weather and water in the atmosphere

types of precipitation

Why does it snow instead of rain? Air temperature determines which type of precipitation falls. Rain falls if the air temperature is above freezing (0 C or 32 F). Frozen precipitation falls if the air or ground is below freezing. Frozen precipitation may fall as snow, sleet, or freezing rain. You can see how the different types form in Figure Snow falls when water vapor condenses as ice crystals. The air temperature is below freezing all the way to the ground, so the ice crystals remain frozen. They fall as flakes. Sleet forms when snow melts as it falls through a layer of warm air and then refreezes. It turns into small, clear ice pellets as it passes through a cold layer near the ground. Freezing rain falls as liquid water. It freezes on contact with cold surfaces near the ground. It may cover everything with a glaze of ice. If the ice is thick, its weight may break tree branches and pull down power lines. Hail is another

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why precipitation falls

Millions of water molecules in a cloud must condense to make a single raindrop or snowflake. The drop or flake falls when it becomes too heavy for updrafts to keep it aloft. As a drop or flake falls, it may collect more water and get larger.

what causes weather

Weather occurs because of unequal heating of the atmosphere. The source of heat is the Sun. The general principles behind weather can be stated simply: The Sun heats Earths surface more in some places than others. Where it is warm, heat from the Sun warms the air close to the surface. If there is water at the surface, it may cause some of the water to evaporate. Warm air is less dense, so it rises. When this happens, more dense air flows in to take its place. The flowing surface air is wind. The rising air cools as it goes higher in the atmosphere. If it is moist, the water vapor may condense. Clouds may form, and precipitation may fall.

what is weather

What do temperature, clouds, winds, and rain have in common? They are all part of weather. Weather refers to the conditions of the atmosphere at a given time and place.

clouds and temperature

Clouds can affect the temperature on Earths surface. During the day, thick clouds block some of the Suns rays. This keeps the surface from heating up as much as it would on a clear day. At night, thick clouds prevent heat from radiating out into space. This keeps the surface warmer than it would be on a clear night.

precipitation

Clouds are needed for precipitation. This may fall as liquid water, or it may fall as frozen water, such as snow.

dew point

Youve probably noticed dew on the grass on a summer morning. Why does dew form? Remember that the land heats up and cools down fairly readily. So when night comes, the land cools. Air that was warm and humid in the daytime also cools over night. As the air cools, it can hold less water vapor. Some of the water vapor condenses on the cool surfaces, such as blades of grass. The temperature at which water vapor condenses is called the dew point. If this temperature is below freezing, ice crystals of frost form instead of dew. As you can see in Figure 16.1, the dew point occurs at 100 percent relative humidity. Can you explain why?

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clouds

Clouds form when air in the atmosphere reaches the dew point. Clouds may form anywhere in the troposphere. Clouds that form on the ground are called fog.

how clouds form

Clouds form when water vapor condenses around particles in the air. The particles are specks of matter, such as dust or smoke. Billions of these tiny water droplets come together to make up a cloud. If the air is very cold, ice crystals form instead of liquid water.

classification of clouds

Clouds are classified on the basis of where and how they form. Three main types of clouds are cirrus, stratus, and cumulus. Figure 16.3 shows these and other types of clouds. Cirrus clouds form high in the troposphere. Because it is so cold they are made of ice crystals. They are thin and wispy. Cirrus clouds dont usually produce precipitation, but they may be a sign that wet weather is coming. Stratus clouds occur low in the troposphere. They form in layers that spread horizontally and may cover the entire sky like a thick blanket. Stratus clouds that produce precipitation are called nimbostratus. The prefix nimbo- means rain. Cumulus clouds are white and puffy. Convection currents make them grow upward and they may grow very tall. When they produce rain, they are called cumulonimbus.

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weather and the water cycle

The water cycle plays an important role in weather. When liquid water evaporates, it causes humidity. When water vapor condenses, it forms clouds and precipitation. Humidity, clouds, and precipitation are all important weather factors.

humidity

Humidity is the amount of water vapor in the air. High humidity increases the chances of clouds and precipitation.

relative humidity

Humidity usually refers to relative humidity. This is the percent of water vapor in the air relative to the total amount the air can hold. How much water vapor can the air hold? That depends on temperature. Warm air can hold more water vapor than cool air. You can see this in Figure 16.1.

humidity and heat

People often say, its not the heat but the humidity. Humidity can make a hot day feel even hotter. When sweat evaporates, it cools your body. But sweat cant evaporate when the air already contains as much water vapor as it can hold. The heat index is a measure of what the temperature feels like because of the humidity. You can see the heat index in Figure 16.2.

instructional diagrams

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questions

type of cloud that forms in low, horizontal layers

a. cirrus

b. cumulus

c. dew point

d. heat index

-->  e. stratus

f. freezing rain

g. sleet

measure of what the temperature feels like because of humidity

a. cirrus

b. cumulus

c. dew point

-->  d. heat index

e. stratus

f. freezing rain

g. sleet

type of cloud that is white and puffy

a. cirrus

-->  b. cumulus

c. dew point

d. heat index

e. stratus

f. freezing rain

g. sleet

temperature at which water vapor condenses out of the air

a. cirrus

b. cumulus

-->  c. dew point

d. heat index

e. stratus

f. freezing rain

g. sleet

type of precipitation that consists of small, clear ice pellets

a. cirrus

b. cumulus

c. dew point

d. heat index

e. stratus

f. freezing rain

-->  g. sleet

type of cloud that is thin and wispy

-->  a. cirrus

b. cumulus

c. dew point

d. heat index

e. stratus

f. freezing rain

g. sleet

type of precipitation that falls as liquid water but freezes on surfaces

a. cirrus

b. cumulus

c. dew point

d. heat index

e. stratus

-->  f. freezing rain

g. sleet

Weather occurs because of unequal heating of the atmosphere.

-->  a. true

b. false

The water cycle plays an important role in weather.

-->  a. true

b. false

Warm air always sinks toward Earths surface.

a. true

-->  b. false

If the relative humidity is 100 percent, it must be raining.

a. true

-->  b. false

Cumulus clouds may grow very tall because of high air pressure.

a. true

-->  b. false

Stratus clouds are clouds that form in the stratosphere.

a. true

-->  b. false

Altostratus clouds form higher in the atmosphere than stratus clouds.

-->  a. true

b. false

A single raindrop consists of millions of water molecules.

-->  a. true

b. false

Freezing rain falls through the air as tiny pellets of frozen water.

a. true

-->  b. false

Snow forms when rain falls through a layer of freezing air before it reaches the ground.

a. true

-->  b. false

The amount of water vapor that the air can hold depends mainly on air

a. density.

b. pressure.

c. movement.

-->  d. temperature.

Clouds form when air in the atmosphere reaches the

-->  a. dew point.

b. tropopause.

c. freezing point.

d. convection point.

Cirrus clouds

a. form high in the troposphere.

b. always produce precipitation.

c. are made of ice crystals.

-->  d. two of the above

The prefix nimbo- means

a. tall.

-->  b. rain.

c. cold.

d. snow.

Hail forms only in

-->  a. cumulonimbus clouds.

b. nimbostratus clouds.

c. cirrocumulus clouds.

d. cirrostratus clouds.

Types of precipitation that form when water vapor condenses as ice crystals include

a. freezing rain.

-->  b. sleet.

c. hail.

d. two of the above

Weather factors that are part of the water cycle include

a. humidity.

b. precipitation.

c. cloud formation.

-->  d. all of the above

diagram questions

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