Ch. 16.1

What is weather?

Weather is

The state of the atmosphere at a specific time and place

Sun provides almost all of Earth’s energy

Air Temperature

The measure of the average amount of motion of molecules When temperature is cold molecules move slow When temperature is hot molecules move fast

River Example

Warm=Fast

Cold=Slow

Wind

Wind is air moving in a specific direction Named for the direction it is coming from

Ex. Air that moves from west to east is called a west wind.

Wind moves from areas of high pressure to areas of low pressure

Measuring Wind

Wind direction is measured using a weather vane.

Wind speed is measured using an anemometer.

Wind socks measure both speed and direction.

Humidity

Amount of water vapor present in the air. Warm air holds more water vapor. Cold air holds less water vapor.

Relative Humidity

A measure of the amount of water vapor present in the air compared to the amount needed for saturation at a specific temperature. Saturation is a state in which something is

completely soaked with liquid. Relative humidity is always

described as a percent.

Dew Point

The temperature at which the air is saturated and condensation forms. Dew point occurs when the air can’t hold any

more water vapor. Measured in degrees At 0°C, frost may form

This dew

Not this

Forming Clouds

Clouds are formed when warm air is forced upward, expands, and cools.

Water vapor condenses into tiny droplets on small particles such as dust and salt in the atmosphere.

Billions of these droplets form a cloud.

Stratus Clouds

Form layers, or smooth even sheets in the sky

Fog is an example of a stratus cloud

Cumulus Clouds

Puffy, white clouds, with flat bases Cumulus clouds look like cotton balls

Cirrus Clouds

Cirrus clouds are thin, white feathery clouds made of ice crystals

Cirrus clouds occur at higher elevations in the troposphere

Cirrus clouds can indicate approaching storms

Prefixes of Cloud Names

Cirro- describes clouds at high a elevation Alto- describes middle elevation clouds Strato- describes clouds at low elevation Example: Cirrostratus are smooth clouds at

high elevations of the troposphere.

Precipitation

Clouds associated with rain or snow often have the term Nimbus attached to them.

Example: Cumulonimbus clouds produce thunderstorms.

Example: Nimbostratus clouds are layered clouds that can produce long, steady, rainfall or snow.

Rain

Rain occurs when warm air rises, cools, condenses

The dew point is reached, the air becomes saturated and water droplets fall from the clouds.

Snow

Snow occurs when the air temperature is below freezing.

Water droplets freeze and fall as snowflakes.

Sleet

Sleet forms when the water droplets fall and freeze before they reach the surface of the Earth.

The temperature of the clouds are above freezing.

The temperature close to the Earth surface is below freezing.

Hail

Hail forms in high level clouds where the temperature is below freezing.

The water droplets in the clouds freezes. The surface temperature of the Earth is

above freezing.