Changing Weather

From: Ck-12.org Foundation Earth Science Chapter 16

By: Robert Smith

• weather in a location often depends on what type of air mass is over the location

• A key factor is whether the spot is beneath a front, the meeting place of two air masses

• characteristics of the air masses and their interactions determine whether the weather over an area is constant, or whether there are rapid changes

Air Mass

• air mass is a batch of air that has nearly the same temperature and humidity

• air mass acquires characteristics above an area of land or water known as its source region

• air mass that sits over a region for several days, or longer, it picks up the distinct temperature and humidity

Air Mass Formation

• Air masses form over a large area• Air masses form primarily in high pressure

zones, most commonly in polar and tropical regions

• Temperate zones are ordinarily too unstable for air masses to form

• air masses move across temperate zones so the middle latitudes are prone to having interesting weather

cPk

• symbol cPk is an air mass with a continental polar source region that is colder than the region it is now moving over

Air Mass Movement

• Air masses are slowly pushed along by high-level winds

• temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it

• cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles

Fronts

• Two air masses meet at a front• the two air masses have different densities

and do not easily mix• One air mass is lifted above the other, creating

a low pressure zone• If the lifted air is moist, there will be

condensation and precipitation. • Winds are common

Four Types of Fronts

• stationary front the air masses do not move• may bring days of rain, drizzle, and fog• Winds usually blow parallel to the front, but in

opposite directions

Cold Fronts• a cold air mass takes the place of a warm air

mass, there is a cold front• squall line is a line of severe thunderstorms

that forms along a cold front. Behind the front is the cold air mass

Cold fronts vary by season

• spring and summer: The air is unstable so thunderstorms or tornadoes may form.

• spring: If the temperature gradient is high, strong winds blow.

• autumn: Strong rains fall over a large area.• winter: The cold air mass is likely to have

formed in the frigid arctic so there are frigid temperatures and heavy snows.

Warm Front

• a warm air mass slides over a cold air mass

Occluded Front

• occluded front usually forms around a low pressure system

• starts when a cold front catches up to a warm front

• air masses, in order from front to back, are cold, warm, and then cold again

• If the air mass that arrives third is colder than either of the first two air masses, that air mass slip beneath them both. This is called a cold occlusion

Images of Weather