High wispy clouds
Cirrus Uncinus
Have you ever noticed clouds like those in the upper part of the picture on the right? They are often seen when a warm or occluded front is approaching, generally 12 hours or more before the front’s precipitation arrives.
The clouds are high in the troposphere, the layer of the atmosphere where precipitation and most clouds occur. They are composed of ice crystals and their height is typically 8 to 10 km. They are called cirrus uncinus (from the Latin cirrus, meaning ‘a curl or tuft of hair’, and uncinus, meaning ‘hooked’). At cloud level, the temperature is typically -40 to -50°C.
Even at temperatures as low as this, the first products of condensation are believed to be water droplets, but the droplets are supercooled and freeze very rapidly. Indeed, they freeze too quickly for clouds of water droplets to become observable. The upcurrents responsible for the cloud formation are slow (typically 5 to 10 cm/s). The fall-speeds of ice crystals are, however, greater than this (50 cm/s or more), so the crystals descend.
Supersaturated Air
Air that is saturated with respect to water is supersaturated with respect to ice. Newly-formed crystals can grow, therefore, even when descending, because they find themselves in air that is supersaturated. Once in air that is not saturated with respect to ice, however, they evaporate as they fall. They then become smaller and smaller and eventually disappear, about a kilometre below the level at which they formed initially. As they become smaller, their descent speed decreases.
When the ice crystals grow, latent heat is released. This warms the air sufficiently (about 0.5°C) for buoyant thermals to form. Thus, cloud regeneration takes place, because additional condensation, and therefore additional ice-crystal formation, occurs in these rising thermals. The small tufts of cloud that are often observed near the heads of cirrus uncinus are formed in this way.
The ice crystals are blown along by the wind, and in the layer where they exist (a kilometre or so deep) wind increases with height. Thus, the tops of the clouds blow ahead of the lower parts, so that hook-shaped streaks of cloud are created. The smallest crystals are blown almost horizontally.
In the upper troposphere, narrow currents of fast-moving air called ‘jet streams’ accompany warm and occluded fronts. They are usually many hundreds of kilometres in length and a few hundreds of kilometres in width, and they contain strong vertical and horizontal wind shears. Vertical shears are typically 5 to 10 m/s per km but can be much more. In the jet streams of middle latitudes, winds are strongest at a height of 9 or 10 km and often exceed 50 m/s, especially in winter. They blow from a westerly point. As the tops of cirrus uncinus clouds blow ahead of the lower parts, the tails of the clouds point towards the west.
Vertical Section through a Warm Front of an Active Depression
When an active warm or occluded front approaches a given point, the following sequence of clouds is usually observed: Cirrus uncinus, Cirrostratus, Altostratus, Nimbostratus. Sometimes, the sequence also includes Cirrocumulus and Altocumulus clouds. As the slope of the warm or occluded front is typically 1 in 100 to 1 in 150, Cirrus uncinus clouds normally begin to appear twelve hours or more before the precipitation arrives. The passage of a warm front is marked by a change of cloud type to Stratus or Stratocumulus. The passage of an occluded front is marked by a change from Nimbostratus to Cumulonimbus clouds.
Demonstrations and experiments of supersaturation and supercooling
A supersaturated solution contains more than the normal saturation quantity of a solute. Air is therefore supersaturated if it contains more than enough water vapour to saturate it at its current temperature. Because the atmosphere contains condensation nuclei, significant amounts of supersaturation with respect to water rarely occur. As the example overleaf suggests, however, supersaturation with respect to ice occurs quite often.
To demonstrate supersaturation, try the following:
- Clean an area of a laboratory bench;
- Place a few small crystals of sodium acetate on this area;
- Slowly drip a solution of sodium acetate over the crystals.
This should cause a column of crystals to form. The supersaturated solution may be prepared as follows:
- Place 50g of sodium acetate trihydrate in a small flask;
- Add 5ml of water and slowly warm the flask;
- Make sure the solid is completely dissolved;
- Remove the flask from the heat, cover it with aluminium foil and allow it to cool at room temperature.
