The cloud hole, with a diameter that stretched as far as 1,000 kilometers (620 miles) across, was caused by sinking air associated with an area of high pressure near the surface. Globally, the average sea-level pressure
is about 1013 millibars; at the center of this high, pressures topped 1,040 millibars.
Sea-level pressure maps published by the Australian Bureau of Meteorology on June 5 showed that the shape of the cloud hole matched the shape of the high-pressure area. However, the center of high pressure and the cloud hole didn't match precisely; the center of the high was near the western edge of the clear area, about 100 kilometers from the cloud edge.
In general, winds blow outward and away from areas of high pressure. As a result, areas of high pressure pull air downward. As the air sinks, it also warms, increasing the rate of evaporation and making it difficult for the air to sustain clouds. Areas of low pressure, by contrast, pull air upward and generate clouds and stormy weather.
While low-pressure systems often produce circular cyclonic storms and clouds, high-pressure systems (which are sometimes called anticyclones) can yield large circular areas of clear skies. “You could call it an anti-storm,” quipped NASA Langley atmospheric scientist Patrick Minnis.
NASA image courtesy Jeff Schmaltz, LANCE MODIS Rapid Response. Caption by Adam Voiland with information from Patrick Minnis, NASA Langley; Joanna Joiner, NASA Goddard; Steve Lang; NASA Goddard; and Heather Hyre, NASA Goddard.