The tools of forecasting

  • Figure 1: Image of from "Will it rain today?" illustrating how radar imagery can be viewed.

    Figure 2: Visible satellite image illustrating how fog and low cloud can be seen.

    Figure 3: Infrared satellite image illustrating how fog and low cloud cannot be seen.

    Figure 4: Map illustrating observed temperatures over the UK

  • The tools of forecasting
    24.03.2012 13:27

    Forecasting the weather is essentially analysing the state of the atmosphere. Within the atmosphere different variables have to be considered, such as temperature, cloud cover, precipitation, sunshine and wind. Forecasters use different tools to predict these conditions so as to determine the next day’s forecast as well as forecasts further into the future. The main tools used are radar imagery, satellite imagery, weather observations and knowledge of past events. These are all inputted into computer models which then analyse and predict the most likely expected weather scenario. 

    Radar imagery is used to identify where precipitation is falling. Radars send out pulses of radiation which then reflect off any precipitation, identifying whether precipitation is falling and if so how heavy it is. Figure 1 illustrates how radar imagery can be viewed when forecasting for the UK. Within MeteoGroup a complex computer algorithm is used to then determine the different types of precipitation that is falling . As described on the premium account “Will it rain today?” rain is represented by blue, snow by red, sleet by grey, hail by yellow and freezing rain by pink. The band of blue over the south-east on figure 1 illustrates a band of rain on a cold front. It is heavy in places as illustrated by the darker blue. The movement of precipitation on the animated radar can give short-term indications of where weather features are heading.

    Satellite imagery is used to determine the type and amount of cloud cover. There are two main types of satellite imagery, infrared and visible. Infrared images can be used 24/7 and as such are good for looking at where cloud cover is over night. In addition to this they identify the height of clouds; the brighter the cloud, the higher it is. However, this has its disadvantages as well, because low cloud does not appear clearly and therefore is hard to identify. This is where visible images become very helpful. Although visible imagery only works during daylight hours, areas of fog, sea mist and low cloud can be identified very clearly. This is illustrated by figures 2 and 3, which show fog over parts of central England on the visible image and clear skies on the infrared image.

    Observations are taken from different weather stations around the world, usually on an hourly or 12 hourly basis. Observations can be used to predict the short term temperatures, wind speeds and precipitation types. For example figure 4 illustrates the observed temperatures over the UK at 11am on the 24th March 2012. From these, afternoon and evening temperatures can be forecasted based on the known weather conditions. In this example due to clear skies, it can be forecast that high afternoon temperatures will drop rapidly in the evening as there is little or no cloud cover which would otherwise prevent heat loss. In addition to this observations can be used to forecast long term weather conditions. When forecasting for a longer period of time the initial conditions are essential. As described by chaos theory, when we start forecasting with a finite amount of information, accuracy will gradually decrease as time increases. For this reason accurate initial conditions are essential and the more observations we have the more accurate forecasts will be.

    Finally, knowledge of the area you are forecasting for is important, such as know which different air masses will affect the area, as well as topography, soil type and distance from the coast. A good example of this is Chesham, Buckinghamshire which is a frost hollow. Due to the low parts of the valley and chalky soil, temperatures usually drop several degrees below surrounding areas.

    By: Sally-Jean Webb