Wednesday Sept. 12, 2007

The Optional Assignment is due at the start of class on Friday.  The first 1S1P assignment and the Experiment #1 reports are due next Monday.  Experiment #1 materials need to be returned this week so that they can be cleaned and handed out next week to the students doing Experiment #2.

The Quiz #1 Study Guide (preliminary version) is now available.  Next week's quiz will cover material on the Practice Quiz Study Guide and the Quiz #1 Study Guide.
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 and drizzle in the NE portion of the map above or the rain shower at the location 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 one City A 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 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 City 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 1006.8 mb and 1009.7 mb in the vicinity of Point B.  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.

What kind of weather can you expect in the vicinity of a low pressure center?

A pressure difference will first start air moving toward low pressure (imagine 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.


It is pretty much the opposite situation with surface high pressure centers.  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 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.

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.
Finally we took a brief look ahead at some material we will be covering on Friday and Monday next week.

We are going to try to understand why warm air rises and cold air sinks.
It is always a good idea to have a picture in mind, a hot air balloon for example.
Hot air balloons do sometimes fall from the sky; most everyone in the classroom would understand that gravity was the force responsible for bringing down a hot air balloon.
But what causes a hot air balloon to rise?  We will see that it is a pressure difference force.  Pressure decreases with increasing altitude.  This creates a force that points upward from high toward low pressure.

Understanding rising and sinking air is a 3-step process.  The first step is learning about the ideal gas law.

When you fill a balloon with air you don't really fill it with air.  That is the inside of the balloon is mostly empty space.  The balloon is kept inflated by the rapid motions of the air molecules which are zipping around inside the balloon and colliding with the walls of the balloon.  The outward push from each collision is very weak but the collisions are so numerous and frequent that the total effect is large.

The ideal gas law equation (that we will learn in class on Friday) explains how pressure depends on variables such as the volume of the balloon, the temperature of the air, and the number of air molecules in the balloon.