Weather Instruments and Observations

Aneroid barometers on board

In short: every ship measures pressure with an aneroid barometer. There are three kinds, each with its own job.

Read first: Pressure, Wind and the Coriolis Effect (topic 1), Humidity, Dew Point and Fog (topic 2) and Depressions, Fronts and Synoptic Charts (topic 3). This is topic 4 of Meteorology. Next: Weather Forecasts and Information at Sea.

• Aneroid = “without liquid”. Inside is a sealed metal capsule (cell) with most of the air pumped out. When pressure rises, the capsule is squeezed; when it falls, the capsule swells. Levers turn that small movement into a needle on a dial.
• Units: millibars (mb). 1 mb = 1 hPa (hectopascal).
• The barometer is part of the bridge’s basic weather equipment on every vessel.

The three types

• On the bridge: read the value from the barometer (or precision aneroid) and the trend from the barograph.

Barometer errors and corrections

In short: a raw reading is not the sea-level pressure. Correct it for the instrument’s own error, for temperature and for height before you use or report it.

• Instrument (index) error: every barometer reads a little high or low. Find the error by comparing it with a known correct reading, for example from the harbour master in port. Note it, and apply it to every reading.
• Temperature: heat changes the metal parts and the reading. Correct for it: all three types are corrected for temperature and height (Marine Observer’s Handbook).
• Height (reduction to mean sea level): pressure falls about 1 mb for every 8 m you climb. The barometer on the bridge is above the sea, so it reads low. Add the height correction.
• The full correction table (height and temperature) is in the Marine Observer’s Handbook.
• Why it matters: weather charts, forecasts and other ships all use mean sea level pressure. An uncorrected reading cannot be compared with them.

Worked example

1. Barometer reading on the bridge: 1008 mb.
2. Instrument error from the last check: reads 1 mb low → +1 mb → 1009 mb.
3. Height of the barometer: 16 m → 16 ÷ 8 = +2 mb → 1011 mb at mean sea level.
4. For a precise figure, take height and temperature corrections from the table in the Marine Observer’s Handbook.

Barograph and pressure tendency

In short: one reading tells you where you are. The tendency (the change over 3 hours) tells you where the weather is going.

• Pressure tendency = the change in pressure over the previous 3 hours: rising, falling or steady, and by how many mb.
• The barograph draws the tendency for you. One glance at the trace shows a steady fall, a sudden drop or a rise after a front.
• Barograph care:
  • Use oil-based ink in the pen.
  • Use chart paper matched to the drum speed, so the time scale is right.
• The barograph is less accurate than the barometer: use the two together. Barometer for the value, barograph for the trend.
• Example: 1012 mb at 0900, 1006 mb at 1200 → falling 6 mb in 3 h. A fall of more than 2 mb in 3 h is already significant, and this one is much steeper (see Pressure, Wind and the Coriolis Effect). Tell the Master.

The Stevenson (marine) screen

In short: a white, louvred box that shades the thermometers so they read the true air temperature.

• White to reflect the sun. Louvred sides let air flow through while keeping off direct sun, rain and spray.
• It holds the dry bulb and wet bulb thermometers: the hygrometer.
• Where: hung on the windward side, usually on the bridge wing, so the air reaching it comes straight off the sea.
• Why windward: on the lee side the air has passed over the warm hull, funnel and accommodation. It reads too warm and too dry.
• On the bridge: when the ship alters course and the wind changes side, move the screen (or use the one on the other wing).

The wet and dry bulb hygrometer

In short: two thermometers side by side. The gap between them tells you how much water vapour the air holds.

• Also called a psychrometer.
• Dry bulb: an ordinary thermometer. It gives the air temperature.
• Wet bulb: its bulb is covered by a muslin wick that dips into a small reservoir of distilled water. Water evaporates from the wick and cools the bulb.
  • Use distilled water only: salt or tap water leaves deposits on the muslin and spoils the reading. Keep the muslin clean.
• The dry bulb reads higher than the wet bulb (or the same, in saturated air).
• More water vapour in the air → less evaporation → the wet bulb reads higher, closer to the dry bulb. So the wet bulb shows the moisture in the air.
• Bigger difference between the two → drier air.
• Depression (wet-bulb depression) = dry bulb minus wet bulb.
• Enter the table in NP100 or the Marine Observer’s Handbook with the dry bulb and the depression → dew point and relative humidity. How to use them for fog is in Humidity, Dew Point and Fog.
• Readings close together = air near saturation = fog likely, especially over a cold sea.

Sea temperature

In short: the sea temperature is measured with a sea bucket. Compared with the dew point, it tells you if sea fog is coming.

• Sea bucket: lower the bucket over the side, haul up a surface sample and read the thermometer in it straight away, out of the sun and wind, before the water warms or cools.
• Take the sample well clear of the ship’s own discharges.
• Use: sea temperature at or below the dew point → sea fog likely (NP100). See Humidity, Dew Point and Fog.

The anemometer: apparent and true wind

In short: the anemometer measures the wind across the moving ship. To know the real wind you must take the ship’s own motion out.

• The anemometer (cups or propeller, with a wind vane) sits high on the mast. It gives wind speed and direction.
• It measures the apparent (relative) wind: the true wind combined with the wind made by the ship’s own course and speed.
• True wind = apparent wind with the ship’s motion taken out. Work it out with a vector triangle (ship’s course and speed + apparent wind), or read it from an instrument that takes heading and speed inputs.
• Check: at anchor or stopped, apparent wind = true wind.
• Back-up: estimate the true wind from the sea state with the Beaufort table (NP100). See Pressure, Wind and the Coriolis Effect.
• Sailing yachts also carry an inclinometer (angle of heel) next to the anemometer.

Recording and reporting observations

In short: your readings go in the log, feed your own forecast and, when the weather is dangerous, must be reported to other ships and the authorities.

• Log book: record corrected pressure, tendency, air and sea temperature, wet and dry bulb, wind and weather at the times set by the Master’s standing orders.
• Danger messages (SOLAS V/31–32): a ship that meets a tropical storm, or winds of Beaufort Force 10 or more with no warning received, must report it. The report includes:
  • the barometric pressure, corrected if practicable, the unit, and whether it is corrected or uncorrected;
  • the tendency: the change over the past 3 hours;
  • true wind direction and Beaufort force, sea state, swell, ship’s position, date and time.
  • Example: “BAROMETER CORRECTED 994 MILLIBARS, TENDENCY DOWN 6 MILLIBARS”.
• Voluntary Observing Ships (VOS): under SOLAS V/5, governments encourage the use of accurate meteorological instruments and ship weather reports. The MCA strongly encourages UK companies to volunteer their ships (MGN 610 (M+F), section 8). VOS crews report every 3–6 hours at sea, sent by satellite (Met Office).