24.4 Glaciers and Wind This antique “weather house” shows the humidity, or moisture content, of the air. If the air is humid, a hair inside the house expands and allows the man to come out of the door. When the air is dry, the hair contracts and the woman comes out. Red Sea

24.4 Glaciers and Wind What are some ways in which water condenses in the atmosphere? Humidity Water vapor typically condenses as dew, frost, clouds, or fog.

24.4 Glaciers and Wind The amount of water vapor in the air is called humidity. Warm, tropical air tends to contain more water vapor than cold, polar air does. Humidity

24.4 Glaciers and Wind Relative humidity is the ratio of the amount of water vapor in the air to the maximum amount of water vapor that can exist at that temperature. Relative humidity is always expressed as a percentage. Relative humidity is 60%, for example, when air contains 60% of the maximum amount of water vapor that can exist at that temperature. When relative humidity reaches 100%, the air is saturated. Humidity

24.4 Glaciers and Wind If the air temperature decreases, the atmosphere can hold less water vapor. After sunset, the temperature often decreases, and so the maximum amount of water vapor that the atmosphere can hold also decreases. The relative humidity rises, although the amount of water vapor in the air remains the same. Humidity

24.4 Glaciers and Wind The temperature at which air becomes saturated is called its dew point. If the temperature drops further, water vapor will condense. Dew is water vapor that condenses on Earth’s surface. Frost forms when the dew point of air is below freezing. Frost forms when water vapor in air changes directly from a gas to solid ice crystals. Humidity

24.4 Glaciers and Wind A.Dew can often be seen on spider webs early in the morning after a cool night. B.Frost is visible on this pane of glass. Humidity

24.4 Glaciers and Wind When the air warms, as can happen during the day, the amount of water vapor that air can contain increases. The relative humidity decreases. Humidity

24.4 Glaciers and Wind How do clouds form? Cloud Formation Clouds form as warm, moist air rises, and water vapor condenses in the atmosphere.

24.4 Glaciers and Wind A cloud is a dense, visible mass of tiny water droplets or ice crystals that are suspended in the atmosphere. Cloud Formation

24.4 Glaciers and Wind Water vapor condenses when air cools to a temperature below its dew point. As air rises, air pressure decreases. The air expands and cools, and condensation occurs. Cloud formation is the result of the expansion of air and the resulting drop in temperature. Cloud Formation

24.4 Glaciers and Wind What are the major types of clouds? Classifying Clouds There are three basic cloud forms: stratus, cumulus, and cirrus.

24.4 Glaciers and Wind Scientists classify clouds on the basis of form and height. Clouds are generally grouped into families by the altitude at which they occur. Classifying Clouds

24.4 Glaciers and Wind Classifying Clouds Stratus clouds Cirrostratus clouds Cirrus clouds Cumulonimbus clouds

24.4 Glaciers and Wind Fog is a cloud that is near or touching the ground. Fogs often form when warm, moist air passes over land. Other fogs form when the ground cools at night, cooling the humid air immediately above the ground to the dew point. Classifying Clouds

24.4 Glaciers and Wind Stratus Clouds Flat layers of clouds that cover much or all of the sky are known as stratus clouds. Nimbostratus clouds are low stratus clouds that produce steady and widespread rain or snow. Altostratus clouds, which are middle-level clouds, often form a layer that covers most of the sky. These clouds can produce light rain or snow. Classifying Clouds

24.4 Glaciers and Wind Classifying Clouds Nimbostratus clouds Altostratus clouds Cumulus clouds

24.4 Glaciers and Wind Cumulus Clouds Puffy white clouds that look like piles of cotton balls with flat bottoms are cumulus clouds. Cumulus clouds are the “fair-weather clouds” that you often see on sunny days. Cumulonimbus clouds are dark, towering clouds that produce heavy precipitation, often together with thunder and lightning. Cumulus clouds and cumulonimbus clouds develop vertically. They form less than 2 kilometers from the ground and grow upward. Classifying Clouds

24.4 Glaciers and Wind Cirrus Clouds Thin, white, wispy clouds, often with a feathery or veil-like appearance, are cirrus clouds. You can see cirrus clouds high in the sky on a clear, sunny day. Because temperatures at high altitudes are so low, cirrus clouds are mostly ice crystals. Cirrus and cirrostratus clouds generally produce no rain. Classifying Clouds

24.4 Glaciers and Wind What are the major forms of precipitation? Forms of Precipitation The most common types of precipitation are rain, snow, hail, sleet, and freezing rain.

24.4 Glaciers and Wind Precipitation occurs when water droplets or ice crystals in clouds join together and become large enough to fall to the ground without evaporating. Forms of Precipitation

24.4 Glaciers and Wind Rain and Snow Rain is liquid precipitation that usually forms in either nimbostratus clouds or cumulonimbus clouds. Snow is precipitation in the form of ice crystals called snowflakes. Forms of Precipitation

24.4 Glaciers and Wind Hail, Sleet, and Freezing Rain Hail is precipitation in the form of round, solid pieces of ice more than 5 millimeters in diameter. Sleet is precipitation in the form of ice particles that are usually smaller than 5 millimeters in diameter. Freezing rain is rain that freezes after hitting the surface. Forms of Precipitation

24.4 Glaciers and Wind Freezing rain creates beautiful effects but can be dangerous. Forms of Precipitation

24.4 Glaciers and Wind Assessment Questions 1.If air is saturated with water vapor and the air temperature drops, a.the dew point will rise. b.the humidity will increase. c.water will condense. d.the amount of water vapor in the air will increase.

24.4 Glaciers and Wind Assessment Questions 1.If air is saturated with water vapor and the air temperature drops, a.the dew point will rise. b.the humidity will increase. c.water will condense. d.the amount of water vapor in the air will increase. ANS:C

24.4 Glaciers and Wind Assessment Questions 2.Precipitation formed when rain freezes as it falls is called a.frost. b.snow. c.sleet. d.fog.

24.4 Glaciers and Wind Assessment Questions 2.Precipitation formed when rain freezes as it falls is called a.frost. b.snow. c.sleet. d.fog. ANS:C