FRONTS TWO

    In fronts one, there was a discussion on how to locate fronts using isotherm patters, especially the temperature patterns on constant pressure charts.  There was also a description of what fronts are not.  In this discussion, locating fronts by the use of jet stream positions and vertical wind shears will be described.  Of course, density patterns are implicit in these methods.

     Upper level jet streams can be used to locate fronts.  Pressures will decrease more rapidly with height in cold air than in warm air because at a given pressure, cold air is denser that warm air.  If you take a column of air with a pressure P at sea level, to reach pressure 1/2 P, you must ascend to a certain height.  In a colder column of air with the same pressure P at sea level, the height required to reach 1/2 P will be less.  After all, a column of air will shrink when chilled.  Therefore, the upper level constant pressure surfaces will be lower in cold air than in warm air. 

View this photo  This photo is a vertical cross section.   P0, P1, P2, etc. are constant pressure surfaces where P0 > P1 > P2, etc.  Cold air is to the left.

    

     Implicit in the above paragraph is that upper highs will be located above warm air and upper lows will be located above cold air.  (See figure 1 by clicking on View this photo).  It follows that the strongest horizontal pressure gradients in the upper levels are found between regions with warm air and regions with cold air.   Stronger pressure gradients mean stronger winds.  Therefore, the jet stream extends along areas with the greatest horizontal temperature contrasts.   Looking downwind, to the left of the jet will be cold air and to the right will be warm air.  The surface front will be to the right of the jet.  To get the idea, look at a 700MB chart and a 500MB chart.  Note where the air is the cold and where the air is warm.  Next, look at a 250MB chart.  Note that the jet at 250MB neatly divides the cold air from the warm air indicated at 700MB and 500MB.  What should be noted is that the upper level jets will not intersect a front.  This is a common mistake in map analysis.  However, there is an exception.  The subtropical jet which is often found over the Gulf of Mexico or the Gulf states during the winter can intersect a cold front penetrating into the subtropics or tropics.   In this case, the cold air layer has become so shallow, about 3,000ft or less, that it has little influence on the heights of the upper level pressure surfaces.   

  Because pressures decrease more rapidly with height in cold air than in warm air, the slopes of the constant pressure surfaces will chance with height between cold and warm air.  This change of the slopes of the pressure surfaces with height will cause winds to change speed and direction with height.   That is, a horizontal temperature gradient will cause a vertical wind shear.  Vertical wind shears can be used to identify fronts.  However, vertical wind shear in the lowest 2,000ft or so may be due more to friction with land or sea than due to temperature contrasts. 

Fredmet

Comments

[Ray]

I see you figured out how to make the pictures bigger. Keep up all the posting any plans on doing one for plants?