A surface weather map is built from hundreds of station reports, each drawn as a tiny symbol called a station model. In one glance it gives the temperature, the dew point, the pressure, the wind and the sky. U.S. maps give temperatures in °F and wind in knots (1 knot ≈ 1.15 mph).
Key idea
Around the station circle: temperature at the upper left and dew point at the lower left; a three-digit pressure code at the upper right; the 3-hour pressure change at the lower right; the sky cover as the fraction of the circle filled in; the weather symbol to the left. The wind barb's staff points toward where the wind comes from; on its end, a pennant is 50 knots, a full barb 10 knots and a half barb 5 knots.
Why it works
The pressure code saves space. Sea-level pressures almost always lie between about 950 and 1050 hPa, so the leading 9 or 10 can be left off: the code is the last three digits of the pressure in tenths of a hectopascal. To decode, put a decimal point before the last digit, then put 10 in front if the code is below 500, or 9 if it is 500 or more. The wind barb is drawn like an arrow's feathered tail: the staff starts at the station and points into the wind, the way a weathervane's arrow points.
Worked example: Decoding the pressure
Decode the pressure codes 084 and 986.
- 084: put the decimal before the last digit, 08.4. The code is below 500, so put 10 in front: 1008.4 hPa.
- 986: 98.6, and the code is 500 or more, so put 9 in front: 998.6 hPa.
Answer: 1008.4 hPa and 998.6 hPa
Worked example: Reading the wind
A station's barb has its staff pointing toward the northwest, with one full barb and one half barb. What is the wind?
- The staff points to where the wind comes from: the northwest.
- Speed: one full barb (10 kt) plus one half barb (5 kt) = 15 knots.
- A northwest wind at 15 kt, about 17 mph.
Answer: From the northwest at 15 knots
Worked example: Encoding
A station measures 1023.7 hPa. What code goes at the upper right?
- In tenths: 10237. Keep the last three digits: 237.
Answer: 237
Common mistake
Reading the wind barb as pointing where the wind goes. The staff points toward the direction the wind comes from, just like the name: a "north wind" comes from the north. Check: on a map, barbs around a Northern Hemisphere low should show air circling counterclockwise and slanting inward, which only works if you read the staffs as "from."
| Symbol | Meaning |
|---|---|
| empty circle | clear |
| circle ¼ filled | about 2/8 of the sky covered (few clouds) |
| circle ½ filled | about 4/8 covered (scattered) |
| circle ¾ filled | about 6/8 covered (broken) |
| filled circle | overcast |
| dots, asterisks, ≡ | rain, snow, fog |