Introduction to Meteorology

How to read weather maps On surface maps you will often see station weather plots. Since meteorologists must convey a lot of information without using a lot of words, plots are used to describe the weather at a station for a specific time. When all stations are plotted on a map, a "picture" of where the high and low pressure areas are located, as well as the location of fronts, can be obtained. There are a large number of weather symbols used for station plotting. Some are used for weather elements such as rain, snow, and lightning. Others represent the speed of the wind, types of clouds, air temperature, and air pressure. All of these symbols help meteorologists depict the weather occurring at a weather observing station. This sample plot represents the maximum amount of information about the current weather at an observing station. Hand plotted maps usually contain the full weather information. However, most computer generated surface weather maps omit some data such as clouds types and heights. Before computers, the plotting of weather maps was considered an art. In fact, Aerographers (weathermen) in the U.S. Navy continue to plots maps by hand. A skilled plotter can easily fit the above information under the space covered by a dime. Decoding these plots is easier than it may seem. The station model shown above left is decoded and explained below. Note that this example does not contain all possible weather elements. Following this explanation will be a full station model for you to examine. In the upper left, the temperature is plotted in Fahrenheit. In this example, the temperature is 77°F. Along the center, the cloud types are indicated. These cloud types use the same cloud codes as found in the cloud chart section. The top symbol is the high-level cloud type followed by the mid-level cloud type. The lowest symbol represents low-level cloud. In this example, the high level cloud is Cirrus, the mid-level cloud is Altocumulus and the lowlevel clouds is a cumulonimbus. See block 3. At the upper right is the atmospheric pressure reduced to mean sea level in millibars (mb) to the nearest tenth with the leading 9 or 10 omitted. In this case the pressure would be 999.8 mb. If the pressure was plotted as 024 it would be 1002.4 mb. When trying to determine whether to add a 9 or 10 use the number that will give you a value closest to 1000 mb. On the second row, the far left number is the visibility in miles. In this example, the visibility is 5 miles. Next to the visibility is the present weather symbol. There 95 symbols which represent the weather that is either presently occurring or has ended within the previous hour. In this example, a light rain shower was occurring at the time of the observation. [See block 8] 1 The circle symbol in the center represents the amount of total cloud cover reported in eighths. This cloud cover includes all low, middle, and high level clouds. In this example, 7/8th of the sky was covered with clouds. [see block 6] This number and symbol tell how much the pressure has changed (in tenths of millibars) in the past three hours and the trend in the change of the pressure during that same period. In this example, the pressure was steady then fell (lowered) becoming 0.3 millibars LOWER than it was three hours ago. [see block 10] These lines indicate wind direction and speed rounded to the nearest 5 knots. The longest line, extending from the sky cover plot, points in the direction that the wind is blowing FROM. Thus, in this case, the wind is blowing FROM the southwest. The shorter lines, called barbs, indicate the wind speed in knots (kt). The speed of the wind is determined by the barbs. Each long barb represents 10 kt with short barbs representing 5 kt. In this example, the station plot contains two long barbs so the wind speed is 20 kt, or about 24 mph. [see block 9] The 71 at the lower left is the dewpoint temperature. The dewpoint temperature is the temperature the air would have to cool to become saturated, or in other words reach a relative humidity of 100%. The lower right area is reserved for the past weather, which is the most significant weather that has occurred within the past six hours excluding the most recent hour. [see block 11] Below are the full station models in symbolic and sample plotted format. These include all possible observed weather elements. Refer to these when practicing with the problems given in class and when plotting your station models for your observation exercise. 