Cloud Cover


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Note 1 - The data shown in these graphs was extracted from ISCCP D2 dataset which contains the monthly average cloud cover for grid-cells of equal area. Latitude is in increments of 2.5 degrees but longitude varies to correspond to the reducing distance between lines of longitude as we move towards the poles. The latitude and longitude of the centre of each grid cell are determined according to ISCCP instructions (and using its software) and these tested against the boundaries to the regions shown on these web pages.

Note 2 - This data might not be accurate.
See caution on this page.

Note 3 - The terms low, mid and upper are in accordance with the descriptions from the ISCCP but can be summarised as follows:
Low = dense cold typically grey cloud,
Mid = dense but white,
Upper = thin cloud which is often semi-transparent.

In general low cloud cools the Earth's surface beneath it, the effect of mid level cloud varies and upper level cloud tends to warm (because it allows some solar radiation through but blocks some outgoing radiation as the Earth cools). However, in high latitudes low winter cloud can have a warming effect because it prevents the Earth from cooling. We usually find the same effect at night in all latitudes.

Note 4 - The y-axis on all graphs is the percentage of sky covered by the specified cloud type. It should not be assumed that the sum of the low, mid and upper level cloud cover is the same as the total cloud cover. It is not at all unusual to find cloud at several different levels simultaneously and the Total cloud provided by the ISCCP is determined by a different method to that for determining cloud at each level.

Note 5 - All graphs of monthly values have the date indicated against January of that year. All graphs of 12 month running averages have the year indicated against December of that year, which are in fact the annual averages across the year. On some pages the Total cloud cover is shown per month and as a running 12-month average and these graphs start from January 1984, which is why the line for the 12-month average begins 11 months after the start.