Wed., Feb. 6 notes

Wednesday Feb. 6, 2013
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I'm sorry you weren't able to see the video that went along with "Workin' Woman Blues" sung by Valerie June.

40 sets of Experiment #2 materials were distributed in class today. About 55 people have signed up for the experiment so that wasn't enough for everyone. I should have enough extra materials on Friday for everyone. Expt. #2 reports are due Mon., Feb. 25.

Quiz #1 is one week from today (Wed., Feb. 13). A preliminary version of the Quiz #1 Study Guide is now available. Quiz #1 will cover material on both the Quiz #1 Study Guide and the Practice Quiz Study Guide. Reviews are scheduled for Mon. and Tue. afternoon next week from 4 - 5 pm in Haury (Anthropology) 129.

Another Optional Assignment was handed out in class today. It is due next Monday, but you can always turn it in early.

On Monday we learned how weather data is plotted on a surface map using the station model notation (and you should go back to the end of the Monday notes because there were one or two topics not covered in class). Today we'll start to see what analysis of that data can start to tell you about the weather.

A bunch of weather data has been plotted (using the station model notation) on a surface weather map in the figure below (p. 38 in the ClassNotes).

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 (isobars) 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 more predictable way).

Isotherms, temperature contour lines, are usually drawn at 10^o 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 40^o F isotherm above passes through a city which is reporting a temperature of exactly 40^o (Point A). Mostly it goes between pairs of cities: one with a temperature warmer than 40^o (41^oat Point B) and the other colder than 40^o (38^o F at Point C). 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. The isobars also enclose areas of high pressure and low pressure. Isobars are generally drawn at 4 mb intervals (starting with a base value of 1000 mb). 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 pressure pattern is not as predictable as the isotherm map. Low pressure is found on the eastern half of this map and high pressure in the west. The pattern could just as easily have been reversed.

This site (from the American Meteorological Society) first shows surface weather observations by themselves (plotted using the station model notation) and then an analysis of the surface data like what we've just looked at. There are links below each of the maps that will show you current surface weather data.

Here's a little practice (this figure wasn't shown in class).

Is this the 1000, 1002, 1004, 1006, or 1008 mb isobar? (you'll find the answer at the end of today's notes)

Now we'll look at what you can learn about the weather once you've drawn in some isobars and mapped out the pressure pattern.

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 (we'll learn what the rest are soon, and, actually, you already know what one of them is - warm air rises, that's called convection). You often see cloudy skies and stormy weather associated with surface low pressure.

Everything is pretty much the exact opposite in the case of surface high pressure.

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 doesn't necessarily mean warm weather, strong surface high pressure often forms when the air is very cold.

Here's a picture summarizing what we've learned so far. It's a slightly different view of wind motions around surface highs and low and wasn't shown in class.

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. Portions of the two figures that follow can be found on p. 40c in the ClassNotes.

A picture of a hill is shown above at left. The maps at upper right is a topographic map that depicts the hill (the numbers on the contour lines are altitude). A center of high pressure on a weather map, the figure at bottom left, could have exactly the same appearance. The numbers on the contours lines (isobars) are now pressure values in millibars.

Closely spaced contours on a topographic map indicate a steep slope. More widely spaced contours mean the slope is more gradual. If you trip walking on a hill, you will roll rapidly down a steep hillside, more slowly down a gradual slope.

On a weather map, closely spaced contours (isobars) means pressure is changing rapidly with distance. This is known as a strong pressure gradient and produces fast winds (a 30 knot wind blowing from the SE is shown in the orange shaded region above). Widely spaced isobars indicate a weaker pressure gradient and the winds would be slower (the 10 knot wind blowing from the NW in the figure).

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). Fast winds where to contours are close together and slower winds where they are further apart.

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

This figure is found at the bottom of p. 40 c in the photocopied ClassNotes. 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 answer at the end of today's notes).

Here are the answers to the two questions found earlier in the notes.

Pressures lower than 1002 mb are colored purple. Pressures between 1002 and 1004 mb are blue. Pressures between 1004 and 1006 mb are green and pressures greater than 1006 mb are red. The isobar appearing in the question is highlighted yellow and is the 1004 mb isobar. The 1002 mb and 1006 mb isobars have also been drawn in (because isobars are drawn at 4 mb intervals starting at 1000 mb, 1002 mb and 1006 mb isobars wouldn't normally be drawn on a map)

And here's the answer to the question about 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. Notice also how the wind direction can affect the temperature pattern. The winds at Point 2 are coming from the south and are probably warmer than the winds coming from the north at Points 1 & 3. We'll be looking at how the pressure pattern (because it largely determines the winds) can indirectly affect the temperature pattern.