Chapter 7 Weather and Climate Objectives Explore how the water cycle drives weather. Discuss the conditions that favor thunderstorms and tornadoes. Main Idea Water vapor and changes in temperature are important factors in cloud formation and in precipitation. Vocabulary evaporation - the changing of a liquid to a gas condensation - the changing of a gas to a liquid as heat is removed humidity - a measurement of the amount of water vapor in the air stratus cloud - clouds that appear in blanket-like layers cumulus cloud - big, billowy, puffy clouds that seem to rise from flat bottoms cirrus cloud - have wispy, feather-like shapes tornado - a violent whirling wind that moves across the ground in a narrow path hurricane - a large storm with low pressure at the center How does the water cycle affect weather? A glass of ice cold lemonade on a hot summer day will form drops of water on the outside of the glass. The water on the glass came from the air around the glass called water vapor, or water in the form of gas. When the air cools, the water condenses on the glass. Earth’s water heats up by the Sun’s solar energy, causing evaporation to occur at a faster rate. During evaporation tiny water particles leave bodies of water and enters the atmosphere. When the air cools, tiny particles of water vapor slow down and loose energy. Condensation occurs when they slow down. Fog forms when water vapor suddenly condenses. Fog can be dense enough to make it difficult to see the ground. Water vapor is measured as humidity. The higher the temperature of the air, the more water vapor it can hold. Relative humidity measures the water vapor in the air compared to the total amount it could hold at that temperature. 50 percent relative humidity = air containing half the water vapor it could hold at that temperature. When air cannot hold any more water vapor the relative humidity is 100%. When air is saturated (filled) with water vapor, additional vapor turns into liquid. The temperature at which this occurs is called the dew point. How Clouds Form When warm air rises it turns to cold air. As it cools it’s relative humidity increases. When temperatures reach the dew point the air is saturated. Additional water vapor is condensed into tiny water droplets and form clouds. If temperatures are below freezing the water droplets are turned into ice crystals. Quick Check Classify Which parts of the water cycle contribute to the formation of clouds? Which parts do not? Evaporation and condensation contribute to the formation of clouds. Precipitation and run off do not. Critical thinking How are humidity and relative humidity alike? How are they different? Humidity and relative humidity are alike because they both measure the amount of water in the air. They are different because humidity measures the actual amount of water vapor, relative humidity measures the amount of water vapor compared to the total amount the air could hold at that temperature. What are the types of clouds? There are three basic cloud shapes. Stratus clouds appear in blanket like layers. Cumulus clouds are billowy, puffy clouds that seem to rise from flat bottoms. Cirrus clouds have wispy, featherlike shapes. Clouds are described as high, middle, or low clouds, depending on the altitudes at which they form. high altitude = prefix cirro- middle altitude = alto- suffix –nimbus or the prefix nimbo- produce precipitation For example, cumulonimbus clouds often bring thunderstorms. Cloud Cover Terms such as clear, scattered clouds, partly cloudy, mostly cloudy, and overcast are all used to describe the amount of cloud cover. An empty circle indicates clear skies. Circles with different shaded portions indicate varied amounts of cloud cover. Quick Check Classify Describe the three main cloud shapes. Stratus clouds, cumulus clouds, and cirrus clouds are the three main cloud shapes. Stratus clouds appear in blanket-like layers. Cumulus clouds are billowy, puffy clouds that seem to rise from flat bottoms. Cirrus clouds have wispy, feather-like shapes. Critical Thinking Compare altocumulus and altostratus clouds. Both are formed at the middle level altitudes. Altocumulus clouds are puffy. Altostratus have blanket like layers. What are the different types of precipitation? All forms of precipitation begin as water droplets or ice crystals in clouds; temperature differences determine the type of precipitation that forms. The four main types are rain, sleet, snow, and hail; as precipitation falls, it passes through the lower atmosphere, where the temperature determines which form the precipitation will take as it nears the ground. Clouds and Weather Clouds give clues about the weather you are going to expect. They also provide a hint of the kind of precipitation that may be forming. How Precipitation Forms Puffy cumulus clouds often appear in fair weather. Wispy cirrus clouds indicate changes in the weather. Darker stratus and cumulonimbus clouds