Meteorology · Level R3 · 3 parts · about 9 min
Clouds, humidity and dew point
3 short parts: Humidity · Stability · Clouds. Each part explains the idea, then gives the points to remember, the common mistakes, a worked case and a practice question; the Academy unit adds all 19 check questions.
1 · Humidity
Humidity and dew point
Air contains invisible water vapour. At its dew point it reaches saturation. Further cooling normally forms droplets: dew, fog or cloud.
Water vapour
Air always contains some water vapour, water as an invisible gas. Warm air can hold much more than cold air: the amount needed for saturation rises steeply with temperature. When air holds the most it can at its temperature, it is saturated.
Absolute and relative humidity
Absolute humidity is the mass of water vapour in a volume of air, in grams per cubic metre. Relative humidity (RH) compares the vapour present with the most the air could hold at that temperature, in per cent. At 100 % RH the air is saturated. Cooling unsaturated air without changing its vapour raises its RH until saturation.
Dew point
The dew point is the temperature to which air must cool, with no change in its vapour, to become saturated. Cool it below the dew point and the extra vapour condenses into droplets. This is how dew on the deck, fog and clouds form. The gap between air temperature and dew point is the dew point spread. The smaller the spread, the less cooling is needed to reach saturation.
Measuring it on board
A psychrometer has two thermometers: a dry bulb and a wet bulb wrapped in clean, wet cloth. Ventilate them as instructed. Evaporation cools the wet bulb; drier air gives a larger temperature gap. With a properly wetted, ventilated instrument, equal readings indicate saturation. Its chart gives RH and dew point. Electronic humidity sensors calculate these automatically.
Key figures
- Saturated air
- Relative humidity 100 %; air temperature = dew point
- Warm air holds more
- Much more vapour is needed for saturation at higher temperatures
- Dew point spread
- Air temperature − dew point; small spread = near saturation
- Psychrometer
- Dry and wet bulb; a bigger difference means drier air
Remember
- Warm air can hold much more water vapour than cold air.
- Relative humidity rises as unsaturated air cools towards its dew point.
- At the dew point air is saturated; cooler still, vapour condenses.
- A small dew point spread means the air is close to saturation.
- A psychrometer gives RH and dew point from dry- and wet-bulb readings.
Common mistakes
Wrong: Relative humidity tells you how much water the air contains.
Right: It is a percentage of what the air could hold at its temperature. Unsaturated air shows a higher RH as it cools towards its dew point.
Wrong: The dew point is the temperature at which water boils off the deck.
Right: It is the temperature at which air becomes saturated when cooled, so droplets start to form.
Wrong: If the wet bulb reads lower than the dry bulb, the air is saturated.
Right: With a properly wetted, ventilated instrument, equal readings mean saturation. A larger difference means drier air.
Worked case
Your air temperature is 18 °C and the dew point is 16 °C. What is the dew point spread, and what happens if the air cools to 16 °C without changing its water-vapour content?
Show the answer
The spread is 2 °C. At 16 °C the air reaches saturation: its relative humidity is 100 %. Further cooling normally causes some vapour to condense.
Practice question
What is the dew point?
- The temperature at which cooled air becomes saturated
- The temperature at which water on deck turns to ice
- The temperature of the sea at the waterline at dawn
- The temperature at which a cloud starts to evaporate
Show the answer
Answer: A. Cool air to its dew point with no change in vapour and it becomes saturated. Further cooling condenses droplets.
2 · Stability
Rising air, cooling and stability
Rising air cools. Whether it keeps rising or sinks back decides if you get flat layer cloud or towering showers. This is stability.
Rising air cools
When a small body of air, called a parcel, rises, pressure falls and it expands and cools. Unsaturated air cools about 1 °C per 100 m: the dry adiabatic lapse rate. After saturation, condensation releases heat, reducing the cooling rate: the saturated adiabatic lapse rate. It is often about 0.4–0.7 °C per 100 m in warmer lower-level air, approaching the dry rate in very cold air.
Cloud base
Rising air cools towards its dew point. Where saturation is reached, cloud can form: its lowest level is the cloud base. For heap clouds, estimate base height above the surface observation level as 125 × (temperature − dew point), using °C and giving metres. With 20 °C and a dew point of 12 °C, that is about 1,000 m above that level.
Stable and unstable
A lifted parcel that becomes colder than its surroundings tends to sink back: stable air. A warmer parcel tends to keep rising: unstable air. Air is conditionally unstable when the surrounding temperature falls with height faster than the saturated rate but slower than the dry rate. After saturation, a parcel may need more lifting before it becomes warmer than its surroundings. An inversion, where temperature rises with height, is very stable and acts as a lid.
What the sea does
A sea warmer than the air heats it from below, favouring instability near the surface. A colder sea cools the lowest air and favours stability. Cold air over warm water can develop showers if enough moisture is present and rising air is not capped. Warm, moist air over cool water may develop low cloud or fog.
Signs for a sailor
Stable air: flat layer cloud, haze or poor visibility, drizzle, a steady wind. Unstable air: heap clouds, showers, very good visibility between them, and a gusty wind with sudden shifts. Reduce sail early so you can handle the gusts.
Key figures
- Dry adiabatic lapse rate
- About 1 °C per 100 m (9.8 °C per km)
- Saturated adiabatic lapse rate
- Often 0.4–0.7 °C/100 m; nearer the dry rate in very cold air
- Standard atmosphere
- Temperature falls 0.65 °C per 100 m on average
- Cloud base (heap cloud)
- ≈ 125 × spread in °C: metres above surface observation level
- Inversion
- Temperature rises with height: very stable lid
Remember
- Rising air expands and cools: about 1 °C per 100 m until saturated.
