer of cold air

3. Freezing rain: forms when raindrops freeze instantly when they hit a solid surface

4. Hail: forms when a frozen raindrop or clump of ice crystals is blown back up repeatedly, building up layers of ice before falling to the ground

5. Snow: forms when ice crystals in a cloud collide and clump together.

After you read

• Precipitation forms in areas where air rises and produces condensation. These areas include places near the equator where the sun’s heat raises land temperatures, places with low pressure where storms are common, and places where moist air rises over mountains and then cools.

• Dryness occurs in places where air sinks and warms. These places include areas of persistent high pressure, areas with extremely cold temperatures, and areas on the leeward side of mountains where dry air sinks.

Windward vs. Leeward

Chapter 18 Test: Essay Study Guide

• What types of clouds will form when a body of warm, moist air rises to a region of dry, stable air? Explain why and how they form.

Cumulus clouds will form.Cumulus clouds grow vertically.Warm air rises and cools, causing condensation.Clouds grow upward until temperature and density of

inside of cloud is same as outside air.

• Explain how ice crystals grow in a cloud. Under what conditions will ice crystals fall as snow? Under what conditions will the ice crystals form hail?

Upper layers of clouds contain ice crystals and super-cooled droplets. As super-cooled droplets evaporate, the water vapor is deposited on ice crystals. Heavy crystals fall, clumping together to form snow.

If the snow doesn’t fall through a layer of warm air that melts it into rain, it may reach the ground as snow .

Hail forms when a growing ice crystal is kept aloft by updrafts and continues to grow into a large ice clump. Bonus points: As the hail is carried up it then refreezes adding to its diameter. Show a drawing depicting this concept.