Outgoing Longwave Radiation

Outgoing Longwave Radiation (OLR) is an indicator of both how warm the earth's surface is and how clear the atmosphere is overhead. OLR is determined from the Earth Radiation Budget Experiment (ERBE) and Advanced Very High Resolution Radiometer (AVHRR) sensors which appear on a series of NOAA satellite. Warm surfaces radiate more in the longwave range. Low values of OLR are typically due to clouds in the atmosphere. The radiation from below the clouds gets trapped by them and it is the temperature of the cloud top heights which determine the amount of longwave radiation measured at the Top of the Atmosphere TOA.

The OLR animation shows two maps. The top map is the climatological average for that month, ie the April map is the average of all the Aprill's from 1975 to 1987 when this data was available. The lower map shows the anomalies for each particular month and year. This is the difference between the long term average and the actual data for a single month. The top map shows a regular annual cycle caused by the changing position of the sun throughout the year. In addition the deep convective cloudiness brought on by the southwest monsoon is clear in the months of May thru September. It is interesting to note that the movement of the OLR low over Africa only moves north-south, whereas the OLR low over the Indian Ocean has and east-west component causing it to move right up the Bay of Bengal.

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Click here to see an animated movie of OLR data.

Outgoing Longwave Radiation time series for thirteen years of data.

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