This post was inspired by a fantastic article which was published in the Bulletin of the American Meteorological Society (BAMS) in December 2019 by Dr. Robert Houze of the University of Washington, and his daughter, Rebecca Houze.
I will never forget one of the first times I felt as an undergraduate that I had really arrived at the point where I was learning ‘real meteorology’. As an applied science, meteorology has a lot of core class requirements, including calculus and differential equations, physics, and chemistry. Thus, it is very easy to get bogged down in all of the prerequisites and lose sight of the light at the end of the tunnel- learning about the science of weather that drove you into those classes in the first place. For me, that light at the end of the tunnel was an introductory weather and forecasting class which I took during the spring semester of my sophomore year. In that class, we did a lot of hands-on work plotting weather maps, which was something I really enjoyed. Actually, looking at data on weather maps and figuring out what it means is STILL one of my favorite parts of my job today!
At any surface weather observing station, a multitude of parameters are recorded and reported every hour, if not more frequently. These often include temperature, dew point (a measure of humidity), wind speed, wind direction, present weather, barometric pressure and pressure tendency (is it rising or falling?), cloud cover, and many others. One of the things that makes meteorology such a complex science is the huge number of variables which must be considered. Even just plotting these variables on a map is quite an undertaking.
In order to be able to process the vast amount of data that existed even back in the ‘old days’ before automated weather stations and computer models, it was necessary to form a convention by which the meteorological information could be displayed visually on a map and easily interpreted with the human eye. As Dr. Houze points out in his article, there were four key jobs which were necessary to make a weather forecast back in the early and mid twentieth century: observers who would transmit weather data, plotters who would receive coded weather observations and plot them on a map, analysts who would draw contours and note the locations of fronts and high and low pressure systems, and forecasters, who would use these manually generated maps to actually make a weather forecast. As you can imagine, this process was exceedingly time consuming and these skilled workers were in high demand as the number of weather stations and timing of observations increased exponentially.
This is a sample of a surface map which you might get online today:
One of the first things that you will notice is that there are numbers, colors and symbols all over the map at each of the reporting stations. One might wonder how this helps us understand what the weather is doing, so let’s take a closer look:
At each of these stations, there is information plotted according to the model which is shown above. This is known as the ‘station model’. The station models in the surface map shown above are simplified, and contain some, but not all, of the information in the full model shown here. On the map, the convention is as follows:
- The red number in the top left is the temperature.
- The green number in the bottom left is dew point.
- The shading of the circle represents the aviation flight rules: visual flight rules (VFR), instrument flight rules (IFR), or low instrument flight rules (LIFR). On most surface maps, the shading of the circle represents the observed cloud cover:
- The symbol in between the temperature and the dew point represents the observed present weather. There are numerous symbols which are used here, a sampling of the most common ones are shown here:
- The three letter code on the right of the model represents the station identification code.
- The three digit number above that represents the surface pressure, which is encoded so that it fits on the model as a three digit number. Usually pressures are in millibars and fall in the range of roughly 950 to 1050 millibars. Pressures are plotted to the tenths place and the leading 9 or 10 is omitted. Thus, 1031.0 mb would be encoded on the model as 310.
- Winds are shown with the ‘stick’ pointing to the direction the wind is blowing from, with the flags representing the wind speed:
For most surface maps, plotting these parameters allows meteorologists to identify key features and weather patterns. From here, the data can be analyzed to further help with the visualization, and important features such as fronts can be placed on the map. The map below has isobars, or lines of constant pressure, analyzed, which shows the locations of high and low pressure systems. Also one can see the locations of key fronts, or temperature boundaries, along with some actual surface observations (although thinned out due to the scale of the map) for reference.
Although one might think that in this highly technological day and age, computers have now replaced hand plotting and analysis of weather maps, this is not the case! Although the data is plotted by computer, forecasters at the Storm Prediction Center still routinely do hand analysis of weather maps. This labor-intensive exercise is well worth it, as it helps meteorologists identify very small scale features, such as temperature boundaries, localized areas of higher humidity, and wind shifts which can become very important when making a weather forecast. Identifying where key features are at the present time, known as situational awareness, is one very important tool which meteorologists use.