Friday Sep. 25, 2009
click here to download today's notes in a more printer friendly format.

3 songs ( "One Love"   "Chanda Mama"   and   "Stand By Me" ) from something I just heard about a week or two ago: the Playing for Change  Songs Around the World CD.

Half of the quizzes have been graded and were returned in class.  The rest of the quizzes should be graded by Monday.

Materials for Expt. #2 were distributed in class today.  I'll bring the remaining few sets of materials to class on Monday.
Here is about where we left off last Monday.


Weather data have been plotted on a surface map using the station model notation.  Isobars have been drawn to reveal the large scale pressure pattern.  Today we will be learning about how particular features in the pressure pattern can affect or determine the weather in their vicinity.
1.
We'll start with the large nearly circular centers of High and Low pressure.  Low pressure is drawn below.  These figures are more neatly drawn versions of what we did in class.



Air will start 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). 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 semester]

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.  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.  (I messed up one of the figures in class so I'm using some more carefully drawn pictures from the Spring 2008 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 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 spiralling inward slightly.

Winds spin counterclockwise and spiral inward around low pressure centers.


This is the figure from the bottom of p. 40c.  The fastest winds (blowing from the SSE) are found in the center of the picture.  The slowest winds are found on the right side of the figure where the contours are far apart.  Note the southerly winds in the middle of the picture would probably be warmer (because they are coming from the south) than the NW winds at the right and left sides of the pictures.

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 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 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 a second way of causing air to rise.  Rising air cools and if the warm air is moist and cooled enough, clouds and precipitation can form.  That's why the clouds were drawn in along the fronts in the middle latitude storm picture above.

We will come back to the topic of fronts again next Monday.  We will, in particular, learn about some of the weather changes that take place as a front approaches and passes through.  We will also look at how fronts can be located on surface weather maps.
Free convection is the 3rd way of causing rising air motions (the figure below is from the Wed., Sep. 16 online notes)

Topographic or Orographic lifting is the 4th way of causing air to rise.  This was discussed briefly in class in response to a question about rainy weather in the Pacific Northwest.

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.