101: Maps

Now that I’ve handled the vertical, it is time to cover the horizontal: Maps!

There are two main types of weather maps, surface analysis maps and upper level maps (which I will get to later). More than likely, everyone has seen a surface analysis (I tend to abbreviate surface as sfc) even if they don’t realize it. These maps display the weather on the ground, hence ‘surface’ map, and are commonly seen on TV weather reports or in the daily newspaper. The official variety of these maps follows a specific structure that stays pretty much the same all across the world. While the exact look and feel of the maps may differ, there are some pieces of data that you can almost be sure to see.

-Isobaric Analysis

The most common feature on sfc maps are isobars, or lines of constant sfc pressure. These contours denote the pressure found at that location in units of millibars (mb, which are equal to the metric unit hectopascals, hPa); however, these values are often abbreviated for clarity by shortening the value to two digits. Therefore, 972.8mb becomes 73, 999.0mb becomes 99, and 1002.4mb becomes 02. It should be noted that the pressure being used here is sea level pressure, therefore the pressure represented with isobars is what the pressure at that location would be if it was located sea at sea level. The process of converting the pressure is called reduction to sea level and requires a hardy equation and several assumptions, so pressure values from stations at high elevations have a tendency to be off a tad. The pressure between any two contours will be somewhere between the values. The placement of each line is determined by the observed pressure of hundreds of ground locations that report the current weather at their site regularly, so drawing isobars is kind of like playing connect the dots, except most dots will fall between lines. For example, say the sfc pressure in NY City is measured to be 1010.3mb and the pressure in Chicago is measured to be 1007.7mb. The 08 line, that is the line at which the pressure is 1008mb, would lie somewhere between the two cities. It is important to remember that since isobars are lines of constant pressure, they will never merge or cross each other. Furthermore, most maps will have pressure lines drawn at 4mb intervals, centered on 1000mb. On any given map, there will be places where the sfc pressure is lower than at any adjacent location, or the pressure will be higher than at all adjacent locations. These points are low pressure centers and high pressure centers, respectively. Each center will likely be surrounded by loops of closed contours, the more closed loops there are, the more intense the low, or high, is.

-Frontal Analysis

            Along with the lines of constant pressure are often lines marking where abrupt pressure change is occurring. These lines are called troughs, fronts, dry lines, and ridges.

-A trough is a line of minimum pressure. On a sfc map, troughs appear as regions where isobars circling a low bulge outward and are denoted by dashed lines.

-One special type of trough are fronts, which follow the same basic definition as troughs, but also represent a region of sharp temperature gradient. Four types of fronts exist: warm fronts, cold fronts, occluded fronts, and stationary fronts. As a warm front passes, one can expect the temperature to warm because the air behind a warm front is typically coming from far south. A cold front is just the opposite, with cold air from up north trailing behind it. Cold fronts are usually much more abrupt and typically far easier to find using isobars. Occluded fronts are generally found near the core of mature low pressure systems and are the result of a warm or cold front catching up to the other and merging, creating an almost zipper like appearance. Finally, stationary fronts have a structure similar to cold fronts, except there is little or no motion of the front, that is, the cold air is not advancing southward. When surface maps are in color, cold fronts will appear as blue lines with triangles pointed in the direction the front is moving. Warm fronts will be in red with bumps pointed in the direction of motion; occluded fronts will be purple lines with alternating triangles and bumps facing the same direction. Stationary fronts will alternate between short blue lines with a triangle and short red lines with a bump, with the triangle facing into the warm air and the bump facing into the cold air. Fronts are a very important part of sfc maps, yet they can be hard to plot correctly, so it would be wise to not solely rely on them.

-Dry lines are a feature generally found only in the central U.S. and maybe south central Canada. They are similar to fronts, except instead of being a region of strong temperature gradient, they are regions of strong dew point gradient, essentially means a strong gradient in moisture. On the east side of a dry line is air that is near the ground and flowing north from the Gulf of Mexico. On the west side is very dry air that is flowing eastward from over the Rockies and the high desert in northern Mexico. Since the terrain slopes upward as you go from east to west, the dry air will end up riding on top of the low lying moist air. Where the land intersects this boundary is the dry line. Dry lines are often associated with severe weather on the planes so they’re an important feature to look out for. They are drawn as orange lines with closely spaced small open bumps, however occasionally a sfc map will not use this convention and simply mark it as a trough, so in that case, look to either side of the trough, if observations indicate a big difference in dew point on opposing sides, it’s probably a dry line.

-Last are ridges. These feature share a lot with troughs as far as their appearance on sfc maps, but they are regions of high pressure. High pressure systems are usually not as sharply defined as lows, thus ridges are often quite vague. In fact, ridges are often omitted completely from maps, but when they’re not, they are denoted by a zigzag line.

-Surface Observations

            Observations from weather stations on the ground (or on ships or buoys) are the raw data from which isobars and fronts are determined. The format for each set of data is called the station model. It is a lot easier to see one than to try to describe it in words, so the picture below should help.

            Note that the present weather and total sky cover entries have lots of possibilities, below are a few other images with some examples.

 

-Notes

            Occasionally additional text notes will be added to the map in order to inform the reader of some significant weather or hazard. For example, tropical cyclones have text boxes associated with them that gives a short summary of the storm. There might also be text indicating a developing storm or high winds.

 

Below are some maps from a few days ago. They are all from the same time, but of slightly different formats. Also included are an array of satellite images and a radar composite, also from the same time, to show how features on the map correlate to features on imagery. Lastly, I’ve drawn some annotations to point out some of the key features.

(I'll post these images at higher resolution in the Gallery for those who are interested)