2 3 Cloud abbreviations: St or Fs = Stratus or Fractostratus; Ci = Cirrus; Cs = Cirrostratus; Cc = Cirrocumulus; Ac = Altocumulus; As = Altostratus; Sc = Stratocumulus; Ns = Nimbostratus; Cu or Fc = Cumulus or Fractocumulus; Cb = Cumulonimbus 4 Present Weather 0 00 10 20 30 40 50 60 70 80 90 1 2 (Block 8) 3 4 Clouds generally dissolving or becoming less developed during past hour State of sky on the whole unchanged during past hour Patches of shallow fog at station not deeper than 6 feet on land More or less Lightning visible, Precipitation continuous shallow no thunder heard within sight, but fog at station not not reaching the deeper than 6 feet ground on land Rain (not freezing, not showers) during past hour, not at time of obs Snow (not falling as showers) during past hour, not at time of obs Slight or moderate Slight or moderate duststorm or duststorm or sandstorm, has sandstorm, no decreased during appreciable change past hour during past hour Fog at distance at time of obs but not at station during past hour Cloud development not observed or not observable during past hour 7 8 9 Duststorm or sandstorm within sight of station or at station during past hour Precipitation within sight, reaching ground, but distant from station Precipitation Thunder heard but Squall(s) within within sight, no precipitation sight during past reaching the at the station hour ground, near to but not at station Funnel cloud(s) within sight during past hour Rain and snow (not Freezing drizzle falling as or rain (not showers) during showers) during past hour, not at past hour, not at time of obs time of obs Showers of rain during past hour, but not at time of obs Showers of snow, or of rain and snow during past hour, but not at time of obs Thunderstorm (with or without precip) during past hour, but not at time of obs Slight or moderate duststorm or sandstorm, has increased during past hour Severe duststorm or sandstorm, has decreased during past hour Severe duststorm or sandstorm, no appreciable change during past hour Severe duststorm or sandstorm, has increased during past hour Slight or moderate Heavy drifting drifting snow, snow, generally generally low low Slight or moderate Heavy drifting drifting snow, snow, generally generally high high Fog in patches Fog, sky discernable, has become thinner during past hour Fog, sky not discernable, has become thinner during past hour Fog, sky discernable, no appreciable change during past hour Fog, sky not discernable, no appreciable change during past hour Fog, sky discernable, has begun or become thicker during past hour Fog, depositing rime, sky discernable Intermittent drizzle (not freezing), slight at time of obs Continuous drizzle (not freezing), slight at time of obs Intermittent drizzle (not freezing), moderate at time of obs Continuous drizzle Intermittent (not freezing), drizzle (not moderate at time freezing), thick of obs at time of obs Continuous drizzle Slight freezing (not freezing), drizzle thick at time of obs Moderate or thick Drizzle and rain, freezing drizzle slight Drizzle and rain, moderate or heavy Intermittent rain (not freezing), slight at time of obs Continuous rain (not freezing), slight at time of obs Intermittent rain (not freezing), moderate at time of obs Continuous rain (not freezing), moderate at time of obs Continuous rain (not freezing), heavy at time of obs Moderate or heavy Rain or drizzle freezing rain and snow, slight Rain or drizzle and snow, moderate or heavy Intermittent fall Continuous fall of of snowflakes, snowflakes, slight slight at time of at time of obs obs Intermittent fall of snowflakes, moderate at time of obs Continuous fall of Intermittent fall snowflakes, of snowflakes, moderate at time heavy at time of of obs obs Continuous fall of Ice needles (with snowflakes, heavy or without fog) at time of obs Granular snow (with or without fog) Isolated starlike snow crystals (with or without fog) Ice pellets (sleet, U.S. definition) Slight rain shower(s) Moderate or heavy rain shower(s) Violent rain shower(s) Slight shower(s) of rain and snow mixed Slight snow shower(s) Moderate or heavy snow shower(s) Slight shower(s) of soft or small hail, with or without rain, and/or snow Moderate or heavy shower(s) of soft or small hail, with or without rain and/or snow Slight shower(s) of hail, with or without rain and/or snow, not assoc with thunder Moderate of heavy shower(s) of hail and/or rain/snow, not associated with thunder Slight rain at time of obs; thunderstorm during past hour not at time of obs Moderate or heavy rain at time of obs; TS during past hour not at time of obs Slight snow and/or Moderate or heavy rain/hail at time snow and/or rain/ of obs; TS during hail at time of past hour not at obs; TS past hour time of obs not at obs time Slight or moderate thunderstorm without hail but with rain and/or snow at obs time Slight or moderate thunderstorm with hail at time of obs Heavy thunderstorm without hail but with rain and/or snow at time of obs Thunderstorm combined with duststorm or sandstorm at time of obs Heavy thunderstorm with hail at time of obs Drizzle (not freezing, not showers) during past hour, not at time of obs Intermittent rain (not freezing), heavy at time of obs Moderate or heavy shower(s) of rain and snow mixed Haze 6 Widespread dust in Dust or sand Well developed suspension in the raised by wind, at dust devil(s) air, not raised by time of obs within past hour wind, at time of obs Light fog Clouds generally Visibility reduced forming or by smoke developing during past hour 5 Slight freezing rain Showers of hail, or of hail and rain during past hour, but not at time of obs Fog, sky not discernable, has begun or become thicker during past hour Fog during past hour, but not at time of obs Fog, depositing rime, sky not discernable 5 6