Aviation Weather: Surface Once you have looked at the METARs for your departure and arrival airports, you need to look at the overall weather effecting the state, region and/or nation. Several sources on the Internet provide maps showing fronts, pressure systems, precipitation and such. These maps normally provide the big picture needed. The first map featured is the most recent United States surface update. Included are pressure areas, isobars, fronts, radar echo returns and standard station data. We will discuss all of these and more to ensure you leave with a firm grasp on the information found on surface maps. The United States National Weather Service (NWS)surface map is a little bigger than the space we have. So we will use a portion of one such map for our study. To get the current map with all its data, click anywhere on the image below. A new window will open with the current map.
This section of map gives us a wealth of information about the national surface conditions at the time the chart was built. First, we will discuss areas of High and Low pressure. High pressure areas, designated with a large blue "H" here, such as the one over Arizona denote domes of heavy air sinking toward the surface. Such sinking air is not conducive to the development of thunderstorms or other severe weather. Large domes of high pressure normally mean clear skies and good weather. In this case, the high is beginning to break down due to the proximity of the three low pressure areas surrounding it, each designated with the large red "L." The numbers under the high and low pressure designators indicates the barometric pressure in millibars (mb) at its highest and lowest points. Another indication that the high over Arizona is breaking down is the small difference between it, at 1020 mb, and the low in extreme southern Nevada, at 1014 mb. Low pressure areas normally indicate stormy and even severe weather. They also tend to be associated with weather fronts. A Front is a line drawn between differing air masses. A prime example is the front draped across Texas from the 1004 mb low in the northwest Texas. This front separates cooler, drier air, behind the blue line, from the warmer, moist air ahead of it in Louisiana and Arkansas. There are three types of fronts, each with a standard look. Cold Fronts, like that in Texas, are designated by a blue line with small blue triangles pointing in the direction of movement. Warm Fronts are designated by a red line with red semicircles indicating the direction of movement. Finally, there is the Occluded Front, which is designated by a magenta line with alternating magenta triangles and semicircles. Occluded fronts occur when a cold front overruns a warm front. They are normally found near the area of low pressure, especially when the low and cold front take on a marked comma shape. An example is the short stretch of occluded conditions between the Texas low and the start of the blue cold front. Not shown on this chart is a stationary front. They are identified by red semi-circles pointed in the direction the warm air is pushing and blue triangles pointed in the direction the cold air is pushing. Stationary fronts tend to retain the characteristics of the dominant front before they reached an equal state. If it was the cold front, look for squall lines and embedded thunderstorms. If it was the warm front, low clouds and heavy rains may dominate. This map also shows< strong>Radar Echoes, which are overlaid on the surface map. This gives you a good indication of large scale precipitation patterns. You can see that the Texas low and associated cold front are generating rain from southern Texas to the Dakotas. The scale at bottom left of the image shows the range of intensities, with one indicating minor precipitation and six indicating heavy, possibly severe storms. We look much more closely at radar returns in the section on thunderstorms. The thin blue lines on the map with associated numbers are called Isobars. They denote areas of equal barometric pressure. On there own, they do not mean a lot, but when analyzed with nearby isobars, they can indicate significant weather information. For instance, if an area of low pressure connected to a front has a lot of tightly packed isobars around it and stretching down the front, one can expect strong to even dangerous winds. Absent those tightly packed isobars, such as with the Texas low, surface winds should not be a major issue. Now to surface station reports.
Dotting the surface map we have been referencing are items which look like the image seen here. This is a Standard Surface Report and like most weather related reports it is full of useful information. If you have the key that is. To try and explain each of the numbers or symbols would take far more room than we have available here. In addition, you can get the METAR for the station and get the same and even more information., However, if you want to know what everything means, click on the image. A full explanatory chart will open in a new window examining each position and all of the possible data units which might be placed there. Finally, it is possible to also obtain short-term surface weather Forecast Maps. These give you an idea of where the fronts, precipitation and such may be heading. Here is an example of a portion of a 12-hour surface forecast map. For the current 12-hour national surface forecast map, click anywhere on the image.
(Intended for Flight Simulation Use Only) |
