Section 2 Air Masses and Fronts Objectives Identify the four kinds of air masses that influence weather in the United States. Describe the four major types of fronts. Explain how fronts cause weather changes. Explain how cyclones and anticyclones affect the weather. Chapter 3

Section 2 Air Masses and Fronts Air Masses Changes in weather are caused by the movement and interaction of air masses. An air mass is a large body of air where temperature and moisture content are constant throughout. Cold Air Masses Most of the cold winter weather in the United States is influenced by three air masses formed over the polar region. Chapter 3

Section 2 Air Masses and Fronts Air Masses, continued Chapter 3 Cold and wet Cold and dry Cold and wet

Section 2 Air Masses and Fronts Air Masses, continued Warm Air Masses Three warm air masses influence the weather in the United States. Chapter 3

Section 2 Air Masses and Fronts Air Masses, continued Chapter 3 Warm and wetWarm and dry Warm and wet

Section 2 Air Masses and Fronts Fronts Front - the area in which two types of air masses meet Chapter 3

Section 2 Air Masses and Fronts Cold Front - where cold air moves under warm air, which is less dense, and pushes the warm air up. Chapter 3

Section 2 Air Masses and Fronts Warm Fronts - where warm air moves over cold, denser air. Chapter 3

Section 2 Air Masses and Fronts Occluded Front - forms when a warm air mass is caught between two colder air masses. This front has cool temperatures and large amounts of rain and snow Chapter 3

Section 2 Air Masses and Fronts Stationary Front - forms when a cold air mass meets a warm air mass, often brings many days of cloudy, wet weather Chapter 3

Section 2 Air Masses and Fronts Air Pressure and Weather Cyclones are areas that have lower pressure than the surrounding areas do. Air moves in and rises. Air moves counter-clockwise, toward the center. Anticyclones are areas that have high pressure. Air moves clockwise, away from the center. Chapter 3 Strong winds can be whipped between the two areas.

Facts you will need to know for the quiz:

Cold Front 1. Cold pushes under warm 2. Cumulous clouds 3. Stormy, cloudy weather 4. High winds 5. Temperature changes from warm to cold

Warm front 1. Warm moves over cold 2. Stratus clouds 3. Clouds form, possibly storms 4. Light winds 5. Temperature changes from cold to warm

Occluded front 1. Warm is caught between 2 cold, warm is pushed up 2. Cumulonimbus clouds 3. Very stormy and cloudy 4. Strong winds, then lighter winds 5. Temperature changes from cold to warm to cold

Stationary front 1. Warm pushes against cold, both are equally strong 2. Nimbostratus clouds 3. Many days of clouds, possibly rain 4. Light winds 5. Temperature does not change

M = maritime Air mass is formed over water, is has high humidity (it is wet).

C = continental Air mass is formed over land, is has low humidity (it is dry).

P = polar Air mass is formed in the north, is has lower temperature.

T = tropical Air mass is formed in the south, is has higher temperature.

Water falling from clouds is called precipitation. Precipitation What is weather? 1 1 Precipitation occurs when cloud droplets combine and grow large enough to fall to Earth, 0.2 mm. That is 100 times larger than the tiny “misty” droplets that make up a cloud. The water needs a “seed”, a speck of dust or salt on which to attach.

The size of raindrops depends on several factors. One factor is the strength of updrafts in a cloud, which can keep drops suspended in the air where they can grow larger. Precipitation What is weather? 1 1 The rate of evaporation as a drop falls to Earth also can affect its size.

Air temperature determines whether water forms rain, snow, sleet, or hail—the four main types of precipitation. Precipitation What is weather? 1 1

Precipitation What is weather? 1 1 Drops of water falling in temperatures above freezing fall as rain. Snow forms when the air temperature is below freezing. Snow is formed when water goes directly from vapor to solid forming unique 6-sided shapes.

Sleet forms when raindrops pass through a layer of warm air, melts, and then refreezes near Earth’s surface, forming ice pellets. Precipitation What is weather? 1 1

Hail is precipitation in the form of lumps of ice. Hail What is weather? 1 1 Hail forms in cumulonimbus clouds of a thunderstorm when water freezes in layers around a small nucleus of ice. They are tossed up and down by rising and falling convection currents. Hail may be the most dangerous form of precipitation.