Humidity, Dew Point and Fog

Humidity and relative humidity

In short: air always carries some invisible water vapour. Warm air can hold more of it than cold air.

Read first: Pressure, Wind and the Coriolis Effect (topic 1 of Meteorology). This is topic 2. Next: Depressions, Fronts and Synoptic Charts. The wet and dry bulb hygrometer itself is covered in Weather Instruments and Observations.

• Humidity = the amount of water vapour in the air. You cannot see vapour; you only see it once it condenses into droplets.
• How much vapour air can hold depends on its temperature: the warmer the air, the more it can hold.
• Relative humidity (RH) = the vapour the air actually holds, as a % of the most it could hold at that temperature.
• 100% RH = the air is saturated. It cannot take any more vapour.
• Key idea: you do not need to add water to saturate air. Cooling it is enough: the same vapour in colder air gives a higher RH, until it reaches 100%.

The dew point

In short: the dew point is the temperature at which the air becomes saturated. Cool the air below it and water appears.

• Dew point = the temperature to which air must be cooled, with no change in its vapour, to become saturated.
• Cooled below the dew point, the extra vapour condenses into droplets: dew on the deck, cloud aloft, or fog at the surface.
• The closer the air temperature is to the dew point, the moister the air and the less cooling it takes to form fog.
• Exam wording: say “the temperature at which air becomes saturated”. Do not say “the point at which air becomes water”.

Finding the dew point: wet and dry bulb

• The dry bulb is an ordinary thermometer: it gives the air temperature.
• The wet bulb is kept wet. Water evaporates from it and cools it, so it reads lower. The drier the air, the more evaporation and the bigger the gap.
• Wet-bulb depression = dry-bulb reading minus wet-bulb reading.
• Enter the hygrometric table with the dry-bulb temperature and the depression. It gives the dew point (and the relative humidity). The table is in NP100 (The Mariner’s Handbook) or the Marine Observer’s Handbook.
• Small depression = air close to saturation. Zero depression = saturated air.
• Example: dry bulb 15 °C, wet bulb 13 °C → depression 2 °C → dew point about 11.5 °C (read the exact figure from the table).

What fog is

In short: fog is a cloud sitting on the surface. By definition it cuts visibility to less than 1000 m.

• Fog = tiny water droplets suspended in the air at the surface, reducing visibility to less than 1000 m (Met Office).
• If visibility is 1000 m or more, it is mist (droplets) or haze (dry particles), not fog.
• Fog forms when air near the surface is cooled to its dew point. At sea and on the coast, the two types to know are advection (sea) fog and radiation (land) fog.
• On the bridge: fog means restricted visibility under COLREG. That changes your duties: see the last section below.

Advection (sea) fog

In short: warm, moist air drifts over a colder sea, cools to its dew point and turns to fog. This is the fog that matters most at sea.

• How it forms: warm, moist air moves (is “advected”) over a colder surface, usually a cold sea. The sea cools the air from below until it reaches its dew point.
• It is the most dangerous fog for shipping: it forms over open water and can cover a wide area.
• It can form with a steady wind: the wind is what carries the warm air over the cold water.
• When: mostly in spring, when the air is warming but the sea is still cold after winter. In the UK it is frequent in March.
• How long: it is persistent and can last a long time. The sun does not clear it, because the sea temperature hardly changes between night and day. It usually clears only when the wind or the air mass changes.
• On the bridge: warm, humid air blowing towards colder water (a cold current, or a sea still cold in spring) is the warning sign. Check the sea temperature against the dew point.

Radiation (land) fog

In short: on a clear, quiet night the ground cools, chills the air above it to the dew point, and fog forms over land.

• How it forms: at night, under high pressure (clear skies, light winds), the ground loses heat. The air touching the ground cools with it. If it falls to its dew point, radiation fog forms.
• When: typical in winter, when nights are long.
• Where: over land, often in valleys and near rivers such as the Thames. It can drift out over rivers, estuaries and harbours.
• How long: less persistent than sea fog. It normally clears with the sun, as the ground warms during the morning.
• On the bridge: expect it when entering or leaving a river port at night or early morning in winter, under a High.

Advection fog vs radiation fog

Predicting fog at sea

In short: find the dew point and compare it with the sea temperature. Sea at or below the dew point = sea fog likely.

The sea temperature test

1. Read the dry and wet bulb and work out the wet-bulb depression.
2. Get the dew point from the table in NP100 or the Marine Observer’s Handbook.
3. Take the sea temperature.
4. If the sea temperature is equal to or lower than the dew point, the air in contact with the sea will cool to saturation: sea fog is likely (NP100).

• Example: dew point about 11.5 °C, sea temperature 11 °C → the sea is below the dew point → expect sea fog.
• Other sign: dry and wet bulb readings close together mean the air is nearly saturated, so fog is likely.
• Repeat the readings regularly: a shrinking gap between dew point and sea temperature is your early warning.

On the bridge when fog is likely

• Call the Master as the standing orders require, and tell the engine room: engines ready for immediate manoeuvre (COLREG Rule 19(b)).
• Proceed at a safe speed for the visibility (COLREG Rule 6).
• Keep a proper lookout by sight, hearing and radar (COLREG Rule 5). Use radar plotting or ARPA for every target (COLREG Rule 7).
• Start fog signals: a power-driven vessel making way sounds one prolonged blast at intervals of not more than 2 min (COLREG Rule 35).
• Switch on the navigation lights (COLREG Rule 20) and fix the position often.