What is a weather map
Drawing theory of the weather map
They are cards of everyday life. Everyone has seen hundreds of them - in the newspaper, on television, or on the Internet. But what all the lines and symbols on a weather map mean is still Spanish to some. It is actually quite simple: the air pressure and the fronts are shown on a weather map, and sometimes the temperature as well. If you want to read the map correctly, you just have to know a few typical characters.
Depending on the purpose for which a weather map was made, it looks a little different. But two things almost always appear in it: isobars and fronts. Because these elements can be used to illustrate the structure of a weather situation.
A weather map
The air pressure on the ground is always entered in a weather map for normal end users. The pressure can be recognized by the isobars, the lines of equal air pressure. For a better understanding, it helps to think of topographic maps: The contour lines (isohypses) have a very similar function to the isobars.
On the isobars there are numbers that indicate the air pressure in the unit hectopascal (sometimes the outdated pressure unit millibars is still used). Where the isobars form a circle, there is usually a T or an H in the middle - the two letters stand for low pressure area and high pressure area. A typical low over the Atlantic has a core pressure of 950 to 980 hectopascals in winter. A high can reach values well over 1040 hectopascals. The mean air pressure is in between and is 1013.25 hectopascals.
The isobars and the wind
Gust collar just behind a cold front
Not only the air pressure can be read from the isobars. The lines also show roughly how the wind is blowing. In principle, the air should flow from high to low pressure - if it weren't for the rotation of the earth. This leads to the wind being deflected. The Coriolis force is responsible for this, an apparent force within a rotating system. The Coriolis force always deflects the wind to the right in the northern hemisphere and to the left south of the equator. The consequence of this is that the wind around the lows in the northern hemisphere blows counterclockwise. Around the highs it goes the other way around, i.e. clockwise.
At some points on the weather map, the isobars are very close to each other. There, the contrasts in air pressure are great and the wind is particularly strong. If you only consider the horizontal pressure difference and the Coriolis force, then the wind should blow parallel to the isobars. But it is not quite like that. The wind direction on the ground is always shifted a few degrees towards the deep core. This is due to the friction caused by air vortices - it is generated by the surface roughness. The turbulent friction shifts the balance of forces. As a result, the air on the ground flows on a spiral path towards the bottom.
The weather fronts
Many fronts are drawn on weather maps - starting from the low pressure areas. Warm fronts are indicated by thick black or red lines with semicircles on them, cold fronts by thick black or blue lines with jagged edges. Often the passage of a weather front is associated with precipitation. Because at the fronts, air masses of different temperatures and densities border one another (see Figure 2), which leads to the formation of clouds.
On a warm front, warm, light air pushes itself over colder air, while on a cold front it is the other way around. There, because of the greater density, an air mass with a lower temperature presses itself under an air mass with a higher temperature. The weather phenomena on a cold front are usually more severe. If the front is pronounced, there can be showers, thunderstorms and gusts of wind there at all times of the year.
Sometimes the ground temperatures are also shown on a weather map, for example by means of colored areas - in this case the air masses separated by fronts can be easily recognized by the different colors.
The fronts move with the wind and clouds. The warm front of a low always pulls up before the cold front. However, because the cold front usually moves faster, it will eventually catch up with its counterpart over time. The cold air mass of the cold front raises the warm air mass between the two fronts and connects with the cool air mass that is moving in front. In this way, an occlusion is formed (from the Latin occludere for “to close”). It can be recognized on weather maps by the alternating round and jagged markings; sometimes this line is colored purple. An occlusion front often occurs when a low pressure area begins to dissolve. However, this does not have to be a sign of weak weather conditions: For example, there can be heavy rain and a lot of wind on the occlusal front of a hurricane low.
Weather maps are drawn sometimes for the observed weather, sometimes for the predicted weather. If the map is about past meteorological conditions, then the data used come from the meteorological services' measuring stations. The air pressure often has to be converted for the display. Because many stations are hundreds to thousands of meters above sea level, where the air pressure is lower than on the coast. If you were to use these values for the map, you would see a depression on every mountain range. That is why you enter the air pressure that, according to the theoretical calculation, would prevail at sea level.
The forecast is visualized by meteorologists with very similar weather maps. The data on these maps comes from simulations carried out by the weather services with their computer models. Based on the forecast values, the weather map is drawn in the same way as it is for the observed weather.
There are many other weather maps for meteorological purposes. For example, the experts produce detailed maps with numerous measured values from the weather stations - these values include, for example, wind speed, wind direction and degree of coverage. In addition, meteorologists examine the conditions in higher air layers very carefully. After all, the essential part of the weather does not take place on the ground, but far above it up to heights of around 10 to 15 kilometers. In the weather maps for the newspaper one only focuses on the conditions on the earth's surface because we humans live there.
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