Weather Stations, Instruments, and Weather Maps

Weather observation is the systematic measurement of atmospheric conditions at weather stations using instruments such as thermometers, barometers, rain gauges, anemometers and wind vanes, and weather maps use symbols and lines to show the results across a region. In brief: each instrument measures one element: a thermometer measures temperature, a barometer air pressure, a rain gauge precipitation, an anemometer wind speed, a wind vane wind direction. Readings are taken at standard heights and times in a standard enclosure, collected by national services and plotted on synoptic charts, where lines called isobars join places of equal pressure.

Reliable forecasts depend on accurate and comparable observations. This guide describes weather stations, the main instruments and how they are read, how weather maps and symbols work, compares the instruments in a table, works through unit conversions and a rain gauge calculation, outlines the history of weather observation and corrects common mistakes.

What is a weather station?

A weather station is a site with instruments for measuring the elements of weather. Observations must be made in a standard way so that places can be compared: the thermometers are kept in a white louvered box called a Stevenson screen, about 1.25–2 m above short grass, which shields them from direct sunshine and rain while letting air flow through; the rain gauge is placed in an open place away from trees and buildings, with its rim at a set height; and the anemometer is mounted on a mast at 10 m above the ground. Automatic stations record data continuously and send them by radio or satellite. Observations come from land stations, ships, buoys, aircraft, weather balloons (radiosondes) that measure the upper air, radar (for rain) and satellites.

What do the instruments measure?

• Thermometer: temperature, usually in °C. A maximum–minimum thermometer records the highest and lowest temperatures in a period.
• Barometer: air pressure. A mercury barometer balances the pressure against a column of mercury (about 760 mm at standard pressure); an aneroid barometer uses a sealed metal capsule that changes shape. Units: hPa (millibars); average sea-level pressure is 1,013 hPa.
• Rain gauge: a funnel leading to a collecting vessel with a scale; the amount is given in millimeters of depth.
• Anemometer: wind speed, most often with rotating cups; the speed is given in km/h, m/s or knots (1 knot = 1.852 km/h). The Beaufort scale describes wind effects from force 0 (calm) to force 12 (hurricane).
• Wind vane: wind direction, shown as the direction from which the wind blows (a south-westerly wind blows from the southwest).
• Hygrometer or psychrometer (wet and dry bulb thermometers): humidity. The difference between the wet-bulb and dry-bulb temperatures is used to find relative humidity.
• Sunshine recorder and cloud estimates: hours of sunshine and the cloud cover in oktas (eighths of the sky).

How are weather maps and symbols read?

A weather (synoptic) map shows the weather over an area at a given time. Isobars are lines joining points of equal sea-level pressure, drawn at intervals of 4 hPa, and they show the high (H) and low (L) pressure systems: close isobars mean strong winds. Fronts are drawn as lines with triangles (cold fronts), semicircles (warm fronts) or both (occluded fronts). Station models and symbols show temperature, wind (a line pointing toward the direction the wind comes from, with barbs for speed), cloud cover (a circle filled in proportion), and present weather (a dot for rain, a star for snow, a zigzag arrow for thunderstorms). Maps of forecasts for the public use simple pictures: a sun, cloud or raindrop, with temperatures. Satellite images and radar maps are other tools.

Instruments compared

Worked example: readings and conversions

1. Max–min thermometer: the maximum is 24.5 °C and the minimum 11.0 °C. The daily range is 24.5 − 11.0 = 13.5 °C, and the simple mean is (24.5 + 11.0) ÷ 2 = 17.75 °C, or 17.8 °C.
2. Temperature scales: °C = (°F − 32) × 5/9, so 68 °F is (68 − 32) × 5/9 = 36 × 5/9 = 20 °C.
3. Wind speed: 20 knots × 1.852 = 37.0 km/h. Also 36 km/h ÷ 3.6 = 10 m/s.
4. Pressure: a barometer shows 29.92 inches of mercury, which is 29.92 × 33.864 = 1,013 hPa, the standard sea-level pressure.
5. Rain gauge: a funnel has a diameter of 10 cm, so its area is π × 5² = 78.5 cm². A volume of 15.7 mL (15.7 cm³) collected corresponds to a depth of 15.7 ÷ 78.5 = 0.20 cm = 2.0 mm of rain.
6. Wind direction: a vane pointing to the northeast indicates a northeasterly wind, which comes from the northeast and blows toward the southwest.

How did weather observation develop?

The Italian scientist Evangelista Torricelli invented the mercury barometer in 1643, and Gabriel Fahrenheit and Anders Celsius developed temperature scales in the 18th century. In the 19th century the telegraph allowed the collection of observations in a central office, and the first weather maps and warning services were established, with Robert FitzRoy in Britain setting up forecasts in the 1860s. Radiosondes were introduced in the 1930s, weather radar after the Second World War, and the first weather satellite, TIROS-1, in 1960. Today, a global network of stations, buoys, aircraft and satellites feeds computer forecast models.

Which mistakes are common?

Mistake 1: Putting a thermometer in direct sunlight

It would read the heating of the instrument, not the air. Use a shaded, ventilated screen.

Mistake 2: Confusing the direction of the wind

The wind direction is the direction from which it blows. A northerly wind blows from the north.

Mistake 3: Placing a rain gauge under trees or next to buildings

This leads to under-measurement. The gauge needs open ground.

Mistake 4: Mixing units

Knots, km/h and m/s are different, and hPa and millibars are equal, but inches of mercury are not. Convert before comparing.