The met instruments usually carried aboard merchant vessels and warships are the following:

Barometer. This instrument measures the atmospheric pressure (usually in millibars). They can either be mercury barometers or aneroid barometers. The first type uses a column of mercury to measure to measure the pressure, whilst the aneroid uses a metal diaphragm.

A. Mercury barometer. B. Electric aneroid barometer. C. Aneroid barometer.

Thermometer. This is used to measure the ambient air temperature as well as the temperature of the sea water.

Rubber bucket with thermometer inside for taking sea temperatures.
A. Ambient air temperature.
B. Sea water temperature.

Hygrometer. The hygrometer consists of a “wet” and a “dry” bulb thermometer, mounted in a louvered container called a Stevenson screen. It is used to measure the humidity of the air, ie the water carrying capacity of the air.

Hygrometer in a Stevenson Screen.

Anemometer & wind vane. These two instruments can either be mounted as two separate instruments or is a single combined unit. The anemometer measures wind speed whilst the wind vane measures the direction of the wind.

Anemometer & Wind vane.

Barograph. A barograph is used to measure and to record the air pressure over a period of time, ie a week.

Barograph.

The seafarer also makes use of weather maps which he receives in the form of a facsimile via radio from ashore. A weather map can also be constructed from weather data collected by the met offices ashore and gives a graphic picture of the weather in an area. It shows isobars (lines of equal pressure), the presence of fronts and wind.