Understanding Weather
Preview
Section 1 Water in the Air
Section 2 Air Masses and Fronts
Section 3 Severe Weather
Section 4 Forecasting the Weather
Concept Mapping
Section 1 Water in the Air
Bellringer
Observe two glasses of water. One is filled with ice
water, and one is filled with warm water. Why do
water droplets form on the outside of the cold glass?
Where do the water beads come from? Why don’t the
water beads form on the warm glass? Have you seen
this happen before on other containers? On your
soda can? On a coffee cup or a soup bowl?
Write your answers in your science journal.
Objectives
• Explain how water moves through the water cycle.
• Describe how relative humidity is affected bytemperature and levels of water vapor.
• Describe the relationship between dew point andcondensation.
Section 1 Water in the Air
Objectives, continued
• List three types of cloud forms.
• Identify four kinds of precipitation.
Section 1 Water in the Air
The Water Cycle
• The condition of the atmosphere is affected by theamount of water in the air. Water in liquid, solid, andgaseous states is constantly being recycled throughthe water cycle.
• The water cycle is the continuous movement ofwater from sources on Earth’s surface into the air,onto and over land, into the ground, and back to thesurface.
Section 1 Water in the Air
Humidity
• Humidity is the amount of water vapor in the air.
• The air’s ability to hold water vapor changes as thetemperature of the air changes.
Section 1 Water in the Air
Humidity, continued
• Relative Humidity is the amount of water vapor inthe air compared to the maximum amount of watervapor that the air can hold at a certain temperature.
• Calculate the relative humidity by using the formula:
relative humidity (%)
actual water vapor content (g/m3)
saturation water vapor content (g/m3)
Section 1 Water in the Air
Humidity, continued
• Measuring Relative Humidity A psychrometer is aninstrument that is used to measure relative humidity. Apsychrometer consists of two thermometers, one ofwhich is a wet-bulb thermometer.
• The difference in temperature readings between thethermometers indicates the amount of water vapor inthe air.
Section 1 Water in the Air
Condensation
• Condensation is the process by which a gas,such as water vapor, becomes a liquid.
• Dew Point The dew point is the temperature atwhich a gas condenses into a liquid.
Section 1 Water in the Air
Clouds
• A cloud is a collection of small water droplets orice crystals suspended in the air, which formswhen the air is cooled and condensation occurs.
• Clouds are classified by form, and by altitude.
Section 1 Water in the Air
Clouds, continued
• Cumulus Clouds are puffy, white clouds thattend to have flat bottoms.
• Stratus Clouds are clouds that form in layers.
• Cirrus Clouds are thin, feathery, white cloudsfound at high altitudes.
Section 1 Water in the Air
Precipitation
• Rain is the most common form of precipitation.
• Sleet and Snow Sleet forms when rain fallsthrough a layer of freezing air. Snow forms whentemperatures are so cold that water vapor changesdirectly to a solid.
• Hail are balls or lumps of ice that fall from clouds.
Section 1 Water in the Air
Section 2 Air Masses and Fronts
Bellringer
How would you describe the air you are breathing
right now? Is it warm or cool? Humid or dry? Is it
stale, sweet, or salty? The air you are breathing right
now was hundreds of miles away yesterday. Do you
know how or why air moves from one place to
another?
Write your response in your science journal.
Objectives
• Identify the four kinds of air masses that influenceweather in the United States.
• Describe the four major types of fronts.
• Explain how fronts cause weather changes.
• Explain how cyclones and anticyclones affect theweather.
Section 2 Air Masses and Fronts
Air Masses
• Changes in weather are caused by the movementand interaction of air masses.
• An air mass is a large body of air where temperatureand moisture content are constant throughout.
• Cold Air Masses Most of the cold winter weather inthe United States is influenced by three polar airmasses.
Section 2 Air Masses and Fronts
Air Masses, continued
• Warm Air Masses Four warm air masses influencethe weather in the United States.
Section 2 Air Masses and Fronts
Fronts
• The area in which two types of air masses meet iscalled a front.
• Cold Fronts A cold front forms where cold airmoves under warm air,which is less dense, andpushes the warm air up.
• Warm Fronts A warm front forms where warm airmoves over cold, denser air.
