AIR MASSES AND FRONTS

TWO FACTS ABOUT WEATHER AND WHY WEATHER EXISTS:

• Weather exists because of unequal heating of the surface of the earth by the sun.• The heating of the lower atmosphere by energy transfer

from the surface also contributes to the various weather conditions found on Earth.

WHAT CAUSES CHANGES IN WEATHER?

• Changes in weather are caused by movement and interaction of air masses.

• An air mass is a large body of air that has similar temperature and moisture content throughout.

• Air masses are characterized by their moisture content and temperature.

• The moisture content and temperature of an air mass are determined by the area that the air mass forms over.

• The area over which an air mass forms is called a source region

AIR MASS• Air Mass is an extremely large body of air whose

properties of temperature and moisture content (humidity), at any given altitude, are fairly similar in any horizontal direction.• can cover hundreds of thousands of square miles.• there can be small variations

SOURCE REGIONS

• Source Regions are simply geographic areas where an air mass originates.

• uniform surface composition - flat• light surface winds• The longer the air mass stays over its source region, the

more likely it will acquire the properties of the surface below.

HOW DO SOURCE REGIONS AFFECT AIR MASSES?

• The Gulf of Mexico is a example of a source region.• The Gulf of Mexico is a warm region that has a lot of water

in it that is constantly evaporating into the air, so an air mass that forms over with Gulf of Mexico will also be warm and wet.

SOURCE REGIONS AND AIR MASSES

• There are many types of air masses• Each type of air mass 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 represents the moisture content and

the second letter represents the temperature that is characteristic of the air mass.

• m= maritime• c= continental• P=Polar• T=Tropical

: 4 GENERAL AIR MASS CLASSIFICATIONS CATEGORIZED ACCORDING TO THE SOURCE REGION• polar latitudes P - located poleward of 60 degrees north

and south• tropical latitudes T - located within about 25 degrees of

the equator• continental c - located over large land masses--dry• marine m - located over the oceans----moist

THESE COMBINATIONS CAN BE MADE TO DESCRIBE THE DIFFERENT TYPES OF AIR MASSES.

• cP continental polar cold, dry, stable• cT continental tropical hot, dry, stable air aloft--unstable

surface air• mP maritime polar cool, moist, and unstable• mT maritime tropical warm, moist, usually unstable

AIR MASS MAP

AIR MASSES THAT AFFECT THE US INCLUDE:

• cP -- wintertime bitter cold can extent to Southern US and even Florida causing crop damage. Require long, clear nights, which means strong radiational cooling of air near the surface. A stable air mass. Little moisture added so air is dry

• mP -- Winter cP air moves over a region such as the NE Pacific, picking up some warmth and moisture from the warmer ocean. In the case of the Pacific NW mountains force the air to rise causing rain.

AIR MASSES THAT AFFECT THE US INCLUDE:• mT -- wintertime source for the SW US is the subtropical East Pacific Ocean. mT air

that influences weather east of the Rocky Mountains comes from the Gulf of Mexico, but only influences winter weather in the SE states. Occasionally, slow moving weather systems in SW flow aloft can draw up moisture at mid and low levels producing precipitation.

• cT -- Continental tropical air usually only influences the US in summertime as warm, dry air is pumped up off of the Mexican Plateau. It is usually fairly stable and dry, and if it becomes stagnant over the midwest, results in a drought. Deaths associated with the 1995 heat wave in the midwest were the result of cT and mT air which stagnated over the central and eastern part of the US this last summer .

COLD AIR MASSES

• The majority of the cold winter weather in the United States is influenced by three polar air masses.

• The continental polar (cP) air mass forms over northern Canada. This air mass brings extremely cold winter weather to the US, and in the summer this air mass will cause cool, dry weather.

• A maritime polar (mP) air mass that forms over the Northern Pacific Ocean will bring cold wet weather, like rain and snow, to the west coast of the US in the winter. This air mass will bring cool, foggy weather in the summer.

• A maritime polar air mass that forms over the Northern Atlantic Ocean will bring cool, cloudy weather and precipitation to the New England states in the winter and cool weather and fog in the summer.

WARM AIR MASSES

• There are four warm air masses that can influence the weather in the United States

• Maritime tropical (mT) air masses that form over the Pacific Ocean are milder than the maritime polar air masses that form over the Northern Pacific Ocean.

• Maritime tropical air masses that form over the Atlantic Ocean and the Gulf of Mexico will move north across the East Coast and into the Midwest bringing hot humid weather, hurricanes, and thunderstorms in the summer and mild cloudy weather in the winter.

