Now that we have learned how surface weather data are plotted on a map, we will look at some of the analyses of the data that can be done. 

Here's a relatively simple example of a surface map.  Pressure, wind, temperature, cloud cover, and weather data are shown. 


Plotting the surface weather data on a map is just the beginning.  You really can't tell, for example, what is causing the cloudy weather with rain (the dot symbols) and drizzle (the comma symbols) in the NE portion of the map above or the rain shower along the Gulf Coast.  Some additional analysis is needed.  A meteorologist would usually begin by drawing some contour lines of pressure to map out the large scale pressure pattern.  We will look first at contour lines of temperature, they are a little easier to understand (the plotted data is easier to decode and temperature varies across the country in a fairly predictable way).


Isotherms, temperature contour lines, are usually drawn at 10 F intervals. They do two things: (1) connect points on the map that all have the same temperature, and (2) separate regions that are warmer than a particular temperature from regions that are colder.  The 40o F isotherm highlighted in yellow above passes through a city which is reporting a temperature of exactly 40o.  Mostly it goes between pairs of cities: one with a temperature warmer than 40o and the other colder than 40o.  Temperatures generally decrease with increasing latitude: warmest temperatures are usually in the south, colder temperatures in the north.


Now the same data with isobars drawn in.  Again they separate regions with pressure higher than a particular value from regions with pressures lower than that value.    Isobars are generally drawn at 4 mb intervals.  Isobars also connect points on the map with the same pressure.  The 1008 mb isobar (highlighted in yellow) passes through a city at Point A where the pressure is exactly 1008.0 mb.  Most of the time the isobar will pass between two cities.  The 1008 mb isobar passes between cities with pressures of 1009.7 mb at Point B and 1006.8 mb at Point C.  You would expect to find 1008 mb somewhere in between those two cites, that is where the 1008 mb isobar goes.

The pattern on this map is very different from the pattern of isotherms.  On this map the main features are the circular low and high pressure centers. 

Just locating closed centers of high and low pressure will already tell you a lot about the weather that is occurring in their vicinity.

1.
We'll start with the large nearly circular centers of High and Low pressure.  Low pressure is drawn below. 



Air will start moving toward low pressure (like a rock sitting on a hillside that starts to roll downhill), then the Coriolis force will cause the wind to start to spin (we'll learn more about the Coriolis force later in the semester). In the northern hemisphere winds spin in a counterclockwise (CCW) direction around surface low pressure centers.  The winds also spiral inward toward the center of the low, this is called convergence.  [winds spin clockwise around low pressure centers in the southern hemisphere but still spiral inward, don't worry about the southern hemisphere until later in the course]


When the converging air reaches the center of the low it starts to rise.  Rising air expands (because it is moving into lower pressure surroundings at higher altitude), the expansion causes it to cool.  If the air is moist and it is cooled enough (to or below the dew point temperature) clouds will form and may then begin to rain or snow.  Convergence is 1 of 4 ways of causing air to rise.  You often see cloudy skies and stormy weather associated with surface low pressure.


Surface high pressure centers are pretty much just the opposite situation.  Winds spin clockwise (counterclockwise in the southern hemisphere) and spiral outward (in both hemispheres).  The outward motion is called divergence.

Air sinks in the center of surface high pressure to replace the diverging air.  The sinking air is compressed and warms.  This keeps clouds from forming so clear skies are normally found with high pressure (clear skies but not necessarily warm weather, strong surface high pressure often forms when the air is very cold). 

2.
The pressure pattern will also tell you something about where you might expect to find fast or slow winds.  In this case we look for regions where the isobars are either closely spaced together or widely spaced. 


Closely spaced contours means pressure is changing rapidly with distance.  This is known as a strong pressure gradient and produces fast winds.  It is analogous to a steep slope on a hillside.  If you trip, you will roll rapidly down a steep hillside, more slowly down a gradual slope.

The winds around a high pressure center are shown above using both the station model notation and arrows. The winds are spinning clockwise and spiraling outward slightly.  Note the different wind speeds (25 knots and 10 knots plotted using the station model notation)



Winds spin counterclockwise and spiral inward around low pressure centers.  The fastest winds are again found where the pressure gradient is strongest.


