Tuesday Feb. 12, 2008

The Practice Quiz was returned today.  Answers to the Practice Quiz questions have been posted online.  If you didn't take the Practice Quiz, pick up a copy in class so that you can become familiar with the format and level of difficulty.

The Expt #1 reports were collected today.  Several people still haven't returned their materials.  Please do so as quickly as you can, the graduated cylinders are needed for Expt. #2.  Some Expt. #2 materials should be available in class on Thursday.

The first Optional Assignment was collected.  A new Optional Assignment was handed out.  It will be due at the beginning of class next Tuesday (Feb. 19).

The Quiz #1 Study Guide should appear online in the next day or two.  Quiz #1 is Thursday Feb. 21.
We started with some practice at decoding surface weather observations plotted using the station model notation.

The value of the relative humidity (RH) is not plotted.  When the air temperature and the dew point temperature are equal, however, you can say the relative humidity is 100%.
Now we will put what we have learned to use and plot a bunch of weather data on a surface map:


Plotting the surface weather data on a map is just the beginning.  For example you really can't tell what is causing the cloudy weather with rain (the dot symbols are rain) 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.



Isotherms, temperature contour lines, are 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 City A 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 (such as near Point B).  Temperatures generally decrease with increasing latitude.


Now the same data with isobars drawn in.  Again they separate regions with pressures 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  passes through a city  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 two cities with pressures of 1006.8 mb and 1009.7 mb.  You would expect to find 1008 mb about halfway 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.
1.
Let's first look at the wind motions associated with a surface low pressure center (the figures were redrawn after class for clarity):

The pressure differences will first start air moving toward low pressure (like a rock sitting on a hillside that starts to roll downhill).  Then something called the Coriolis force will cause the wind to start to spin (we'll learn more about the Coriolis force later in the semester). 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]


The convergence causes the air to rise at the center of the low.  Rising air expands and cools.  If the air is sufficiently moist clouds can form and then begin to rain or snow.  Thus you often see cloudy skies and stormy weather associated with surface low pressure.

With surface high pressure everything is pretty much just the opposite:


Winds spin clockwise and spiral outward.  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.  The clear eye in the center of a hurricane is produced by sinking air.
2.
The pressure pattern will also tell you something about how fast you might expect the wind to blow.  In this case we look for regions where the isobars are either closely spaced together or widely spaced.  (the following figures were redrawn after class)

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 tumble 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 spiralling inward slightly.

Winds spin counterclockwise and spiral inward around low pressure centers.

Try to determine the directions of the winds at Points 1, 2, and 3 in the figure below (found at the bottom of p. 40c in the photocopied Class Notes).  Where will the fastest and slowest winds be found?  Would you expect to find that the temperatures at Points 1, 2, and 3 were equal or different?

When you thought about these questions for awhile, click here to see the answers.
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.

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.

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.

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.  The warm low density air is lifted out of the way by the cold air. 

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 warm air rises again (more gradually) and clouds form.  The clouds generally are 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 another way of causing air to rise.  Rising air cools and if the warm air is moist, clouds and precipitation can form.

Now we will return to the surface weather map we have been analyzing.


The weather data plotted on the map indicate clearly the presence of cold and a warm fronts (we learn later about some of the criteria used to located fronts).  Now we can begin to understand what is causing the rain shower along the Gulf Coast (clouds caused by an approaching cold front) and the cloudy rainy weather in the Northeast (an approaching warm front and also perhaps some convergence into the low pressure center).