What are Fronts?

As mentioned in the previous lesson, air masses are incredibly huge.  Two or three of them are capable of covering the continental U.S.  As these air masses move around they collide.  The boundary between two air masses is called a front.  

A FRONT IS THE BOUNDARY BETWEEN TWO AIR MASSES.

Since air masses have unique temperatures and humidity, an approaching front means the weather is about to change dramatically.  

An approaching front means the following changes are about to happen:

  • increased wind speeds
  • Change in temp
  • Change in humidity
  • Precipitation likely

Types of Fronts

Cold Fronts

  • Cold fronts form at the leading edge of a cold air mass.  
  • Here, cold air is pushing warm air out of the way.  
  • Cold fronts are represented on weather maps by blue triangles.  
  • The cold air is always located behind the triangles, and the triangles point in the direction the cold air is moving.

 

AN APPROACHING COLD FRONT MEANS SEVERAL THINGS ARE ABOUT TO HAPPEN:

  1. CLOUDS FORM AT THE FRONT
  2. TEMPERATURE DROPS RAPIDLY
  3. RAIN FALLS IMMEDIATELY BEHIND THE FRONT
  4. AFTER THE FRONT PASSES, SKIES ARE COLD AND CLEAR

 

 

Warm Fronts

  • Warm fronts form at the leading edge of a warm air mass.  These are places where warm air is pushing cold air out of the way.  
  • Warm fronts are represented on weather maps by red semi-circles, with the warm air behind the bumps.  
  • Again, the red semi-circles indicate the direction that the warm air is moving.

AN APPROACHING WARM FRONT MEANS SEVERAL THINGS ARE ABOUT TO HAPPEN:

  1. CLOUDS AT THE FRONT.
  2. RAIN FALLS AHEAD OF THE FRONT
  3. As the front moves away, warm air moves into the area, the temperature will rise and skies will clear.

 

 

 

Stationary Fronts

  • Stationary fronts form in areas where warm air masses and cold air masses smash into each other from opposite directions.  
  • The forces of warm and cold air pushing against each other cancels out all movement, so stationary fronts stay still (hence the name stationary).
  • The warm air mass is always behind the semi-circles, and the cold air mass is always behind the triangles.

 

Occluded Front

An occluded front forms when warm and cold air masses moving in the same direction combine.  

This creates a confusing mixture of warm and cold air all pushing in one direction.  

They are represented on weather maps with semi-circles and triangles pointed in the same direction.

 

 

 

Mid Latitude Lows

MLL.jpeg

In the United States most of our weather is not as simple as individual fronts moving across the country.  Instead, most of our weather is dominated by weather systems called Mid Latitude Lows.  Mid Latitude Lows are low pressures systems made up of three different fronts.

THE LEADING EDGE IS ALWAYS A WARM FRONT TRAVELING EAST / NORTHEAST WITH RAIN AHEAD OF IT

THE TAILING LEG IS ALWAYS A COLD FRONT MOVING EAST WITH RAIN BEHIND IT

The top is a small occluded front moving E.

These systems have low pressure so the whole thing rotates counterclockwise and moves across the country towards the NE.