16-4 Water in the Atmosphere 1 16-4 Water in the Atmosphere 1. Describe humidity and how it is measured. 2. Explain how clouds form. 3. Name the three main types of clouds.

Introduction Key Concept: Water is always moving between the atmosphere and Earth’s surface.

The water cycle is the movement of water between the atmosphere and Earth’s surface.

Water vapor enters the air through evaporation Water vapor enters the air through evaporation. Evaporation is when a liquid becomes a gas. Liquid water evaporates to become water vapor.

The water cycle includes all forms of precipitation, including rain and snow. Water moves from the atmosphere to Earth’s surface by precipitation.

water

Key Concept: Relative humidity can be measured with an instrument called a psychrometer.

Humidity is a measure of how much water vapor is in the air Humidity is a measure of how much water vapor is in the air. Warm air can hold more water vapor than cold air.

Relative humidity is a percentage measurement Relative humidity is a percentage measurement. Relative humidity is the percentage of water vapor in the air compared to how much humidity the air can hold.

A psychrometer is an instrument that measures relative humidity A psychrometer is an instrument that measures relative humidity. A psychrometer has two thermometers. One thermometer has a wet cloth covering it. You can tell the relative humidity of the air by comparing the temperatures on the two thermometers.

psychrometer Relative humidity humidity

How Clouds Form (page 563) 2. Explain how clouds form. Key Concept: Clouds form when water vapor in the air condenses to form liquid water or ice crystals.

Condensation is when a gas becomes a liquid Condensation is when a gas becomes a liquid. For example, water vapor condenses to form liquid water. This happens in the atmosphere when clouds form.

There are two things needed for condensation to occur in the atmosphere: There must be cooling of the air. There must be particles—tiny solids—in the air.

Cold air holds less water vapor than warm air Cold air holds less water vapor than warm air. When air cools, water vapor condenses into little drops of water or ice crystals. The temperature at which this happens is called the dew point.

Tiny particles must be in the air for condensation of water to take place. Water needs something to condense onto.

condensation true

Types of Clouds (pages 564–566) 3. Name the three main types of clouds. Key Concept: Scientists classify clouds into three main types based on their shape: cirrus, cumulus, and stratus. Clouds are further classified by their altitude.

Cirrus (SEER us) clouds are the wispy clouds that look like feathers in the sky. Cirrus clouds only form high in the sky.

Cumulus (KYOO myuh lus) clouds look like fluffy, rounded piles of cotton. Cumulus clouds can produce thunderstorms. Thunderstorm clouds are called cumulonimbus clouds.

Stratus (STRAT us) clouds are flat layers of clouds Stratus (STRAT us) clouds are flat layers of clouds. Stratus clouds usually cover most of the sky. Stratus clouds that produce rain are called nimbostratus clouds.

Clouds that form near the ground are called fog Clouds that form near the ground are called fog. Fog often forms when the ground cools at night. Fog is common near bodies of water, such as a lake.