LAB J: Weather Maps and Humidity
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PART I – Introduction
All over the United States, hundreds of weather observations are taken several times during the day. These stations collect information such as temperature, pressure, dew point, etc. Meteorologists need a way to get all of this detailed information into the smallest area possible on a weather map, so that several stations can be plotted for the same observation time on the same map. This gives them a ‘big picture’ of what the weather is doing at one particular moment in time.
A simplified example of a station model plot used to represent meteorological conditions near the surface is shown in Figure 1 below. The station model depicts current weather conditions, cloud cover, wind speed, wind direction, visibility, temperature, dew point temperature, atmospheric pressure adjusted to sea level, and the change in pressure over the last three hours. Nine weather variables commonly reported on the evening news are plotted in Figure 1, which is a simplified version of the sample weather station shown on p. 113 in your lab manual.
Simplified Station Model
The information recorded on a weather station model can be decoded using the “Weather Map Decoding Key” starting on p. 113 of your lab manual. Turn to p. 113 to continue with this pre-lab activity. Use the example of the “Sample plotted report” to go with the following description.
The easiest place to begin decoding weather information is with the center circle. The circle in the station model is centered on the latitude and longitude of the city where the weather observations are made. The letter “N” is written on the circle. Find “N” in the description below the station models on p. J-11 and you will see that it represents the total amount of sky coverage. On the following pages of the weather map decoding key, find Table E. A completely darkened in center circle indicates a completely overcast sky.
The weather data reported for a city is always recorded in the same position on the station models. For example, the number 31 is in the upper left corner from the center circle and corresponds to “TT” on the symbolic station model. Find “TT” in the description on p. 113 and you will see that it stands for current air temperature in oF.
The two small stars (**) directly to the left of the center circle corresponds with the “ww” on the symbolic station model. Find “ww” in the description on p.113 and you will see that it stands for present weather conditions. On the following pages look for Table H. It is a two-page table that contains all the abbreviated weather symbols found on weather maps. Skim through the symbols until you find **. In brief, this stands for “snowing.”
To the far left of the two ** symbols on the sample station model, you will see the number ¾. This corresponds to “VV” on the symbolic station model and decodes down below as “visibility in miles.” Thus, visibility at this time is ¾ miles.
The number 30 is in the lower left corner of the sample station model. It corresponds to “TdTd” on the symbolic station model. Looking for “TdTd” on p. J-11, you will see that it stands for the dew point temperature in oF.
Turn your attention to the upper right corner of the center circle on the sample station model. You will see the numbers “247.” These numbers correspond to the “PPP” on the symbolic station model. This stands for barometric pressure and the directions to decode the actual air pressure is written on p. 113 and repeated for you here.
Surface pressure (PPP) is the station pressure adjusted to sea level. The units are coded in mb. Use the following procedure to decode barometric pressure:
1. Place a decimal point to the left of the last number. For example, 247 becomes 24.7.
2. Next place a 9 or 10 in front (to the left of the first digit). If the number falls between 56.0 and 99.9, place a 9 before the first digit. If the number falls between 00.0 and 55.9, place a 10 in front of the first digit. Thus 24.7 would be 1024.7 mb.
Directly above the center circle on the sample station model are two symbols. These two symbols represent cloud types at various heights. The lowest one (closest to the circle) corresponds to the “CM” on the symbolic station model which stands for “middle clouds”. Find Table C on the pages following p. 113. The symbol for “CM” is found as the fourth symbol down from the top on the right column. It decodes as “Ac of a chaotic sky, usually at different levels; patches of dense Ci usually present also.” That’s a lot to write down! You will find the abbreviations for the cloud types in Table B. To simplify, you just need to decode the specific cloud type. Thus, “Ac” becomes “altocumulus” and your answer for “CM” is middle clouds = altocumulus.
The only information on a weather station model which changes position relative to the center circle is the line coming out of the circle. This “flag” represents “dd” on the symbolic station, which is wind direction. Think of the center circle as an archery target. An arrow is shot into the center of the target (see Figure 2a). The line is the arrow. Next, think of the center circle as a compass (see Figure 2b). Wind direction is always reported as the direction the wind is blowing from! In this example, the wind is blowing from the NW.
The flags at the end of the wind direction line correspond to “ff” on the symbolic station model. This decodes out as “wind speed” which can be found on Table F. You can report the information as either knots per hour (kph) or miles per hour (mph) as long as you label it.
All of the above examples show the specific ways in which detailed weather information can be recorded on a very small space on weather maps to allow meteorologists to evaluate current weather conditions and to make forecasts for the near future.
Below is a table showing the data decoded for the sample weather station shown below. This is the same sample weather station used in the lab manual on p. 113.
Symbol Represents Conditions
|Sky coverage (see Table E)||completely overcast|
|31||Current air temperature in oF||31 oF|
|**||Present weather (see Table H)||snowing|
|3/4||Visibility in miles||¾ miles|
|30||Dew point temperature in oF||30 oF|
|247||Barometric pressure in mb||1024.7 mb|
|Middle clouds (see Table C)||altocumulus|
|High clouds (see Table C)||dense cirrus|
|True direction from which wind is blowing||from NW|
|Wind speed in knots or miles per hour (see Table F)||26-31 mph|