# Presentation: 274555

## Air masses

- and
- Fronts

- UPDATED: 28 FEB 2000

## An air mass is an extremely large body of air whose 

**An **_**air mass**_** is an extremely large body of air whose **
**properties of temperature and moisture are fairly**
**similar in any horizontal direction at any given altitude. **

## A source region is a large area of a uniform surface (land, sea or ice) of uniform temperature and humidity.

**c -Continental air mass originating  over land or ice.  **
**m -Maritime air mass originating over water.  **

**Air masses have different characteristics according to their source region. **
**A- Arctic- originating near the North or South Poles.**
**P- Polar- originating from 40oto 60o North and South.**
**T- Tropical- originating from 25o North and South.**
**E- Equatorial- originating from 10o North and South.**

**Air masses are further classified according to Thermodynamic Process or temperature compared to the surrounding air.**
**k -air mass is colder then the underlying air.**
**w -air mass is warmer then the underlying air.**

## Slide 4

## Visible satellite picture showing the modification of cP air as it

**Visible satellite picture showing the modification of**** ****cP**** ****air as it**
**moves over the**** warmer Gulf of Mexico and the Atlantic Ocean****.**** **

**cPk**

## mTw air mass

**mTw air mass**

**HOW TO IDENTIFY A COLD FRONT**

- - SHIFTING OF THE WIND DIRECTION.  COLD FRONTS ARE LOCATED IN THE 	        AREA OF MAX TROUGHING OR COUNTERCLOCKWISE ROTATION OF THE WINDS.
- - SHARP TEMPERATURE CHANGES OVER A SHORT DISTANCE.

- - CHANGE IN THE AIR MOISTURE CONTENT.  INDICATED BY DIFFERENCE BETWEEN TEMPERATURE AND THE DEWPOINT.
- - CHANGE IN THE SURFACE PRESSURE.
- - WEATHER ASSOCIATED WITH A COLD FRONT.  SHOWERS AND THUNDERSTORMS

## A front is a zone of transition between two air masses

- of different density and temperature.

## Slide 8

## A COLD FRONT IS THE LEADING EDGE OF A MASS OF COLD AIR THAT IS UNDERRUNNING WARMER AIR.

**A COLD FRONT IS THE LEADING EDGE OF A MASS OF COLD AIR THAT IS UNDERRUNNING WARMER AIR.**
**FRICTION AT THE SURFACE CAUSES THE COLD FRONT TO HAVE A STEEPER SLOPE THEN THE WARM FRONT.**
**WINDS BACK (ROTATE COUNTERCLOCKWISE) WITH HEIGHT.**

## Warm air

**Cold**

_**SLOPE**_**-** DISTANCE IN THE VERTICAL 	TO THE  	DISTANCE IN THE HORIZONTAL**  **
**	****FAST MOVING COLD FRONT-   1:50**
- 	**SLOW MOVING COLD FRONT - 1:100**

## AVERAGE SPEED IS 15 KTS.  

- AVERAGE SPEED IS 15 KTS.
- WEATHER IS LOCATED AT AND BEHIND THE SURFACE FRONT

## AVERAGE SPEED IS 25 KTS

- WEATHER IS LOCATED AHEAD OF THE SURFACE FRONT

## COLDER AIR RUSHES AHEAD OF THE COLD FRONT LIFTING THE WARMER AIR IN FRONT OF THE SURFACE COLD FRONT.  WARMER AIR IS UNSTABLE CAUSING THE AIR TO RISE UNTIL IT BECOMES A THUNDERSTORM.

**COLDER AIR RUSHES AHEAD OF THE COLD FRONT LIFTING THE WARMER AIR IN FRONT OF THE SURFACE COLD FRONT.  WARMER AIR IS UNSTABLE CAUSING THE AIR TO RISE UNTIL IT BECOMES A THUNDERSTORM.**

## POSSIBLE 

- POSSIBLE

## A weakening cold front over land intensifies into

- a vigorous front as it crosses the warmer waters
- of the Gulf Stream.
- Warmer air at the surface causes instability.

## A warm front is the line of discontinuity

- where the foward edge of an advancing mass of relatively
- warm air is replacing a retreating relatively colder air mass.

## Warm

**Warm**

**Cold**

**SLOPE- for a warm front averages **
**	****    ****between 1:150 and 1:300**
**Average Speed of a Warm Front Is 10 Kts**

## mTw air mass

**mTw air mass**

**HOW TO IDENTIFY A WARM FRONT**

- - MUCH HARDER TO LOCATE THEN A COLD FRONT.
- - WINDS SHIFT FROM SOUTHERLY BEHIND THE FRONT TO EASTERLY AHEAD OF THE FRONT.

- - WINDS VEER (ROTATE CLOCKWISE) WITH HEIGHT.
- - WEATHER ASSOCIATED WITH A COLD FRONT.  FOG, DRIZZLE AND RAIN

## Weather expected is fog and low ceilings.

**Weather expected is fog and low ceilings.**

## Possible embedded thunderstorms

**Possible embedded thunderstorms**

## A stationary front has essentially no movement.

- Surface winds blow parallel to the
- stationary front, but in opposite directions.

## Cold

**Cold**

**Warm**

**Stationary fronts oscillate between the warmer and colder air.**

## An occluded front is a composite of two fronts.

- They form when a cold front overtakes a warm front
- and one of these two fronts is lifted aloft.

## COLD OCCLUSION

**COLD AIR**

**COOL AIR**

**WARM AIR**

**CROSS-SECTION VEIW OF THE TRIPLE POINT**

**COLD OCCLUSIONS FORM WHEN A COLD FRONT UNDERRUNS THE WARM FRONT FORCING IT ALOFT.  **
**THIS IS THE MOST COMMON TYPE OF OCCLUSION.  **
**THE CLOUDS AND WEATHER ASSOCIATED WITH A COLD OCCLUSION ARE USUALLY THE SAME AS A COLD FRONT.  WITH THE SHOWERS OCCURING JUST AHEAD OF THE SURFACE FRONT. **
**CLEARING OF THE CLOUDS USUALLY OCCURES AFTER THE PASSAGE OF THE **
**COLD FRONT, BUT SOMETIMES CAN LAST UNTIL THE PASSAGE OF THE WARM FRONT ALOFT.**

- COLD AIR

- COOL AIR

- WARM AIR

## WARM OCCLUSION

**CROSS-SECTION VEIW OF THE TRIPLE POINT**

**COOL AIR**

**COLD AIR**

**WARM AIR**

**WARM OCCLUSIONS OCCURE WHEN COOL AIR OVERTAKES COLDER AIR AND IS FORCED ALOFT.**
**THE WEATHER ASSOCIATED WITH A WARM FRONT HAS THE  CHARACTERISTICS OF BOTH A COLD FRONT AND WARM FRONT.  **
**THE CLOUDS A HEAD IF THE FRONT ARE SIMILAR TO THE CLOUDS OF A WARM FRONT.  THE INTENSITY OF THE WEATHER DEPENDS ON THE DISTANCE FROM THE APEX OR TRIPLE POINT.**
**THE WEATHER CHANGES RAPIDLY AND ARE USUALLY MOST SEVERE DURING THE INITIAL STAGE OF DEVELOPMENT.**

- COLD AIR

- COOL AIR

- WARM AIR

## Meteorology Today

- Aerographer's Mate Second Class, Volume 2
- USA Today's homepage
- and
- University of Illinois homepage

- References:

## This

- Concludes
- Training