Tutorial 20:
The Midlatitude Wave Cyclone
Concept:  These storm systems consist of two fronts, a warm and a cold, that together pivot about a center of low pressure.  The whole system moves from west to east being driven by the Westerlies.  These systems are common to our climate during the winter months.

Origin:  These storm systems have their origin in the area of the Polar Front; approximately 60° latitude.  Meanders, or bends, in the Polar Front Jet cause the underlying Polar Front to move.  The part of the Polar Front driven toward lower latitudes becomes a cold front.  The part driven poleward becomes a warm front.  A well developed wave cyclone is depicted in the following diagram.

Midlatitude Wave Cyclone

NOTE:  DOTTED LINE SHOWS EXTENT OF PRECIPITATION GENERATING CLOUD COVER


Weather Observations and the Passage of a Wave Cyclone:  In this section, we will describe observed changes in weather as a wave cyclone passes overhead.  On an exam, I will describe weather observations and from them you will be expected to identify what type of front has passed.  We will keep track of temperature, current weather (rain or no), cloud cover, wind direction, and barometric pressure.  We will describe conditions as we read from east to west.  This is the same as standing in one place and having the weather system pass overhead from west to east.  Four major zones are depicted in the above diagram, A, B, C, and D.  Conditions will be described for each.

Region A: Preceding the Warm Front.

1)  Current Temperature:  The temperature must be cooler than what is coming otherwise the approaching front would not be classified as warm.

2)  Current Weather:  The gray tone indicates cloud cover.  Note the station that has an R associated with it.  That indicates rainfall.  We will characterize typical warm front storms as being gentle and of long duration.  We say this because uplift is gradual making for gentle rain and the area of cloud cover is very wide, making for long duration rainfall.

3)  Cloud Cover:  The solid black station markers indicate 100% cloud cover.

4)  Wind Direction:  The predominant wind direction shown in this region is from the east.  Air from the east is generally neither cold or warm, but cool.  Remember that the "sticks" attached to the stations should be considered arrows that were shot into the station.  Ask yourself where the archer must have been standing in order to have struck the station.

5)  Pressure Change:  You may see, by looking at the isobars, that as the front approaches from the west, pressure will be dropping.
 

Region B: Behind the Warm Front.
1)  Current Temperature:  The temperature has changed from negatives and single digits to the teens.  The temperature has warmed.

2)  Current Weather:  The stations indicate that the rain has stopped.

3)  Cloud Cover:  Skies are now generally clearing.

4)  Wind Direction:  The stations now indicate that winds are from the south.  After all, in our hemisphere, warm air comes from the south.  Just think of air coming up from the Carolinas for instance.

5)  Pressure Change:  The center of low pressure is now closer to observers at B, so pressure is still dropping.
 

Region C:  Immediately Before the Cold Front.
1)  Current Temperature:  Air temperatures are still warm since the cold front has not yet passed.

2)  Current Weather:  The weather stations indicate thunderstorms.  The narrowness of the cloud band and the rapidity with which cold fronts travel cause cold front storms to be intense but of short duration.

3)  Cloud Cover:  The stations, again, indicate 100% cloud cover.

4)  Wind Direction:  Winds are still from the south.

5)  Pressure Change:  Pressure is still dropping as we have passed between more isobars closer to the center of low pressure.
 

Region D:  Behind the Cold Front.
1)  Current Temperature:  Temperatures have now dropped to the negative teens.

2)  Current Weather:  The map shows that skies clear following the passage of the cold front.  The reason is that cold air, being dense, is pulled downward by gravity.  Sinking air doesn't allow for cloud formation or rain.

3)  Cloud Cover:  None.

4)  Wind Direction:  Winds now come from the west or northwest.  Cold air comes from higher latitudes, so this makes sense.

5)  Pressure Change:  Note how that the center of low pressure is moving away and that we have passed through progressively higher value isobars.  Pressure now goes up.
 

Summary of Giveaway Changes: You need to become familiar with the CHANGES associated with the passage of each front.  Specifically, what are the giveaway changes going from A to B and then from C to D?
Warm Front: Changes From A to B.
1)  Temperature Change:  Temperature changes from cool to warm.

2)  Current Weather:  Weather changes from gentle, long duration rain to generally clearing skies.

3)  Cloud Cover:  From 100% to partially cloudy.

4)  Wind Direction:  An important shift occurs from east to south.

5)  Winter warm fronts also have a characteristic sequence of precipitation types.

Cold Front:  Changes from C to D.
1)  Temperature Change:  Temperature changes from warm to cold.

2)  Current Weather:  Weather changes from intense, short duration rain to clearing skies.

3)  Cloud Cover:  From 100% to very clear.

4)  Wind Direction:  An important shift occurs from south to west, northwest, or north.

5)  Pressure Change:  The pressure change trend shifts from decreasing to increasing.