Noticed a lot of people doing their own forecasting and analysing of weather patterns, so I thought I might share a bit of knowledge about how to
forecast using clouds.
The most obvious place to start, is using cirrus clouds and contrails to foresee possible weather.
Cirrus clouds can be broken into four small categories. There is cirrus, cirrocumulus, cirrostartus and contrails. Each of these clouds consist of ice
crystals, and they occur at heights greater than 20,000ft. It is possible for them to occur lower, but the environment is rarely cold enough in most
parts of the world.
Cirrus clouds are characterised by a few different things. First off they can appear as thin wispy strands, hook shapes, or filaments (see below image
for example). Secondly, they can also be seen in large twisted masses, and finally, they are also seen at the top of cumulonimbus clouds
(thunderstorms), in the shape of an anvil. Contrails are also considered in the meteorological world, as cirrus clouds, providing they have persisted
for half an hour. Cirrus we use in forecasting, is generally accompanied by cirrostratus, and is invading the sky from the west.
The second type of high cloud is cirrocumulus. Cirrocumulus clouds are characterised by small tufts or cells, occuring in sheets (called mackerel sky,
when the sheet covers the entire sky as the resemble the scales of a fish). Unfortunantly cirrocumulus is not a very handy tool in forecasting, but it
does make a spectacular picture
The third type of high level cloud, cirrostratus, is the high level cloud which is most useful for forecasting (fiorecasting using clouds that is). It
is characterised by its thin, sheet like appearance. It may appear quite thick on some days, and on other it may be almost transparent, producing the
halo and contrail shadow effects. The cirrostraus we use to forecast, will generally appear in a sheet, advancing from a westerly direction.
Since cirrus clouds are full of water, in its solid form, a large number of them is usually a good indicator that a cold front, with some wet weather
is on the way. A contrail that persists prior to any cirrus clouds being observed is also a good indicator that the upper atmosphere is moistening
rapidy. As contrails consist of ice, they also add moisture to the atmsophere, helping to fuel the cloud development.
As a cold front approaches, rapidly moving cold air is thrust into areas of receding or slow moving warm air. High level clouds like cirrus and
particularly cirrostratus form before any other clouds, as the warm moist air is thrust up, by the introduction of cold air, which in turn creates an
unstable atmosphere. As the water vapour rises, it condenses into ice crystals, and saturates the air, causing cirriform clouds to appear and invade
the sky. In some cases, cirrus and cirrostratus clouds will appear 2 days before the front is due to hit, depending on the speed it is travelling.
Occasionally cirrostratus and cirrus clouds will invade the sky as if a cold front was approaching, but the weather may not change. If that occurs,
its a good idea to check your local synoptic chart to see if there is a cold front to the north or south of your location
Hopefully this helps a little bit for those people interested in doing their own forecasting, and I am happy to answer any questions (or
challenges...i dont write things clearly sometimes) regarding this.
Happy weather watching