METR 104:
Our Dynamic Weather

(Lecture w/Lab)
Meteograms Dr. Dave Dempsey
Dept. of Geosciences
SFSU

Meteograms. Meteograms show a time series (that is, a sequence over time) of surface weather conditions observed at a particular weather station. You have been provided an example as a handout. Below is a summary of the information shown by a typical meteogram.

Quantity Plotted Description
TEMP
(topmost graph)
Temperature and dew-point temperature graph, showing temperature as the upper curve and dew point as the lower curve, in °F.

Dew-point temperature is an indirect measure of the amount of water vapor in the air. (This isn't obvious, is it!) Higher dew-point temperature (or just "dew point") mean more water vapor is in the air. For physical reasons that we'll learn later in the course, dew-point temperature can't exceed the temperature.

MAXT Reported at midnight local standard time (PST in our case), this is the maximum temperature recorded during the previous 24 hours.
MINT Reported at midnight local standard time, this is the minimum temperature recorded during the previous 24 hours.
WX "Present weather" reports, plotted as a weather symbol (see the on-line key to decoding surface weather-station models, under "Weather Symbols", for examples).
PREC Precipitation, showing precipitation totals in inches; at 12Z the values are generally 24-hour totals, while at 0Z, 6Z and 18Z, they are 6-hour totals; at any other times they are generally 3 hour totals.
VIS Visibility (horizontally), in miles, over at least half of the horizon.
WGST Wind gusts, in knots (if reported). (Wind speeds are typically reported as 2-minute averages. Instantaneous departures from that average that exceed the average by 10 knots or more are reported as wind gusts.)
WIND Winds and cloud cover. The circle is used to report the extent of cloud cover (see key to decoding surface weather-station models, under "Cloud Coverage").

The wind direction is represented by the "stem" sticking out of the circle. Think of it as a pollywog, with a fat head and a tail (the stem), swimming in the direction toward which the wind is blowing. Alternatively, think of the stem as pointing toward the direction from which the wind is blowing. (A double circle with no stem represents a calm wind, which of course has no direction.)

Wind speed is represented by "barbs" sticking out of the stem at or near the end of the stem. There are three kinds of barbs: (1) a short barb; (2) a long barb; and (3) a narrow, filled-in triangular flag. A short barb represents 5 knots of wind speed; a long barb represents 10 knots; and a triangular flag represents 50 knots. Multiple barbs in combinations of these three can represent any wind speed (within a range of plus or minus 2 knots) from less than three knots (no barbs at all—just a stem) to hundreds of knots. (See key to decoding surface weather-station models, under "Winds".) [A knot is one nautical mile per hour. A nautical mile is 1.15 regular miles, so a know is 1.15 miles per hour.]
CLD
(middle graph)
Cloud chart: This chart gives cloud layer information. The vertical axis is a logarithmic scale of the cloud base height in feet ranging from the 100 to 50,000 feet. Each cloud layer base is plotted as horizontal lines. Clear skies are plotted as a 'C'. Scattered cloud layers (1/8th to 3/8th coverage) are plotted as a single short dash "-". Broken cloud layers (4/8th to 7/8th coverage) are plotted as two short dashes "- -". Overcast layers are plotted as a single long dash "---". The actual cloud ceiling is displayed immediately below the chart in 100's of feet if the ceiling is below 10,000 feet.
PRALT
(bottom graph)
Sea-level pressure graph: plots sea level pressure in millibars (or altimeter setting if sea-level pressure is not reported—never mind what that is, though pilots need to know it; it's similar to sea-level pressure).
Time
(along bottom of meteogram)
The observation hour in UTC. (The time range covered by the meteogram is displayed in the title across the top of the plot.)

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