Why is forecasting cloud cover so difficult?

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  • Figure 1: A visual satellite image at 10am illustrating thick cloud cover.

    Figure 2: A visual satellite image at 3pm illustrating pools of clear sky developing.

    Figure 3: A visual satellite image illustrating how temperatures vary depending on cloud cover.

    Figure 4: Image to illustrate different cloud levels

  • Why is forecasting cloud cover so difficult?
    19.03.2012 16:39

    Clouds can be defined as a visible mass of liquid water and ice particles suspended above the Earth’s surface. They influence the Earth’s temperature in two different radiative ways: the solar albedo and the thermal greenhouse effect. Clouds have a high albedo and therefore reflect the suns radiation causing a cooling effect. However, they also work like a barrier and can trap heat close to the surface. For this reason different amounts of cloud can lead to different temperatures. 

    Over the past week the UK has been affected by a high pressure system positioned over central Europe. This has lead to higher temperatures as light winds push warm air from France northwardly across the UK. However, it has also lead to lots of uncertainty in cloud cover. 

    A high pressure system over the UK suggests clear skies but if cloud becomes trapped it can remain for a number of days. On Tuesday last week sunny spells and patchy cloud were forecast but in this sort of situation models can struggle to pinpoint breaks in the cloud. In some areas the sun did not burn through some fairly thick cloud cover at all readily. 

    Figure 1 is a visual satellite images illustrating the cloud cover at 10am and figure 2 at 3pm; it is clear that the cloud is starting to break as pools of clear sky develop. However, the incoming solar radiation was not strong enough to allow the sun to burn through across the whole UK. Figure 3 illustrates how the presence of cloud can vary the temperature.

    Clouds develop at three main levels, as illustrated in figure 4. High level clouds such as cirrus form above a height of about 6km and often cause the sun to look hazy. Medium level clouds such as altostratus and altocumulus occur between 4km and 6km and are often the cause of very grey skies, particularly in the presence of the former. 

    Finally, low level clouds form below 4km – stratus, stratocumulus, cumulus, cumulonimbus and nimbostratus, for example. Forecast models sometimes have difficulty in distinguishing these clouds but forecasters are skilled at interpreting satellite images. Even so, slow moving banks of low cloud and fog are notoriously difficult to pinpoint.

    Cloud modelling is made hard by factors such as varying size of droplets, the height of the cloud and the scale at which the model is predicting. As climate change becomes such a key topic there is a push from organisations such as the Intergovernmental Panel of Climate Change (IPCC) to improve the accuracy of cloud modelling. 

    Global climate models (GCMs) are now able to model in both two and three dimensions. This helps to understand how clouds may differ over time and at different levels of the atmosphere. However, models are constructed on a grid format at differing resolutions, and as those resolutions change so must the calculations and statistics that drive the forecast. Understanding the affects on clouds in a 1km grid square is not the same as a 5km gird square.

    As high pressure begins to influence us again this week, it will remain largely dry and mild with sunny spells. However, fog and low cloud may again be troublesome. 

    By: Sally-Jean Webb