- Cloud forms where rising air reaches its dew point.
- Stable air sinks back: layer cloud, haze, steady wind.
- Unstable air keeps rising: heap cloud, showers, gusts.
- A warmer sea favours instability near the surface; a colder sea favours stability.
Common mistakes
Wrong: Rising air cools because it gets closer to cold space.
Right: It cools because it expands as the pressure around it falls.
Wrong: Unstable air means bad visibility.
Right: Unstable air usually gives very good visibility between showers. Stable air can trap haze or moisture and reduce visibility.
Wrong: Cold air over a warm sea gives calm, settled weather.
Right: Heating from below favours instability near the surface. Showers can develop if there is enough moisture and no stable lid.
Worked case
Behind a cold front, 12 °C air with a 6 °C dew point, measured at sea level, flows over warmer water. Heap clouds are growing, but the wind eases between them. Would you judge your sail area by those quieter spells? Explain, and estimate the cloud base.
Show the answer
No. Heating from below favours instability near the surface. The growing heap clouds warn of possible showers, gusts and sudden wind shifts. The cloud base is about 125 × (12 − 6) = 750 m above sea level. Choose a sail area you can handle in the strongest gusts.
Practice question
Why does rising air cool?
- It moves away from the warm sea
- It expands as pressure falls
- It loses heat to the clouds above
- It mixes with the cold upper wind
Show the answer
Answer: B. Pressure falls with height, so a rising parcel expands. Expanding air cools.
3 · Clouds
Cloud types and what they tell you
Clouds have ten main types, called genera, grouped by height. Their shapes and changing sequence help you read the weather.
How clouds form
Clouds form when air cools to saturation, usually as it rises. Convection is rising air heated from below. Orographic lifting forces air up over high ground. Frontal lifting carries warmer air over colder air. Convergence occurs where air streams meet near the surface and force air upward, often around a low-pressure centre. Cooling from below can also produce fog or low stratus.
Names and levels
The WMO Cloud Atlas lists ten genera in three levels. In middle latitudes: high 5–13 km (cirrus, cirrocumulus, cirrostratus), middle 2–7 km (altocumulus, altostratus, often nimbostratus), low below 2 km (stratus, stratocumulus). Cumulus and cumulonimbus have low bases but can grow far upwards. The name parts help: cirro- high, alto- middle, strato- layer, cumulo- heap, nimbo- rain.
Layers or heaps
Layer clouds (stratus, altostratus) usually mean stable air, lifted gently over a wide area or cooled from below. Heap clouds (cumulus) mean unstable, rising air. Small flat cumulus on a sunny day mean fair weather. Cumulus that keep growing tall by late morning warn of showers in the afternoon.
A front coming
A classic sign: cirrus thickening into a milky cirrostratus with a halo round the sun, then a grey altostratus, then a low dark nimbostratus with steady rain. With a falling barometer, this sequence often comes ahead of a warm front, sometimes a day in advance. The Pressure systems unit follows the whole front.
Warning clouds
A cumulonimbus, a huge tower often with an anvil top, brings heavy showers, squalls (sudden strong winds), lightning and sometimes hail or waterspouts. Turreted altocumulus castellanus in the morning warn of thunderstorms later. Smooth, lens-shaped lenticular clouds over high ground mark waves in stable air and often warn of strong wind aloft. Practise recognition in the Weather lab: clouds.
Key figures
- High clouds (5–13 km)
- Cirrus, cirrocumulus, cirrostratus
- Middle clouds (2–7 km)
- Altocumulus, altostratus; nimbostratus spreads above and below
- Low clouds (below 2 km)
- Stratus, stratocumulus; cumulus and cumulonimbus have low bases
- Warm front sign
- Cirrus → cirrostratus (halo) → altostratus → nimbostratus
- Danger cloud
- Cumulonimbus: squalls, lightning, hail, waterspouts
Remember
- Clouds form when air cools to saturation, usually by rising.
- Ten genera in three levels; name parts tell height and shape.
- Layer cloud: stable air. Heap cloud: unstable air.
- Cirrus → cirrostratus halo → altostratus → nimbostratus often means a warm front.
- Cumulonimbus warns of heavy showers, sudden strong winds and lightning.
Common mistakes
Wrong: Any cirrus means rain tomorrow.
Right: Cirrus alone does not forecast rain. Watch for it thickening into a veil, a halo and lower cloud, with the barometer falling.
Wrong: Cumulus clouds always mean showers.
Right: Small flat cumulus mean fair weather. Watch for cumulus that keep growing tall.
Wrong: A halo around the sun is just a pretty effect.
Right: It shows ice crystals, often in cirrostratus, part of the cloud sequence ahead of a warm front.
Worked case
At anchor you see thin cirrus at breakfast. By noon the sky is a milky veil with a halo round the sun, and the barometer has fallen 3 hPa since morning. What may be coming, and what do you do?
Show the answer
The thickening high cloud, halo and falling pressure suggest an approaching warm front. These signs do not fix its arrival time or wind direction. Check the latest forecast for timing and wind changes, record the pressure regularly, and check that your anchorage will remain sheltered.
Practice question
Which cloud is a high cloud of ice crystals that can give a halo?
- Altostratus
- Cirrostratus
- Stratocumulus
- Nimbostratus
Show the answer
Answer: B. Cirrostratus is a high veil of ice crystals and often gives a halo. Altostratus dims the sun without a halo.