High wispy clouds
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.
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.