Section 2 Air Masses and Fronts
Fronts, continued
• Occluded Front An occluded front forms when awarm air mass is caught between two colder airmasses. An occluded front has cool temperatures andlarge amounts of rain and snow.
• Stationary Front A stationary front forms when acold air mass meets a warm air mass. A stationaryfront often brings many days of cloudy, wet weather.
Section 2 Air Masses and Fronts
Air Pressure and Weather
• Cyclones are areas that have lower pressure thanthe surrounding areas do.
• Anticyclones are areas that have high pressure.
Section 2 Air Masses and Fronts
Bellringer
Write a one-paragraph description of a thunderstorm.Describe the weather conditions immediately before,during, and after the storm. How does the storm affecteach of your senses?
Write your paragraph in your science journal.
Section 3 Severe Weather
Section 3 Severe Weather
Objectives
• Describe how lightning forms.
• Describe the formation of thunderstorms, tornadoes,and hurricanes.
• Describe the characteristics of thunderstorms,tornadoes, and hurricanes.
• Explain how to stay safe during severe weather.
Thunderstorms
• Lightning is an electric discharge that occursbetween a positively charged area and a negativelycharged area. Thunderstorms are very activeelectrically.
• Thunder is the sound that results from the rapidexpansion of air along the lightning strike.
Section 3 Severe Weather
Tornadoes
• A tornado is a small, spinning column of air that hashigh wind speeds and low central pressure and thattouches the ground.
• A tornado starts out as a funnel cloud that pokesthrough the bottom of a cumulonimbus cloud andhangs in the air. The funnel cloud becomes a tornadowhen it makes contact with Earth’s surface.
Section 3 Severe Weather
Hurricanes
• How a Hurricane Forms A hurricane begins as agroup of thunderstorms moving over tropical oceanwaters. Winds traveling in two different directionsmeet and cause the storm to spin.
• Damage Caused by Hurricanes Hurricanes cancause a lot of damage when they move near or ontoland. Wind speeds of most hurricanes range from120 to 150 km/h.
Section 3 Severe Weather
Severe Weather Safety
• Thunderstorm Safety Lightning is one of the mostdangerous parts of a thunderstorm. If you areoutside, stay away from trees, which can get struckdown. If you are in the open, crouch down.
• Tornado Safety If there is a tornado warning foryour area, find shelter quickly. The best place to gois a basement or cellar.
Section 3 Severe Weather
Severe Weather Safety, continued
• Flood Safety The best thing to do during a floodis to find a high place to wait out the flood.
• Hurricane Safety If you live in an area wherehurricanes strike, your family should have a disastersupply kit that includes enough water and food tolast several days.
Section 3 Severe Weather
Section 4 Forecasting the Weather
Bellringer
If you did not have the benefit of the weatherforecast on the news, radio, or television, howwould you forecast the weather?
Record your answer in your science journal.
Objectives
• Describe the different types of instruments used totake weather measurements.
• Explain how radar and weather satellites helpmeteorologists forecast the weather.
• Explain how to interpret a weather map.
Section 4 Forecasting the Weather
Weather-Forecasting Technology
• High in the Sky Weather balloons carry electronicequipment that can measure weather conditions ashigh as 30 km above Earth’s surface.
• Measuring Air Temperature and Pressure A toolused to measure air temperature is called athermometer. An instrument used to measure airpressure is a barometer.
Section 4 Forecasting the Weather
Weather-Forecasting Technology, continued
• Measuring Wind Direction Wind direction can bemeasured by using a windsock or a wind vane.
• Measuring Wind Speed An instrument used tomeasure wind speed is called an anemometer.
• Radar and Satellites Radar is used to find thelocation, movement, and amount of precipitation.Weather satellites that orbit Earth provide images ofweather systems.
Section 4 Forecasting the Weather
Weather Maps
• Who Makes the Maps The National WeatherService produces weather maps based on informationgathered from about 1,000 weather stations across theUnited States.
Section 4 Forecasting the Weather
Weather Maps, continued
• Reading a Weather Map Weather maps that you seeon TV include lines called isobars. Isobars are lines thatidentify areas of equal, high, or low air pressure.
Section 4 Forecasting the Weather
Understanding Weather
Concept Mapping
Use the terms below to complete the concept map onthe next slide.
waterair masses
hailcirrus
cumulusclouds
humidityfronts
weather