• A continental tropical (cT) air mass that forms over the deserts of northern Mexico and the southwestern United States will move northward and bring clear, dry, and hot weather in the summer.

AIR MASSES CAN CONTROL THE WEATHER FOR A RELATIVELY LONG TIME PERIOD: FROM A PERIOD OF DAYS, TO MONTHS. MOST WEATHER OCCURS ALONG THE PERIPHERY OF THESE AIR MASSES AT BOUNDARIES CALLED FRONTS.

• http://www.classzone.com/books/earth_science/terc/content/visualizations/es2001/es2001page01.cfm?chapter_no=visualization

WHAT ARE FRONTS?

• Air masses that form from different source regions often do not mix.

• This is because they have different densities

• Since warm air is less dense than cold air, the warm air generally rises over the cold air

• The area where two air masses meet is called a Front

• There are four different kinds of fronts

• Cold Front, Warm Front, Occluded Front, and a Stationary Front

WHAT MAKES THE FRONTS DIFFERENT?

• Front is the transition zone between air masses with distinctly different properties.

• The differences in density are most often caused by temperature differences, although air masses have different humidity as well.

• We identify fronts by the movement of this transition zone and the properties that move over a geographical location

THE FOUR DIFFERENT FRONTS: COLD FRONT

• A cold front is formed when cold air moves under less dense warm air and quickly pushes it up.

• Cold fronts can move quickly and bring storms and heavy precipitation.

• Cooler weather usually follows a cold front because a the air mass behind a cold front is normally cooler and drier than the air mass that it is pushing out.

COLD FRONT

THE FOUR DIFFERENT FRONTS: WARM FRONT

• A warm front forms when warm air moves over the top of colder more dense air, and then gradually replaces the cold air.

• Warm fronts usually cause drizzly rain and are followed by clear warm weather.

WARM FRONT

THE FOUR DIFFERENT FRONTS: OCCLUDED FRONT

• When a warm air mass is caught between two cold air masses an occluded front forms.

• The coldest air mass will push up the warm air mass and continue to move forward until it meets the other cold air mass.

• The coldest air mass will usually push the less cold air mass up above it, but sometimes the two cold air masses will mix

• Occluded fronts bring cool temperatures and large amounts of precipitation.

OCCLUDED FRONT

THE FOUR DIFFERENT FRONTS: STATIONARY FRONT

• When a warm air mass meets a cold air mass and neither air mass has enough energy to push the warm air mass over the cold air mass a stationary front forms.

• This causes the two air masses to remain separated.

• A stationary front can form when the winds are not strong enough to keep the air masses moving.

• Stationary fronts will often bring many days of cloudy, wet weather.

STATIONARY FRONT

AIR PRESSURE AND ITS AFFECT ON THE WEATHER

• We have learned that wind is the movement of air due to differences in air pressure.• Differences in air pressure can also affect the weather• Differences in air pressure can result in the formation of

cyclones and anticyclones

WHAT ARE CYCLONES AND ANTICYCLONES?• A cyclone is an area with lower pressure than its surrounding areas, and has winds that

spiral towards the center, where two air masses converge and rise.

• Air enters the area of low pressure from all directions and because of the Coriolis effect the winds bend to the right which creates a counterclockwise rotation (in the northern hemisphere) around the low.

• Near the center of the system the air converges and is forced aloft where a divergence occurs and takes the air away from the system.

• If not for the divergence of air at the top of the system the system would soon fill with air and the pressures would equalize which would cause the system to dissipate.

SO WHAT IS AN ANTICYCLONE?

• An anticyclone is the rotation of air around a high-pressure center in a clockwise direction (in the northern hemisphere).

• In an anticyclone the convergence of air masses occurs aloft and the colder more dense air is forced downward towards the surface.

• At the surface a divergence occurs and takes the air away from the system which allows the system to remain active.

• The Coriolis effect bends the winds to the right and creates a clockwise rotation in the northern hemisphere and a counterclockwise direction in the southern hemisphere.

IMAGES OF CYCLONES AND ANTICYCLONES

HOW DO CYCLONES AND ANTICYCLONES AFFECT WEATHER?

• As the warm rising air in the center of a cyclone rises it cools which causes the water vapor in the air to condense. This forms clouds and brings rain.

• The rising air in the center of a cyclone causes stormy weather.

• In an anticyclone the cool dense sinking air warms as it sinks towards the surface. The dry sinking air warms which allows it to absorb moisture.

• The sinking air in an anticyclone brings dry, clear weather.

• Meteorologists keep track of cyclones and anticyclones which allows them to more accurately predict the weather.