forecast precipitation. Large cumulus clouds bring heavy rain or snow showers that end quickly. Stratus clouds bring long-lasting precipitation with smaller raindrops or snowflakes. Taller clouds produce larger rain drops. Very tall clouds hold more water and bring heavy downpours. Strong up and down air drafts within these clouds can hurl ice crystals upward over and over causing layer upon layer of ice creating hailstones. Measuring Rainfall It is very important for farmers to know how much rain or snow will fall each year and when it will come. The instrument that is used to measure rainfall is a rain gauge, a funnel-shaped or straight-sided container with a flat bottom. Quick Check Classify Which types of precipitation are associated with cumulus and stratus clouds? Puffy cumulus clouds often appear in fair weather. Large cumulus clouds can bring heavy rain or snow showers which often end quickly. Stratus clouds usually cause long-lasting precipitation with smaller raindrops or snowflakes. Taller clouds are likely to bring larger raindrops. Critical Thinking Why do you think hail forms in tall clouds? The tall clouds have different temperatures at different heights. As ice crystals in the clouds are pushed up high and then fall down again, they freeze. Then they are coated with water and freeze again. What is a thunderstorm? A thunderstorm, the most common kind of severe storm, form in cumulonimbus clouds and produce strong winds and heavy rains. Thunderstorms begin with fast rising air due to intense heat. This updraft holds water droplets and ice crystals in the clouds. When it gets too heavy to support the ice or rain, it falls as precipitation. Downdrafts in the cloud cause falling air to brush against rising air, causing an electric charge, which is lightning. Lightning can go from: cloud to ground, ground to cloud, or cloud to cloud. The lightning heats the air around it, expanding it suddenly and then contracting it as it cools, creating a sound wave we know as thunder. Tornadoes The most violent thunderstorms can produce tornadoes. Tornadoes form when cold, dry air mixes with warm, moist air. On very hot days, rising air causes powerful updrafts and the air will then begin to spin. If the updraft is strong enough, air rushes in at high speed, and the air pressure in the center drops. As more air rushes in, the pressure drops even more, and the spinning increases. Soon, a funnel forms that may touch the ground. The speed of the tornado in the clouds is not the same as the speed it moves along the ground. Tornadoes can move either quickly or slowly depending on the surface. They can also change in direction, moving first in one direction then in the other direction. Tornadoes can cause horrible damage. Quick Check Classify Categorize these storms according to degree of danger: tornado, rainstorm, thunderstorm. Explain your ranking. Least dangerous to most dangerous: rainstorm, thunderstorm, tornado; greater wind speeds increase the danger of these storms. Critical Thinking Why should people stay alert during a severe storm, even if they are indoors? Thunderstorms can spawn tornadoes; people should stay away from windows because the wind could cause them to break or flying debris could break the glass; people should listen to emergency announcements about worsening conditions or even flooding, and relocate accordingly, if necessary. What are hurricanes? Hurricanes are large, swirling storms with low pressure at the center. They begin as thunderstorms over tropical oceans near the equator and global winds push the thunderstorms west. Heat and evaporation produce a region of low pressure in the center as winds blow in toward the center and spiral upward. In the Northern Hemisphere, the Coriolis effect causes the wind to flow counterclockwise. The thunderstorms merge into one large storm that gets stronger as it travels over warm waters. Water vapor condenses and releases heat. When winds reach 120 kilometers per hour, the storm is considered a hurricane. Air pressure always presses down on the surface of the ocean. The air pressure in a hurricane is low; the greater pressure surrounding the hurricane causes the level of the sea beneath the hurricane to rise. Quick Check Classify What makes hurricanes so dangerous? Hurricanes are large, swirling storms with low pressure at the center. So they are very dangerous because they can cause very much harm and destruction with the powerful winds, heavy rainfall, and storm surge. Critical Thinking Why do you think hurricanes often lose some of their force after moving over land? They may lose some of their force because they no longer have the warm ocean water that fuels them. How Can We Predict Severe Storms? Because storms can form quickly, they are often difficult to predict. To find and track developing storms, scientists use special tools or devices such as weather satellites, radar, and planes fitted with special equipment.