Weather - Forecasting



The major problem that faces meteorology is how to forecast the weather systems and the weather that they bring. This is always a great deal more difficult than most people would expect.

In order to forecast the weather you will need to understand how the atmosphere works, information on current weather patterns and be able to put both of these together to predict future weather patterns.

Forecasts used to be produced manually (until about the 1960's) and observations were plotted on charts. They were carefully and painstakingly analysed . Forecasters would then use a variety of mtehods to produce charts for the future (up until the next two days or so). Most predications were subjective and depended greatly upon expierence for success. At this time any forecast beyond 24 hours was next to useless!

Today the average 3 day forecast is more accurate than a one day forecast in the 1960's!

However, with modern day techniques it is possible to provide a fairly reliable forecast for days in advance and it is possible to give general predications on local weather patterns.

Numerical Methods

Numerical weather predication is the ability to forecast weather by simulating the atmosphere using a mathematical model. It has almost completely replaced earlier methods of weather forecasting and is agreed to be the only sensible method for forecasting usefully beyond 24 hours. However this massive advance would not have been possible with the sheer computing power of modern computers, completely beyond the technology available to meteorologists in the 60's!

The model uses the laws of physics and mathematics (in particular fluid mechanics) to approximate the extremely complex system of the Earth and its atmosphere. The equations form a very complex system with many uncertainties and approximations. Even if we knew every possible piece of information about the weather at one instant (which is impossible) the equations would be impossible to solve to give future weather with any certainty.

The model relies upon using the sheer computing power of modern computers to run many simulations, each with a slightly different set of initial conditions to look at how weather systems might possibly develop. These many simulations are studied as an ensemble and a sort of average is adapted to produce a reliable forecast.

It is crucial that the initial data fed in is as accurate as possible (rubbish in, rubbish out!). In order to do this weather reports of all types are collected in real time from all over the world. Set observations at specific points are normally made at 3 hourly intervals. Other sources of information from satellites and aircraft report continuously. The computer then essentially does some number crunching simulating the future patterns.

Once the computer has done its business the hard work then begins of extracting useful information from the mass of data produced. These are then broadcast and presented in a variety of ways to those who have a stake in knowing the outcome of the weather.

However the story does not stop there. Even with the massive number crunching power of modern computers, the model cannot hope to predict anything too specific on smaller scales. By necessity the 'cells' that the computer breaks the atmosphere into are quite large (the UK Met office uses a resolution of around 100 km). It requires the skills of a trained human forecaster to be able to predict how these weather systems will impact local weather.

Rules for the British Isles

Wind - The wind blows around 30° across the isobars with low pressure to the left. The wind speed is the strongest when the isobars are closely spaced (and light when they are further apart). It is reduced inland compared with windward coasts but is almost always stronger over hills and mountains.

Temperature - Northerlies are almost always cold, and southerlies warm. A long sea track normally means a westerly or south-westerly and is mild but usually cloudy in winter and cool in summer. A long land track bringing easterly of north-easterly winds is hot in summer but bitterly cold in winter. High pressure brings light winds which means cold weather in winter, and cold nights with frost in spring and autumn, but in summer produces the hottest weather especially if it is centred to the north or north-east. Temperature extremes are never associated with strong winds.

Rain - Continuous precipitation (lasting an hour or more) is most often associated with depressions and weather fronts. Normally fronts are most active where pressure is low or falling. Serious snow most often results from a warm front or occlusion pushing in slowly from the west or south against well-established cold winds. However polar lows moving south can produce large amounts in the north, and winter depressions running east through the English Channel give copious snow over southern England and Wales.

Showers - Showers are always likely in a north-westerly when pressure is low and also to the rear of cold fronts. They are always enhanced if the isobars are cyclonically curved and also when pressure is falling. Showers are inhibited when isobars are anticyclonically curved and also when the pressure is rising.

Often high surface temperatures are required to trigger their parent clouds. This means that inland they are much more frequent from late morning through the afternoon, and are generally less common in winter. Winds off the sea bring showers in by night and day to coastal area when seas are warm and the airstream cool (that usually means autumn and winter). Hilly areas trigger more showers than flat areas.

Thunderstorms - These occur when shower clouds are particularly deep and may be regarded simply as a type of heavy shower. The most widespread and most violent thunderstorms occur in summer when hot and humid low-level air is overridden by cooler air aloft ahead of fronts, usually with falling pressure.

Fog - Fog forms on radiation nights when skies clear and winds are light, which happens most often in anticyclones or ridges of high pressure. Long winter nights can produce freezing fog which the weak sun may not manage to clear during the day. Summer fog on the other hand is short-lived and often the precursor of a hot day.

Drizzle - This often occurs in the warm sector between a warm front and following cold front, but is much more frequent over coasts and hills where warm moist air is forced to ascend, especially from the south-west. Persistent drizzle, low cloud and fog often occur in cold easterly winds across North East England and Eastern Scotland when it is termed haar or fret, especially when the sea is very cold in the spring and early summer. Sometimes overnight fog, thickens enough to produce drizzle, a good indicator that the fog will be slow to clear.