  You should be able to sketch in the direction of the wind at each of the three points and determine where the fastest and slowest winds would be found. (you'll find the answers at the end of this lecture)

3.
The pressure pattern determines the wind direction and wind speed.  Once the winds start to blow they can affect and change the temperature pattern.  The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all or if the wind was blowing straight from west to east.  The bands of different temperature are aligned parallel to the lines of latitude.  Temperature changes from south to north but not from west to east. 

This isn't a very interesting picture.   It gets a little more interesting if you put centers of high or low pressure in the middle.

The clockwise spinning winds move warm air to the north on the western side of the High.  Cold air moves toward the south on the eastern side of the High.  The diverging winds also move the warm and cold air away from the center of the High.

Counterclockwise winds move cold air toward the south on the west side of the Low.  Warm air advances toward the north on the eastern side of the low.

The converging winds in the case of low pressure will move the air masses of different temperature in toward the center of low pressure and cause them to collide with each other.  The boundaries between these colliding air masses are called fronts.  Fronts are a second way of causing rising air motions (rising air expands and cools; if the air is moist clouds can form)

Cold air is moving from north toward the south on the western side of the low.  The leading edge of the advancing cold air mass is a cold front.  Cold fronts are drawn in blue on weather maps.  The small triangular symbols on the side of the front identify it as a cold front and show what direction it is moving.  The fronts are like spokes on a wheel.  The "spokes" will spin counterclockwise around the low pressure center (the axle).

A warm front (drawn in red with half circle symbols) is shown on the right hand side of the map at the advancing edge of warm air.  It is also rotating counterclockwise around the Low.

This type of storm system is referred to as an extratropical cyclone (extra tropical means outside the tropics, cyclone means winds spinning around low pressure) or a middle latitude storm.   Large storms also form in the tropics, they're called tropical cyclones or more commonly hurricanes.

Clouds can form along fronts (often in a fairly narrow band along a cold front and over a larger area ahead of a warm front).  We need to look at the crossectional structure of warm and cold fronts to understand better why this is the case.

The top picture below shows a crossectional view of a cold front




At the top of the figure, cold dense air on the left is advancing into warmer lower density air on the right.  We are looking at the front edge of the cold air mass, note the blunt rounded shape.  The warm low density air is lifted out of the way by the cold air.   The warm air is rising. 

The lower figure shows an analogous situation, a big heavy Cadillac plowing into a bunch of Volkswagens.  The VWs are thrown up into the air by the Cadillac.


Here's a crossectional view of a warm front, the structure is a little different.



In the case of a warm front we are looking at the back, trailing edge of cold air (moving slowly to the right).  Note the ramp like shape of the cold air mass.  Warm air overtakes the cold air.  The warm air is still less dense than the cold air, it can't wedge its way underneath the cold air.  Rather the warm air overruns the cold air.  The front can advance only as fast as the cooler air moves away to the right.

The warm air rises again and clouds form.  Because the warm air rises more gradually, clouds that form are generally spread out over a larger area than with cold fronts. 

In the automobile analogy, the VWs are catching a Cadillac.  What happens when they overtake the Cadillac?



The Volkswagens aren't heavy enough to lift the Cadillac.  They run up and over the Cadillac. 

Fronts are a second way of causing air to rise (winds spiraling into surface centers of low pressure, convergence, was the 1st way)

Next we will spent some time learning about the weather conditions that precede and follow passage of warm and cold fronts. 

A crossectional view of a cold front is shown below:


Here are some of the specific weather changes that you might expect to observe before and after passage of a cold front.  The figure in the picture above is positioned ahead of an approaching cold front.  The person would experience relatively warm conditions, winds would be blowing from the SW, and pressure would be falling.  These conditions are listed in the right most column in the table below.
Weather variable
 Behind, after the cold front has passed through
 As the front is passing through
 Ahead, before the arrival of the cold front
Temperature
 cool, cold, colder*
warm
Dew Point
 usually much drier*
may be moist (though that is often
not the case here in the desert southwest)
Winds
 northwest
 gusty winds (dusty)
 from the southwest
Clouds, Weather
 clearing
 rain clouds, thunderstorms in
narrow band along the front
(if the warm air mass is moist)
 might see some high clouds
Pressure
 rising
 reaches a minimum
 falling

* the coldest air might follow passage of a cold front by a day or two.  Nighttime temperatures often plummet in the cold dry air behind a cold front.

