Tips on Interpreting Fax Maps This is a long article. Here is a section that sumarizes the use of the 500-mb maps (note that without earlier background not included in this demo, some of the discussion may not be clear... (1) These maps usually show wind arrows following the meandering paths around the globe. These are the steering winds that direct the weather toward or away from us. A low on the surface below these winds, along with its associated fronts, will typically follow the path shown in these winds aloft. See schematic animation in G218. Sometimes, however, even though the Low will move in the direction of these high winds aloft, a trailing surface front that aligns with the winds aloft will just stretch out behind it without as much actual lateral translation across the map as its parent Low. (2) When the flow aloft is smooth, firm, and steady over a few days, the progs (forecasts) — which to a large part are based on this behavior aloft — will be more reliable. When the flow is breaking up or splitting, then it will probably be harder for the forecasters to predict what is going to happen on the surface, especially when the break up occurs near a coast which adds further complexity to the development. Remember, there will always be a forecast... and the forecasts are not marked this is a good one or this is somewhat uncertain etc. By looking at these 500 mb maps, we can make some rough guess about the dependability of the forecasts. (3) When the winds aloft are strong, the surface lows move faster and typically have stronger surface winds associated with them. Most, but not all, 500 mb maps also show the wind speeds aloft. When the winds aloft are some 70 knots or more, they would be considered strong. In these cases, we can often more or less readily discern the actual motion of high cirrus clouds that are moving along with this wind. Normally at these high altitudes, at lower speeds we are hard pressed to actually see them move without some real concentrated observations. (4) When the flow aloft forms an inverted shape like the Greek letter capital omega, then the pattern is particularly stable and will resist change. The normal slipping of these waves to the east is interrupted and a pattern can sit in one place for some time. Generally a strong surface High develops in the center of the omega which might then persist in the same general shape and place for a week or so. See blocking High which explains how to recognize one. (5) Note that the isobars within the warm sector between warm and cold fronts, will usually align with the flow of wind aloft. This fact is often useful in predicting the motion of lows or fronts on the surface when you do not have access to the winds aloft map. Put in reverse, it is valuable to take note of the 500 mb map periodically to know which way Lows might move when not indicated on the surface maps and no warm sector isobars are present. (6) Guidelines (see articles referenced under 500-mb map): in the winter, the 5640 contour (drawn in bold in NMC maps) is an excellent indication of the southern extent of surface winds of force 7 or greater. In the summer, this limit marks winds more in the 34 knot range. (7) The surface storm track is usually 300 to 600 nautical miles north of and parallel to the 5640 height contour. (8) Surface storm centers (Lows) and cold fronts move along the surface at speeds of some 1/3 to 1/2 of the 500-mb wind speed. That is, if the 500-mb winds aloft are 60 knots, expect surface Lows to move at some 20 to 30 knots. (9) Surface wind speeds of a surface Low are typically some 50% of the 500-mb winds. (10) See notes on Rossby waves and frontal formation in Britton-Lilly book on Sea state forecasting (Source Book).