Fronts and MassesUNIT 4, LESSON 3

Warm Up – November 5

Right

Now

How does the water cycle affect weather?• Evaporation affects the weather by:

• Adding water vapor in the air which affects humidity

• Condensation affects weather by:

• Forming clouds and fog, cooling the land and affecting visibility

• Precipitation affects weather by:

• Bringing water to the ground, cooling the land

• Think about it! Why is the word continuous used to describe the water cycle?

How do air masses affect weather?• An air mass is a large volume

of air in which temperature and moisture content are nearly the same throughout.

• An air mass forms when air over a large region stays in one area for many days.

• The air gradually takes on the temperature and humidity of the land or water below it

(what kind of heat transfer would this be?)

Watch this video on Air Masses!

Types of Air Masses

Continental Tropical Air Mass

Maritime Tropical Air Mass

Continental Polar Air Mass

Maritime Polar Air Mass

Where do fronts form?• When two different air masses

meet, density differences usually keep them from mixing

• A cool air mass is more dense than a warm air mass, so a boundary called a front forms between the air masses.

• Fronts result in a change in weather as they pass

• For fronts to form, one air mass must run into another air mass

• Described by the type of mass “winning”

• Watch this video on Fronts!

Where do fronts form? Cold Fronts• Cold Fronts form where cold

air moves under warm air

• A cold air mass that is moving quickly can push up a warm air mass.

• If the warm air is moist, clouds will form.

• Storms that form along a cold front are short, but move quickly and bring heavy rain or snow

• Cooler weather follows a cold front.

Where do fronts form? Warm Front• Warm Fronts form where

warm air moves over cold air

• When a warm air mass, follows a retreating cold air mass, the warm air rises over the cold air and its moisture condenses into clouds.

• Warm fronts bring drizzly rain and are followed by warm, clear weather.

Where do fronts form? Stationary Front• A stationary front forms

where cold and warm air stop moving.

• In a stationary front, there is not enough wind for either the cold or warm air mass to move, so the two air masses remain in one place.

• A stationary front usually causes many days of unchanging weather, usually clear.

Turn and Talk – Show what you know!1. Why in the video, did the

narrator compare fronts and masses to two different militaries fighting?

2. When could a stationary front become a warm or a cold front?

3. How are fronts and masses related?

4. Describe the symbols that you saw in the diagrams. What did the symbols represent?

High Pressure System• In a high pressure system, air sinks

slowly down because the air is cooler and denser. As the air nears the ground, it spreads out towards areas of lower pressure.

• Most high pressure systems are large and change slowly.

• When a high pressure system stays in one area for a long time, an air mass may form.

• A high pressure system brings clear skies and clam air or gentle breezes.

Low Pressure System• In a low pressure system, air rises

and has a lower air pressure than the area around it.

• As the air rises, it begins to cool and condense, and forms clouds and rain.

• A low pressure system can develop wherever there is a center of low pressure, such as along the boundary of a warm air mass and cold air mass.

• Rain often occurs at these boundaries, or fronts.

What are pressure systems and how do they interact?

Turn and Talk: Describe the High and Low pressure system….

Study Jams – Fronts and Masses

Describe what’s happening on the weather map!

Do you see high pressure and low pressure systems?

What kind of weather is occurring on this map?

Can you make a prediction about how the weather may change?