Alternatively, a supersaturated solution may be seeded by addition of a small crystal. This will create a solid mass of the chemical.
To demonstrate this:
- Half fill a glass with crystals of sodium thiosulphate (Na2S2O3);
- Heat the glass in a bath of hot water until the crystals melt to form a transparent liquid;
- Filter out any impurities using a funnel and cotton;
- Cover the glass and allow the liquid to cool to room temperature;
- Shake the glass, whereupon the liquid freezes immediately
Notice that the glass feels warm.
Sodium thiosulphate freezes at a temperature of 48°C. Thus, a solution of this chemical is supercooled when at room temperature (around 20°C). The shaking of the glass, or the addition of a small crystal, triggers the freezing process. When freezing occurs, latent heat is released and the glass is therefore warmed.
When the sky is clear at night, land surfaces radiate heat to space and therefore cool.Sea and lake surfaces do not, however, cool by more than a small amount overnight (much less than 1°C). If the air in contact with a surface is cooled to its dew-point temperature, small water droplets form (condensation). If there is no wind, droplets of dew form on, for example, grass. If there is a very gentle breeze, the tiny water droplets are stirred upwards to form a shallow layer of radiation fog, as in the picture of fog at Cardiff shown. Fog can reach 30m high in some cases. This type of fog does not form over the sea because the temperature of the sea’s surface stays much the same day and night. During the day, the sun’s rays heat the ground beneath the fog. Most of the rays are actually reflected from the top of the fog but some reach the surface, otherwise it would not be daylight in the fog! The ground is gradually heated until the dew-point temperature is exceeded. The fog then dissipates (disappears), often very quickly.
This kind of fog forms when cold air flows over water that is more than 9° or 10°C warmer than the air. Over sea water, steam fog is called sea smoke.
Condensation results mainly from the cold air mixing with the air that is in contact with the water surface. However, convection also occurs, because the water is so much warmer than the overlying air. This convection causes the mixed air to rise a metre or more, thus enhancing the process of fog formation. Because of the convection, the water appears to ‘steam’. The photo shown was taken in the Canadian province of Quebec.
1. Morning sunrise- the sun’s rays heat the ground beneath the fog and the water droplets evaporate to become water vapour. This effect spreads up through the fog and the fog dissipates. This is often called ‘burning off the fog’.
2. Strong winds- these can cause turbulence within the fog and disperse the water droplets.
When fog occurs at temperatures below 0°C, its water droplets are supercooled, i.e. they exist in the liquid state at a temperature below the normal freezing point. When these droplets strike obstacles such as fences, masts or vegetation, they freeze almost instantaneously to form milky opaque deposits of ice crystals on the windward sides of the obstacles. These deposits are called rime. In the British Isles, supercooled fogs are uncommon at low levels. They occur much more frequently on the tops of hills and mountains.
Cool air tends to flow downhill. Accordingly, radiation fog is most likely to be encountered in dips, especially where there is a stream or river and therefore a source of moisture.
Convection is controlled by buoyancy relative to a thermal’s surroundings. When ascending bubbles encounter an ‘inversion’, a layer where temperature increases with height, they tend to lose their relative buoyancy and spread out sideways, as shown in the picture to the right.. As cumulus clouds grow higher, their tops become colder. Eventually, when a temperature of about -10°C is reached, the water droplets of the cloud (which are by then supercooled) begin to freeze and become ice crystals. The anvils of cumulonimbus clouds are composed predominantly of ice crystals.
Instead of tea, we heat a beaker of water. First, we put a small crystal of potassium permanganate in a beaker of water. Then (right), we heat the beaker with a bunsen burner positioned under the crystal. Currents should form in the water. Can you explain these currents? How can you demonstrate convection currents in a cup of tea?
