Water Vapor and Humidity

What is Water Vapor?

Water in the form of a gas.

It is invisible, colorless, odorless and tasteless.

The amount of water vapor in the air is called humidity.

What is precipitation? - pageD46

Any form of water particles that falls from the atmosphere and reaches the ground.

Occurs when cloud droplets or ice crystals join together and become heavy enough to fall.

What are the types of percipitation?

RainSnowSleetHail

What is rain?Water droplets form from warm air. As the warm air rises in the sky it cools. Water vapor (invisible water in the air) always exists in our air.

Warm air holds quite a bit of water. For example, in the summer it is usually very humid. When enough of these droplets collect together, we see them as clouds.

When the drops in the clouds get heavy, they fall because of gravity, and you see and feel rain.

What is snow?Formed when water vapor changes directly to ice without first becoming a liquid, high in the atmosphere at a temperature of less than 32°F and then falls to the ground.

What is sleet?Rain drops that freeze into ice pellets before reaching the ground.

It can accumulate like snow and cause a hazard to motorists.

What is hail/freezing rain?Rain that falls onto a surface with a temperature below freezing.

Freezes to surfaces, such as trees, cars, and roads, forming a coating or glaze of ice.

Very hazardous

How can I see water vapor?

Condensation

When the molecules slow down, they form condensation.

The changing from a gas to a liquid.

Transpiration

Plants’ roots absorb water that has seeped into the ground.

Plants “transport” this water through their roots and into their leaves. The leaves then “give off” the water.

EvaporationThe changing from a liquid into a gas

Each day the sun turns trillions of tons of ocean water into water vapor.

Relative humidityHow much water vapor is in the air and how much the air could hold - at a given temperature.

It can affect how a person feels. The higher the relative humidity, the less water can evaporate into the air. The less water (sweat) can evaporate from our skin and make us feel sticky.

Clouds

A large collection of very tiny droplets of water or ice crystals. The droplets are so small and light that they can float in the air.

How are clouds formed?All air contains water, but near the ground it is usually in the form of an invisible gas called water vapor.

When warm air rises, it expands and cools.

Cool air can't hold as much water vapor as warm air, so some of the vapor condenses onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle.

When billions of these droplets come together they become a visible cloud.

What is fog?Fog is a cloud on the ground. It is composed of billions of tiny water droplets floating in the air.

How is fog formed?

Southerly winds bring warm, moist air into a region.

When it warms, moist air flows over much colder soil or snow.

Warm, moist air is cooled from below as it flows over a colder surface.

If the air is near saturation, moisture will condense out of the cooled air and form fog.

With light winds, the fog near the ground can become thick and reduce visibilities to zero.

What are the main cloud types?

StratusCirrusCumulusCumulonimbusNimbus

Cloud Video Clip

StratusStratus clouds are uniform grayish clouds that often cover the entire sky.

They resemble fog that doesn't reach the ground.

Light mist or drizzle sometimes falls out of these clouds.

CirrusThe most common of the high clouds.

They are composed of ice and are thin, wispy clouds blown in high winds into long streamers.

Cirrus clouds are usually white and predict fair to pleasant weather.

CumulusCumulus clouds are white, puffy clouds that look like pieces of floating cotton.

Cumulus clouds are often called "fair-weather clouds".

The base of each cloud is flat and the top of each cloud has rounded towers.

CumulonimbusCumulonimbus clouds are thunderstorm clouds.

High winds can flatten the top of the cloud into an anvil-like shape.

Cumulonimbus clouds are associated with heavy rain, snow, hail, lightning and even tornadoes. The anvil usually points in the direction the storm is moving.

Nimbus

A cloud that produces precipitation.