Bell work 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?  

Fronts

Describe the four major types of 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.

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 similar throughout. Air masses are characterized by their moisture content and temperature.

Air Masses The moisture content and temperature of an air mass are determined by the area over which the air mass forms. These areas are called source regions. There are many types of air masses, each of which is associated with a particular source region.

The characteristics of these air masses are represented on maps by a two-letter symbol. The first letter indicates the moisture content that is characteristic of the air mass. The second letter represents the temperature that is characteristic of the air mass. Air Masses

Air Mass Symbols M= Maritime, forms over the water; wet C= Continental, forms over land; dry T= Tropical, forms near the equator; warm P= polar, forms near the poles; cold mT=wet and warm mP=wet and cold cT=dry and warm cP=dry and cold

Fronts The area in which two types of air masses meet is called a front. The four kinds of fronts—cold fronts, warm fronts, occluded fronts, and stationary fronts

Describe the four major types of 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.

Cold Front Cold dense air pushes warm air out of the way Cold fronts move very quickly and bring short periods of rain/thunderstorms Lower temperatures are behind the front SYMBOL – the direction of the “arrows” points towards the direction the front is MOVING

Cold Front: Warm air is abruptly pushed upward, cooling, condensing moisture into cumulus or cumulonimbus clouds                                                                               

Cold Front Notice how steep the angle is between the two air masses Typically brings sudden, heavy rains and storms

Describe the four major types of 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.

Warm Front Warm air moves up the cold front as it slowly displaces the cold air Warm fronts move slowly, and bring many days of steady precipitation Higher temperatures are behind the front SYMBOL – direction of “half-moons” is the direction the front is moving

Warm Front WARM FRONT: when a warm air mass moves into a colder,denser air mass. Warm air rides up and over the colder air Notice the angle of slope between the two air masses.                                                                               

Warm Front The weather during a WARM FRONT starts with cirrus clouds about 24-48 hours before the rain begins Cirrus clouds are “at the front of the front”

Warm Front As more warm air is pushed upward, more moisture condenses forming cirrostratus clouds

Warm Front As warm more warm air is pushed up, heavier clouds form mid-way up over the cold air Altostratus and stratus

Warm Front The final cloud type in a warm front is the nimbostratus “nimbo” = “rain”

Warm front: rain or snow is steady over several hours or days

Describe the four major types of 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.

Occluded Fronts An occluded front forms when a warm air mass is caught between two colder air masses. The coldest air mass moves under and pushes up the warm air mass. The coldest air mass then moves forward until it meets a cold air mass that is warmer and less dense.

Occluded Fronts The colder of these two air masses moves under and pushes up the warmer air mass. Sometimes, though, the two colder air masses mix. An occluded front has cool temperatures and large amounts of rain and snow.

Stationary Front The air from the warm front and cold front meet, but do not move These fronts have the same weather as warm fronts SYMBOL – warm and cold fronts are moving in opposite directions, thus making a stationary condition

Stationary front A stationary front forms when a cold air mass meets a warm air mass. Both air masses do not have enough force to lift the warm air mass over the cold air mass. The two air masses remain separated. A stationary front often brings many days of cloudy, wet weather.

Describe the four major types of 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.