Science ~ chapter 8weather Miss Nelson
Section 1 Water in the Atmosphere
Anticipatory set What happens to the mirror in your bathroom when you take a hot shower?
Anticipatory set What happens to the mirror in your bathroom when you take a hot shower? Warm, moist air from the shower comes in contact with cool surface of the mirror. The air cools and can hold less water vapor. As a result, water vapor condenses on the mirror. Clouds form in the same way!
standards S 6.4.e – Students know differences in pressure, heat, air movement, and humidity result in change in weather
The big idea What is humidity and how is it measured? How do clouds form? What are the three main types of clouds?
Key terms • Water cycle – the continual movement of water among Earth’s atmosphere, oceans, and land through evaporation, condensation, and precipitation • Evaporation – the process by which water molecules in liquid water escape into the air as water vapor • Humidity – the amount of water vapor in a given volume of air • Relative humidity - the percentage of water vapor in the air compressed to the maximum amount of water vapor that air can contain at a given temperature • Psychrometer – an instrument used to measure relative humidity, consisting of a wet-bulb thermometer and a dry-bulb thermometer
Key terms • Condensation – the process by which molecules of water vapor in the air become liquid water • Dew point – the temperature at which condensation begins • Cirrus – wispy, feathery clouds made mostly of ice crystals that form at high levels • Cumulus – fluffy, white clouds, usually with flat bottoms that look like rounded piles of cotton • Stratus – clouds that form in flat layers and often cover much of the sky
The Water cycle View the diagram on page 301 of your textbook Read Water in the Atmosphere on page 300 of your textbook
humidity Humidity is a measure of the amount of water vapor in the air Air’s ability to hold water vapor depends on its temperature Warm air = more water vapor than cool air
Relative humidity • Weather reports usually refer to the water vapor in the air as relative humidity • Relative humidity is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold at a particular temperature • Air with a relative humidity of 100 percent is said to be saturated
Measuring relative humidity Relative humidity can be measured with an instrument called a psychrometer
humidity Read Humidity on pages 301-302 of your textbook
How clouds form Clouds form when water vapor in the air condenses to form liquid water or ice crystals Process is called condensation
How clouds form Two conditions are required for condensation: • Cooling of the air • Presence of particles in the air
The role of cooling • As air cools, the amount of water vapor it can hold decreases • The water vapor condenses into tiny droplets of water or ice crystals • The temperature at which condensation begins is called the dew point • Dew point above freezing = water droplets • Dew point below freezing = ice crystals
The role of particles • For water vapor to condense, tiny particles must be present • The water vapor needs a surface on which to condense • In cloud formation, most of these particles are: • Salt crystals • Dust from soil • Smoke
Types of clouds • Scientists classify clouds into three main types based on their shape: • Cirrus • Cumulus • Status • Clouds are further classified by their altitude
Cirrus clouds • Wispy, feathery clouds • Comes from the word meaning “a curl of hair” • Form only at high levels where temperatures are very low • As a result, are made of ice crystals • Have feathery “hooked” ends • Look like the scales of a fish
Cumulus clouds • Look like fluffy, rounded piles of cotton • Word means “heap” or “mass” in Latin • Usually indicate fair weather • When you add the suffix –nimbus, which means “rain,” you get clouds that produce thunderstorms
Stratus clouds • Form in flat layers • Strato means “spread out” • Usually cover most of the sky • Are a uniform, dull, gray color • As they thicken they produce rain or snow • Called nimbostratus clouds
Altocumulus and altostratus • Part of a cloud’s name may be based on its height • Clouds that form between 2-6 kilometers above the surface have the prefix alto- (means high) • Altocumulus and altostratus are the two main types of these clouds
fog • Clouds that form near the ground are called fog • Often forms when the ground cools at night after a warm, humid day • More common near bodies of water or low-lying marshy areas
Types of clouds Read Types of Clouds on pages 304 – 306 of your textbook
Check for understanding What is humidity? How is it measured?
Check for understanding What is humidity? Humidity is a measure of the amount of water vapor in the air. How is it measured? Humidity is measured using “relative humidity,” which is the percentage of water vapor that is actually in the air compared to the amount of water vapor the air can hold at a particular temperature. Relative humidity can be measured using a psychrometer.
Guided practice How do clouds form? What are the three main types of clouds?
Guided practice How do clouds form? Clouds form when water vapor in the air condenses to form liquid water or ice crystals. What are the three main types of clouds? The three main types of clouds are cirrus, cumulus, and stratus.
Independent practice Complete Weather 8-1 Independent Practice