Met Instruments

Meteorology · Chapter 23 · DGCA CPL and ATPL

Every number in a METAR comes from an instrument. Which one measures what, and in which unit?

Remember

Pressure: mercury or aneroid barometer, barograph (records); hPa = mb, or inches of mercury (1013.25 hPa = 760 mm = 29.92 in).
Temperature °C: dry and max (mercury), min (alcohol). Dew point and RH (%): dry and wet bulbs, hygrometer, psychrometer, hygrograph; mixing ratio g/kg.
Rain: rain gauge, self-recording gauge, hyetograph (mm or cm); snow gauge, melted. Wind: vane (direction FROM, ° true clockwise), anemometer (kt), anemograph.
Cloud base (ft or m): searchlight/laser + alidade, ceilometer/ceilograph, ceiling balloon. Cloud movement: nephoscope, 8 points.
Visibility (m, km) and RVR (m): transmissometer, AVRA, scopograph, forward scatter; landmarks by eye.
Upper winds: theodolite + hydrogen pilot balloon, RAWIN (° and kt, km/h or m/s). Upper P, T, RH: radiosonde; dropsonde in cyclones. Clouds: satellites, radar.

Where do the numbers in a weather report come from?

The met enclosure

A met office's instruments, one element at a time. Tap Next.

The Stevenson screen A white, louvred box on legs keeps the thermometers in the shade with air flowing through. Inside: dry- and wet-bulb thermometers, maximum and minimum thermometers and often a hygrograph. Temperatures in °C, humidity in %.
The rain gauge A gauge in the open catches rain; the depth is measured in millimetres (or cm). Snow is melted and measured as water.
The barometer Pressure is read indoors on a mercury or aneroid barometer, or recorded by a barograph, in hectopascals (hPa, the same as millibars) or inches of mercury.
The wind mast On a mast in the open, a wind vane gives the direction the wind blows from, in degrees from true north measured clockwise, and an anemometer the speed in knots. An anemograph records both.

Memory hook: Screen: temperature, humidity. Gauge: rain (mm). Barometer: hPa. Mast: wind (° true, kt).

Takeaway: Every surface observation has its instrument: thermometers and humidity instruments in a shaded, ventilated Stevenson screen; a rain gauge in the open; a barometer indoors; a wind vane and anemometer on a mast in the open. Each element has its unit: °C, %, mm, hPa, degrees true and knots.

How is air pressure measured?

Barometers

Three instruments for one element. Tap Next.

Mercury barometer A glass tube, empty at the top, stands in a dish of mercury. The air pressing on the dish holds the column up; the higher the pressure, the taller the column. The standard 1013.25 hPa holds about 760 mm (29.92 inches).
Aneroid barometer "Aneroid" means without liquid. A sealed, partly emptied metal capsule is squeezed as pressure rises and swells as it falls; levers turn a pointer. The aircraft altimeter works the same way.
Barograph A self-recording aneroid: a pen draws the pressure on a chart round a clock-driven drum, so the forecaster can see the trend (rising, falling, the twice-daily tide). Units for all three: hectopascals (hPa), the same as millibars (mb), or inches of mercury.

Memory hook: Mercury · aneroid · barograph (records). hPa = mb; 1013.25 hPa = 760 mm = 29.92 in.

Takeaway: A mercury barometer balances the air's push on an open dish against a column of mercury (760 mm at 1013.25 hPa). An aneroid barometer uses a sealed, partly emptied capsule that squeezes as pressure rises and moves a pointer. A barograph is a self-recording aneroid that draws pressure on a turning drum. Units: hPa (= mb) or inches of mercury.