Surface and Upper Air Charts

When you look at isobars and isotherms, it will be easy for you now to see where there are low-pressure systems. In the figure you just saw, low-pressure system was obvious as the area in the center of isobars that decreased in air pressure as you moved toward the center. In fact, whether maps often show two letters indicating high-pressure or lowpressure. If you see the H, is a high pressure air mass. If you see the letter L, it is a lowpressure air mass. Any elongated sections of low-pressure are known as troughs. Any elongated section of high-pressure is known as a rich

When you study isotherms on a map, you can often predict where the wind is blowing. If a series of isotherms are very close together, there is a large temperature difference over a short space on the map. Of course, this will be an area where wind will travel across a shorter temperature gradient. The same is true for isobars; if these are close together, winds will be high. We will study surface maps in a minute. You can see however, that these isobars and isotherms provide a good pictorial representation of a lot of the data collected in different parts of the world.

SURFACE AND UPPER AIR CHARTS

Now we can study real weather maps. While these are interesting, the real goal of them is to predict what type of weather you can expect in the next hours, days, or perhaps a week or more in length. Meteorologists often show these maps on the news; now you will be able to read them too.

What you can now imagine is that people collect information about the current temperature and pressure in their area. This data is put together to create isobars and isotherms. In addition, we have radar to show there are current storms in areas of precipitation. All of this information is put together to make predictions. Many of these weather maps are color-coded so you can see that red areas indicate warmer temperatures while green and blue areas on the map indicate cooler temperatures.

You've probably heard of cold fronts and warm fronts. A front is the edge of any moving air mass. Symbols on the surface map will tell you not only if it is a cold front or a warm front, they will tell you the direction the air mass happens to be moving. A stationary front will also be marked in a way that makes it easy to see that the front is not moving.

Figure 28 shows you clearly what the symbols mean on the surface map. Warm fronts are labeled red with half circles on the line with the front is coming. Cold fronts of blue triangles marked at the edge of the cold front. Stationary fronts have both types of symbols on them. Cold fronts often mean bad weather and storms, while warm fronts often mean that warm and sunny weather is coming.

Figure 28.

Often, you can look briefly at the surface map and be able to say where there is precipitation expected. Precipitation most likely comes when the front is advancing; the precipitation is seen at the edge of the front as it enters a certain area. You might also see special symbols on the map indicating thunderstorms, rain prediction, or coming snowfall. In the northern hemisphere, many fronts operate from a westerly direction to an easterly direction, although you will see some North or southward tendency. Figure 29 demonstrates a simple map of the United States with warm and cold fronts easily depicted:

Figure 29.

By looking at the weather map and because you now understand the flow of air, you can predict the direction of airflow. In the northern hemisphere, a low-pressure system will rotate in a counterclockwise direction. In reality, you need to remember that because of the Coriolis effect, it will flow in a rightward or easterly direction; because of the low pressure in the center, this will cause a spin in the counterclockwise direction as shown in figure 30:

High-pressure systems always rotate clockwise, because the air is expanding outward and not moving inward as you would see in a low-pressure system. This is only true in the northern hemisphere. The rotation of high-pressure and low-pressure systems will be the reverse in the southern hemisphere. You will need to look at isobars to predict just how fast the wind is blowing and in what direction. While there are upper-level maps and surface level maps, your weather is mostly determined by surface level maps, which indicate where the air is flowing near the ground.