Cold Fronts

There are two different types of cold fronts, active and inactive. 
 The difference is based on their speed of movement and where the 
weather is occurring in relation to the front itself.

Active Cold Fronts.  Active cold fronts are a boundary that has 
cold air behind, warm air ahead.  They are very slow moving,
 moving at an average speed of 15 knots.  This is because the 
upper-level winds are running parallel to the front in the upper 
air (Figure 4-1).  It provides no significant "push" to move the 
front along.  Most of the significant weather associated with the 
front, thunderstorms and cumuliform clouds occur right at or 
just behind the front as well. 

On the surface, temperatures fall sharply after passage of the 
front.  This is due to tight thermal gradient on the backside of 
the front, also referred to as "thermal packing".  It shows a 
strong change of temperature over a short distance.  Surface dew 
points will likewise fall, since cold air holds less moisture 
than warm air.  Visibility is usually fair to poor before passage, 
is generally poor during passage due to precipitation, and good 
after passage since the cold air has no precip and does not 
support fog as well as warm air.  The main way to find the exact 
frontal passage is by surface pressure.  Ahead of the front, 
pressures will fall sharply, since all fronts lie in troughs.  
Following passage, pressures sharply rise as higher pressure 
moves in.  Surface winds will be gusty, usually from the southwest 
before passage, rotating clockwise to become gusty northwest 
after passage (known as veering winds)

Inactive Cold Fronts.  Inactive cold fronts are again a boundary 
that has cold air behind, warm air ahead.  They are different 
from active cold fronts because they are fast moving, at an 
average speed of 25 knots. The upper-level winds are perpendicular 
to the front, which provides the push to move the front rapidly.  
Because of this, the weather associated with the front occurs out 
ahead of the front, up to 150 miles ahead.  This front often 
produces squall lines or lines of strong thunderstorms ahead of 
it. 

On the surface, temperatures fall more gradually after passage 
than do active cold fronts.  The thermal packing is moderate, 
and leads to a slower temperature change.  The majority of the 
cold air mass is well behind the front, whereas the strong cold 
air is right at the edge of the active cold front.  Surface dew 
points also fall though again not as rapidly as the active cold 
front.  Visibility is good outside of areas of precipitation.  
Ahead of the front, pressures fall more gradually, rising after 
passage as higher pressure moves in.  Surface winds will be 
gusty, usually from the southwest before passage, rotating 
clockwise to become gusty west to northwest after passage 
(veering winds)