Every number in a station model comes from an instrument. Thousands of stations, ships, buoys, aircraft and weather balloons measure the air at the same moments around the world, and those numbers are the starting point for every forecast.
Key idea
A thermometer measures air temperature (in a shaded, ventilated shelter about 1.5 to 2 m above the ground); a barometer, air pressure; a hygrometer, humidity or dew point; a cup anemometer, wind speed; a wind vane, wind direction; a rain gauge, precipitation. A radiosonde, carried by a weather balloon, measures temperature, humidity and pressure (and, tracked by GPS, the wind) from the ground to about 30 km.
Why it works
Each instrument is designed to measure the air itself, not something else. A thermometer in sunshine would measure sunlight on its own glass, so it sits in the shade with air flowing past it. A wind vane is a lopsided arrow: the wind pushes hardest on the big tail fin, swinging it downwind, so the arrowhead points into the wind, toward where it comes from. Balloon launches happen worldwide at the same two times each day (00 and 12 UTC), so forecasters get a snapshot of the whole atmosphere at one instant.
| Instrument | Measures | Units on U.S. maps |
|---|---|---|
| thermometer | temperature | °F (°C elsewhere) |
| barometer | pressure | hPa, inHg |
| hygrometer | humidity, dew point | %, °F |
| anemometer | wind speed | knots, mph |
| wind vane | wind direction (from) | degrees or compass points |
| rain gauge | precipitation | inches (mm elsewhere) |
| radiosonde | temperature, humidity, pressure and wind aloft | hPa levels, °C, knots |
Worked example: Reading a wind vane
A wind vane's arrow points toward the southwest. What is the wind?
- The arrowhead points into the wind, toward where it comes from.
- So the wind blows from the southwest: a southwest wind, blowing toward the northeast.
Answer: A southwest wind
Worked example: A rain gauge in two units
A rain gauge collects 1.25 inches of rain. How many millimeters is that? (1 inch = 25.4 mm)
- $1.25 \times 25.4 = 31.75$ mm, about 32 mm.
Answer: About 32 mm
Worked example: How long is a balloon flight?
A weather balloon rises about 5 m/s. How long does it take to reach 30 km, where it bursts?
- Time = $30{,}000 \div 5 = 6000$ s.
- $6000 \div 60 = 100$ minutes, a little under two hours.
Answer: About 100 minutes
Common mistake
Thinking a wind vane points where the wind is going. It turns to point into the wind, toward the direction the wind comes from, which is how winds are named: a north wind blows from the north. Check: if smoke from a chimney drifts east, the vane on the roof points west.