Chapter: The Atmosphere in Motion Table of Contents Chapter: The Atmosphere in Motion Section 2: Earth’s Weather

Earth’s Weather 2 A. Weather 1. Weather describes the current condition of the atmosphere, including temperature, cloud cover, wind speed, wind direction, humidity, and air pressure.

Earth’s Weather 2 B. Temperature 1. Temperature is a measure of the average movement of molecules and is measured with a thermometer. 2. The faster the molecules move, the higher the temperature goes up. 3. The two scales commonly used to measure temperature are the Celsius and Fahrenheit scales.

Earth’s Weather 2 C. Energy Transfer Fast-moving molecules transfer energy to slower-moving molecules when they bump into each other. This is called conduction. It is conduction that transfers heat from Earth’s surface to those molecules in the air that are in contact with it.

Earth’s Weather 2 C. Energy Transfer 3. The heated air rises. As it rises, it starts to cool. 4. If air becomes cooler than the surrounding air, it will sink. The process of warm air rising and cool air sinking is called convection. 5. Convection is the main way that heat is transferred throughout the atmosphere.

D. Atmospheric Pressure Earth’s Weather 2 D. Atmospheric Pressure 1.Air pressure decreases with altitude in the atmosphere. This is because as you go higher, the weight of the atmosphere above you is less.

D. Atmospheric Pressure Earth’s Weather 2 D. Atmospheric Pressure 2. Temperature and pressure are related. When air is heated, its molecules move faster, and the air expands. This makes the air less dense, which is why heated air gets moved upward. 3. Less dense air also exerts less pressure on anything below it, creating lower pressure. Therefore, rising air generally means lower pressure and sinking air means higher pressure

Earth’s Weather 2 E. Humidity 1. Humidity is the amount of water vapor in the atmosphere. The graph shows how temperature affects how much moisture can be present in the air.

Earth’s Weather 2 E. Humidity 2. When air is holding as much water vapor as it can, it is said to be saturated and condensation can occur. The temperature at which this takes place is called the dew point.

Earth’s Weather 2 F. Relative Humidity 1. Relative humidity is a measure of the amount of water vapor that is present compared to the amount that could be held at a specific temperature. 2. When air is holding all of the water vapor it can at a particular temperature, it has 100 percent relative humidity.

Earth’s Weather 2 G. Clouds Clouds form when air rises, cools to its dew point, and becomes saturated. Water vapor then condenses onto small particles in the air. If the temperature is not too cool, the clouds will be made of small drops of water. If the temperature is cold enough, clouds can consist of small ice crystals. Clouds are classified according to the height above Earth’s surface at which they form.

Earth’s Weather 2 H. Low Clouds The low-cloud group consists of clouds that form about 2,000 m or less in altitude. These clouds include the cumulus, layered stratus, and nimbostratus cloud. Fog is a type of stratus cloud that is in contact with the ground.

Earth’s Weather 2 I. Middle Clouds Clouds that form between about 2, 000 m and 8,000 m are known as the middle-cloud group. Their names often have the prefix alto- in front of them, such as altocumulus and altostratus. Sometimes they contain enough moisture to produce light precipitation.

J. High and Vertical Clouds Earth’s Weather 2 J. High and Vertical Clouds 1. Some clouds occur in air that is so cold they are made up entirely of ice crystals. These are known as the high-cloud group. 2. They include cirrus (SIHR us) clouds and cumulonimbus (kyew myuh loh MIHM bus). These clouds create the heaviest precipitation of all.

Earth’s Weather 2 K. Precipitation When drops of water or crystals of ice become too large to be suspended in a cloud, they fall as precipitation. The difference type of precipitation that falls depends on the temperature of the atmosphere. For example, rain falls when the air temperature is above freezing while the air near Earth’s surface is below freezing, the result might be freezing rain.

Earth’s Weather 2 K. Precipitation 4. Hail is balls of ice that form within a storm cloud. As the ice crystals move, droplets of water freeze around them. 5. Hailstones keep growing until they are too heavy for the winds to keep up. Then they fall to the ground.

Earth’s Weather 2 L. Wind Air moves from high-pressure areas toward low-pressure areas. This movement is called wind. The greater the difference in temperature of pressure between areas, the stronger the wind is. Wind speed is measured by and instrument called an anemometer (an uh MAH muh tur). The fastest wind speed ever measure was 371 km/h. (230 mph)

M. Global Air Circulation Earth’s Weather 2 M. Global Air Circulation 1. In any given year, the Sun’s rays strike Earth at a higher angle near the equator than near the poles. 2. Warm air flows toward the poles from the tropics and cold air flows toward the equator form the poles.

M. Global Air Circulation Earth’s Weather 2 M. Global Air Circulation 3. Because Earth rotates, this moving air is pushed to the right in the northern hemisphere and to the left south of the equator. This is known as the Coriolis (kor ee OH lus) effect.

Earth’s Weather 2 N. Surface Winds Air at the equator is heated by the sun. This air expands, becomes less dense, and gets pushed upward. At about 30 degree latitude, the air is somewhat cooler. This air sinks and then flows toward the equator. As this air flows, it is turned by the Coriolis effect.

Earth’s Weather 2 N. Surface Winds 3. The result is steady winds that blow from east to west. These steady winds are called the trade winds or tropical easterlies.

O. Westerlies and Easterlies Earth’s Weather 2 O. Westerlies and Easterlies 1. Between 30° and 60° latitude north and south of the equator, winds usually blow from the west. 2. These winds form between the cold air from the poles and warmer air closer to the equator. These winds are called the prevailing westerlies.

O. Westerlies and Easterlies Earth’s Weather 2 O. Westerlies and Easterlies 3. Between 30° and 60° latitude north and south regions are known for frequent storms. 4. Similar winds near the poles blow from the east are known as the polar easterlies.

Earth’s Weather 2 P. Jet Streams Jet streams are bands of strong winds that develop at higher altitudes within the zone of the prevailing westerlies. Jet streams are like giant rivers of air. They are important because weather systems move along their paths.

Earth’s Weather 2 Q. Other Winds Slight differences in pressure create gentle breezes and great differences create strong winds. The strongest winds occur when air puches into the center of the low pressure. This can cause severe weather like tornadoes and hurricanes.

Section Check 2 Question 1 How are temperature and pressure related?

Section Check 2 Question 2 Which winds blow in an east-to-west direction and are formed by cool, sinking air?

B. prevailing westerlies C. surface winds D. trade winds Section Check 2 A. polar easterlies B. prevailing westerlies C. surface winds D. trade winds

Section Check 2 Question 3 Which are responsible for producing heavy precipitation and thunderstorms? A. altostratus clouds B. cirrus clouds C. cumulonimbus clouds D. nimbostratus clouds

End of Chapter Summary File