Thursday Feb. 09, 2006

Distribution of the Expt. 2 materials began today.  Collect your data as soon as you can, return the materials, and pick up the supplementary information sheet.  Reports are due on Tuesday, Feb. 28

The Quiz #1 Study Guide is now available online.  The quiz is on Thursday next week.  Remember Quiz #1 will cover material on the Practice Quiz Study Guide also.
We are in the process of learning about surface weather maps and what they tell you about the weather.  We covered surface centers of LOW and HIGH pressure in class on Tuesday.
topics covered today
Today we'll look at what the contour pattern on a map, specifically the isobar spacing, tells you about wind speeds.  The pressure pattern and the winds that it produces can have an effect on the temperature patterns.  Warm and cold fronts can form at the boundaries between air masses with different temperatures and are an additional way of causing rising air motions in the atmosphere.  Finally we'll look at how atmospheric conditions above the ground are depicted on upper level charts and at some of the features found on these charts.

2. Strong and weak pressure gradients and wind speed

contour spacing, pressure gradients, and wind speed
 Closely spaced isobars indicate the pressure is changing rapidly with distance.  This is called a strong pressure gradient.  Conversely, widely spaced isobars mean a weak pressure gradient and slower winds.

steep and gradual gradients on a hill
In some respects weather maps are like topographic maps.  The map above at right represents the hill at left.  Closely spaced contours on the topographic map correspond to a steep slope on the hill.  Widely spaced contours depict the gradual slope on the right side of the hill.  If you were to trip and roll downhill, you would roll faster on the steep slope than on the gradual slope. 
steep and weak pressure gradients on a surface weather map
Here's a more complex weather map example.  The brown arrows indicate the directions of the winds (clockwise and outward around the high, counterclockwise and inward around the low).  Fast 30 knot winds are found in the strong pressure gradient region shaded red.  Slower 10 knot and 15 knot winds are shown in the blue weak pressure gradient region.

3. Temperature patterns, cold and warm fronts
The pictures below are somewhat different from and are hopefully a little clearer than those shown in class. 
Winds can affect the surface temperature pattern.  The following picture shows a band of cold air in the north and a band of warm air in the southern part of the map.  This is the kind of temperature pattern you might expect to see if the wind wasn't blowing (boundaries between air masses of different temperatures would lie parallel to the lines of latitude.
temperature pattern when the winds are calm
Now we will add the winds spinning around the H and L pressure centers in the middle of the picture.  First the H pressure:
temperature pattern near a center of high pressure
The clockwise spinning winds move warm air to the north of the western side of the HIGH.  Cold air moves toward the south on the eastern side of the high.

In the case of LOW pressure:
temperature pattern near a low pressure center & fronts

The temperature  pattern is the opposite, cold air moves southward on the western side of the low and warm air moves northward on the eastern side of the low. 

Surface winds converge in toward the center of low pressure.  Sharp boundaries form beween the air masses with different temperatures as they are brought into contact with each other.  A cold front is low ahead of the advancing mass of cold air on the left side of the picture.  Note the points on the frontal boundary that identify it as a cold front and show its direction of motion.  A warm front is shown on the eastern side of the LOW and is identified with half circles).

A low pressure center with fronts is a type of large scale storm system called a middle latitude storm or an extratropical cyclone.

crossectional view of a cold front
Cold dense air on the left is advancing into warmer lower density air on the right.  The warm low density air is lifted out of the way by the cold air.  Fronts are another way of causing rising air motions in the atmosphere.  Rising air cools and if the warm air is moist, clouds and precipitation can form

Here's what a warm front looks like:
crossectional view of a warm front
Warm air is advancing into colder air.  The warm air is less dense than the cold air and can't lift the cold air out of the way.  Rather the warm air overrides the cold air.  Air is still rising and cooling, so clouds and precipitation can still form.  Clouds often spread out over a large area ahead of a warm front than ahead of a cold front.

If you refer back to the map view of the middle latitude storm you will see clouds drawn in ahead of the cold and warm fronts.

understanding upper level charts

Now we'll take a look at upper level charts, charts that depict atmospheric conditions above the ground.  Before you go any further you need to remember that pressure decreases with increasing altitude in the atmosphere (pressure at any level is determined by the weight of the air overhead, as you move upward there is less and less air left overhead and pressure decreases).  Also, the rate of pressure decrease depends on the air's density (if you move upward through dense air you are quickly moving weight from overhead and putting it underneath you; this causes a rapid rate of pressure decrease). 

pressure contours on an upper level constant altitude chart

One way of depicting upper level conditions would be to measure pressure values at some fixed altitude above the ground.  The pressure pattern could then be plotted on a constant altitude chart using isobars.  Note the lowest pressures would be found in the cold air, higher pressures would be found in the warm air.


height contours on a constant pressure upper level chart

Rather than plotting conditions at a constant altitude above the ground, meterologists measure and plot conditions at a particular reference pressure level above the ground.  Every point on the sloping surface above has the same pressure, 850 mb.  The altitude above the ground is what is changing.  You would find contours of altitude or height contours plotted on a one of these constant pressure charts. 

Note, at the bottom of the figure above, that the the two kinds of upper level charts (constant altitude vs constant pressure) have the same overall pattern.


upper level chart with west to east temperature variations

A slightly more complicated example - a wavy surface instead of a flat sloping surface.

upper level chart with W-E and N-S temperature variation

In this last example we have added a south (warm) to north (cold) temperature change to the west to east temperature variations that were in the last example.  We end up with a wavy surface that slopes from front to back (high in the south, closer to the ground in the north).  The "topographic map" that represents this surface is much different from our earlier examples but resembles more closely what you would see on a real upper level weather map.  Ridge and trough features are clearly defined.  You can now understand why the ridges are called ridges.  They should have called troughs valleys.


upper level chart example
A final example not shown in class that you can use to check your understanding of upper level charts.  This is a 500 mb constant pressure chart not an 850 mb chart like shown in the previous section.  The numbers on the contours are altitudes in meters.  The 500 mb pressure level is found at higher altitude than the 850 mb pressure level. 

Pressure at sea level is typically around 1000 mb.  You find the 850 mb level at about 1500 meters altitude, the 700 mb level at 3000 meters and the 500 mb level at around 5500 meters altitude. 

All four points on the map above have one thing in common: the pressure at all four points is 500 mb.

Points C and D lie at about the same latitude.  Point C is found at higher altitude (5640 m) than Point D (5580 m).  The air below Point C is warmer than the air below Point D.  Similarly the air below Point A is warmer than the air below Point B.

Points A and C both lie in a ridge.  The altitude at Point A (5460 m) is lower than the altitude at Point C (5640 m).  The air below Point A in the north is colder than the air below Point C near the southern edge of the map.  Similarly the air below Point B is colder than the air below Point D.  Both Points B and D lie in a trough.

Pressure is decreasing most rapidly in the cold dense air below Point B.  Point B is closest to the ground.  Pressure is decreasing most slowly with increasing altitude in the warm low density air below Point C.  Point C is furthest from the ground.