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Ch 21 Understanding Weather •Water in the Air •Air Masses and Fronts •Severe Weather Water in the Air •The Water Cycle •Evaporation •Condensation •Precipitation •Runoff Water in the Air •Humidity- the amount of water vapor in the air Water in the Air • Dew point- the temperature to which air must cool to become saturated. • Saturated- when air holds all the water vapor it can at a given temperature Clouds • Cumulus- puffy white clouds with flat bottoms • Stratus- clouds that form in layers • Cirrus- feathery, wispy white clouds at high altitudes Air Masses and Fronts •Cold Front- a fast-moving cold air mass moves under less dense warm air, pushing it upwards •Results in thunderstorms, cooler weather Air Masses and Fronts •Warm Front- warm, less dense air moves over the cold, denser air, bringing drizzly rain and then clear warm weather Air Masses and Fronts •Occluded Front- a warm air mass is caught between two colder masses, resulting in cool temperatures and lots of precipitation Air Masses and Fronts •Stationary Front- cold and warm air masses meet but do not have the force to create storms Severe Weather • Thunderstorms- heavy rainstorms with strong winds, lightning and thunder • Lightning- an electric discharge that takes place between a positively charged area and a negatively charged area • Thunder- the sound caused by the rapid expansion of air along the lightning strike Severe Weather • Tornado- a small, spinning column of air that has high wind speed and touches the ground • Hurricane- a large, spinning tropical weather system with wind speeds of at least 119 km/hr Severe Weather • How does a tornado form? • Wind moving in two different directions causes a layer of air in the middle to spin like a roll of toilet paper. It works it’s way to the bottom of the cloud and eventually to the ground. Severe Weather • How does a Hurricane form? • It’s a group of thunderstorms that form over the oceans. Wind that travels in different directions causes it to spin. Energy is gathered from warm waters. Severe Weather • A weather forecast is a prediction of weather conditions over the next 3-5 days. • A meteorologist is a person that observes and collects data on atmospheric conditions. Forecasting Weather • Meteorologists use a barometer to measure air pressure and an anemometer to measure wind speed. • Weather balloons carry radio transmitters that send measurements to the weather stations