Chapter 13: The Weather

Water in the Atmosphere, Precipitation, and Air Masses

What is the name of someone who studies weather?

A meteorologist

The process by which water molecules in a liquid escape into the air as water vapor is called what?

Evaporation

Evaporation makes the “feels like” temperature cooler because it removes heat from your body. Winds increase evaporation.

A measure of the amount of water in the air is called what?

Humidity

(Also called absolute humidity) It makes the “feels like”

temperature hotter.

What is the amount of water vapor in the air compared to amount the air can hold at that particular temperature?

Relative humidity

Warm air can hold more water than cold air.

What instrument is used to measure relative humidity ?

A (sling) psychrometer

A hygrometer is also an instrument used to measure relative

humidity.

How do clouds form?

Clouds form when water vapor (gas) condenses to form

liquid water or ice crystals.

Only the highest clouds such as cirrus have ice

crystals.

How are clouds named?

They are named according to their shape and altitude.

• Cirro- “curl” High clouds use the prefix

cirro-• Cumulo- “ heap” or “pile”• Strato- “layer” or “sheet”

• Alto- “middle”• Nimbus- or Nimbo- “rain”

What is the process in the water cycle where plants & trees give off water into the atmosphere?

Transpiration

When the water evaporates it is called evapotranspiration.

Where is water stored in the water cycle?

In groundwaterIn lakes, rivers, & oceans

In snow and glaciersIn the atmosphere as water vapor

As cloudsIn plants & trees

The water cycle is driven by the Sun. The Sun causes unequal heating on Earth.

What are puffy middle clouds that grow tall and resemble cotton balls?

Cumulus clouds

They are seen during fairweather.

What are low clouds that appear to be in sheets or layers?

Stratus clouds

When they are at Earth’s surface we call it fog.They can bring light rain or drizzle.

What is the name for ice that has been deposited on a surface when condensation occurs at a temperature below freezing?

Frost

Frosty

What is the process by which molecules of water vapor (gas) becomes a liquid?

Condensation

To form a cloud the tiny water droplets need

particles like dust to cling to. These are called

condensation nuclei.

The dew point

In the morning the atmosphere at ground level is the coolest. If the

air is saturated with water vapor, it will condense on surfaces at the

dew point.

Example: dew on grass

What is the temperature at which condensation occurs?

What type of cloud tells us that a storm may be coming?

cirrocumulus

Cirrocumulus are cumulus clouds with a

high altitude.

What are stratus clouds that bring rain, or snow?

nimbostratus

“Nimbo-” means rain.

What type of cloud brings thunderstorms?

cumulonimbus

These are referred to as “thunderheads.” They are towering cumulus clouds with flat tops

and dark gray or black bottoms. They may weigh several tons.

What are examples of precipitation?

Rain, snow, sleet, & hail.

They differ in that they can be solid (frozen) or liquid, and

they have different sizes.

What is formed when layers of ice are added to ice pellets as they bounce up and down in a cumulonimbus cloud?

Hail

The ice pellets are greater than 5 mm in diameter

and can grow to the size of baseballs.

What is formed when water vapor is converted directly into ice crystals in a cloud?

Snow

If the air is dry, powdery snow falls. If it is humid, the snowflakes join

together in clumps. It can melt before it hits the ground if it passes through warmer air.

What is the recipe for weather?

1. Sun2. Temperature

3. Water4. Air pressure

5. WindsAll five factors interact to bring us our daily

weather.

What forms when raindrops freeze into tiny partciles of ice as they fall through the air?

Sleet

Which air has greater air pressure, warm or cold air?

Cold air has greater air pressure.

Cold air is dense (the molecules are closer together) and exerts greater pressure.

Cold, dense, dry, drops!

Why does warm air rise?

It is less dense than cold air. It exerts less air pressure

than cold air.

The molecules in warm air have more energy (kinetic) and move around faster. It is less dense (less mass per volume) and rises. It

can hold more water than cold air.

Warm, less dense, moist, rises!

What is an air mass?

It is a huge body of air that has similar temperature, humidity,

and air pressure.

Air masses are classified according to humidity and

temperature.

Describe how air masses are classified?

Warm/Humid = Maritime tropical

Cool/Humid = Maritime polarWarm/Dry = Continental tropical

Cool/Dry = Continental polarThere’s an easy way to remember!

Pay close attention.

How can I remember the types of air masses?

• Continental comes over land so it is dry.

• Maritime comes from over the sea (ocean) so it is humid.

• Polar is from the polar regions so it is cool.

• Tropical is from the equator so it is warm.

True or False?Major wind belts in the U.S generally push air masses from east to west.

False

The Prevailing Westerlies affect the U.S. and they flow west to east.

What is the name of the high, wide, and fast wind belt that carries air masses along its path?

The jet stream

The jet stream moves west to east.

What is the name for the area where two different air masses meet and do not mix?

A front

When air masses meet at a front, the collision often causes

storms & changeable weather.

What are the four types of fronts?

1. warm front2. cold front

3. occluded front4. stationary front

When two fronts meet, there is usually a change in weather conditions.

All can bring clouds & precipitation.

What is it called when a rapidly moving cold air mass meets a slowly moving warm air mass?

A cold front

The cold air mass is denser and plows under the warm (moist) air mass lifting it up. Clouds &

precipitation.As it passes it leaves cold, dry air.

What is it called when a warm air slowly overtakes a slow-moving cold air mass?

A warm front

The warm air rises above the cold mass. Warm fronts bring clouds and rain. This is much slower than when a cold front

moves in.

What type of front occurs when a cold air mass & a warm air mass meet and remained stalled over an area?

A stationary front

Stationary means “still.”

Stationary fronts may bring days of clouds and precipitation.

What type of front occurs when a warm air mass is caught between two cooler air masses?

An occluded front

It may cause clouds and precipitation.

What is the name for a swirling center of low air pressure?

A cyclone

1.Winds spiral inward in a cyclone.2. Cyclones cause clouds, winds, and precipitation.

3. Cyclones are called “lows ” & are shown by a circled “L” on a weather map.

4. They spin counterclockwise due to the Coriolis effect.

What is the name for high pressure centers of dry air that is associated with dry, clear weather?

An anticyclone

1. An anticyclone spins clockwise in the Northern Hemisphere.

2. They are called “highs” & are shown as a circled “H” on a weather map.

What instrument is used to measure wind speed?

An anemometer

Wind is measured in miles per hour or kilometers

per hour.

What instrument is used to measure air pressure?

A barometer

There are two types:1. Mercury barometer (contains liquid mercury) that measures in

inches of mercury.2. Aneroid barometer (filled with air) that measures in millibars.

About 30” of mercury = 1016 millibars

THE END