A temperature drop is probably the most obvious change associated with a cold front.  Here in southern Arizona, gusty winds and a wind shift are also often noticeable when a cold front passes.

The pressure changes that precede and follow a cold front are not something we would observe or feel but are very useful when trying to locate a front on a weather map.

In the next figure we started with some weather data plotted on a surface map using the station model notation. 


Before trying to locate a cold front, we needed to draw in a few isobars and map out the pressure pattern.  In some respects fronts are like spokes on a wheel - they rotate counterclockwise around centers of low pressure.  It makes sense to first determine the location of the low pressure center.

Isobars are drawn at 4 mb increments above and below a starting value of 1000 mb.  Some of the allowed values are shown on the right side of the figure.  The highest pressure on the map is 1003.0 mb, the lowest is 994.9 mb.  Thus we have drawn in  996 mb and 1000 mb isobars.

The next step was to try to locate the warm air mass in the picture.  Temperatures are in the 60s in the lower right portion of the map; this area has been circled in red.

The cold front on the map seems to be properly postioned.  The air ahead of the front is warm, moist, has winds blowing from the S or SW, and the pressure is falling.  These are all things you would expect to find ahead of a cold front.

Clouds and a rain shower were located right near the front which is typically where they are found.

The air behind the front is colder, drier, winds are blowing from the NW, and the pressure is falling.  Note how the cold front is positioned at the leading edge of the cold air mass, not necessarily in front of the coldest air in the cold air mass.

Next we follow the same procedure with warm fronts.
Here's the crossectional view


Here are the typical weather conditions in advance of and following the frontal passage.
Weather Variable
 Behind, after the front has passed through
 As the front is passing through
 Ahead, before the front arrives
Temperature
 warmer
cool
Dew point
 may be moister
drier
Winds
 from S or SW
from E or SE
Clouds, Weather
 clearing
wide variety of clouds well ahead of the front,
may be a wide variety of types of precipitation also.
Pressure
 rising
 reaches a minimum
 falling

And here is the surface map analysis:


Note the extensive cloud coverage and precipitation found ahead of the warm front.  There is a pretty good temperature and dew point difference on opposite sides of the warm front and a clear shift in wind directions.  Pressure is falling ahead of the warm front and rising behind.

There was also pretty clear evidence of a cold front on this map. 


Finally, here is the surface map that we began this lecture with.   We were trying to figure out what was causing the clouds in the NE portion of the map and what was causing the rain shower along the Gulf Coast.   A warm front and a cold front have been added to the isobaric analysis.



The warm front is probably what is producing most of the widespread cloudiness and precipitation in the NE portion of the map (rising air motions caused by surface winds converging into the low pressure center are also contributing).  The cold front is producing the showers along the Gulf Coast.

Processes that cause rising air motions are important.  Rising air expands and cools.  If the air is moist and is cooled enough, clouds can form.

Convergence is the first process that causes rising air motions.

As we've just seen, both warm and cold fronts cause air to rise.


Warm air is lifted by the cold dense air behind an advancing cold front.  Warm air overruns cool retreating air along a warm front.

Free convection, something we have already covered, is the 3rd process.




Topographic or Orographic lifting is the 4th way of causing air to rise.


When moving air encounters a mountain it must pass over it.  You often find clouds and rain on the windward side of the mountain where the air rises.  Drier conditions, a rain shadow, is found on the leeward side where the air is sinking (assuming that the winds mostly blow in the same direction over the mountain).
Here is the answer to the question found earlier in the notes concerning wind directions and wind speeds




The winds are blowing from the NNW at Points 1 and 3.  The winds are blowing from the SSE at Point 2.  The fastest winds (30 knots) are found at Point 2 because that is where the isobars are closest together (strongest pressure gradient).  The slowest winds (10 knots) are at Point 3.