Cloud Patterns on SSEC Maps

The patterns of precipitation climatology showed the area along the Equator getting rainfall almost all of the time. North and south of the Equator were broad zones of little or no rainfall. These are the desert areas of the world.

That same pattern is evident in the maps of weather, as in this example of March 20, 1999. This date is almost half way between winter and summer.

Along the equator you can see a line of patchy clouds extending all the way across the map. There are small breaks in this line. Over the Indian Ocean the line broadens out to form a rather big patch that covers all of Indonesia and portions of northern Australia.

This line of clouds along the Equator is the ITCZ, the Intertropical Convergence Zone. In the ITCZ warm air from north and south converges causing the air to rise and form thunderstorms. It is estimated there are 40,000 thunderstorms around the world every day. You can see many of them along the ITCZ.

Note the location of the ITCZ relative to the continents. Over the year the ITCZ moves north and south with the Sun. We saw this in the climate maps and we can see it in these weather maps. On this map from mid-March the ITCZ is slightly south of the equator. Some of the drier areas in the southern hemisphere are covered by clouds.

Below are two samples showing the patterns of weather in summer and winter. The top image is dated 2 August 1999 at 000 UTC and the bottom image is 15 January 1999 at 000 UTC.

In the summer image the clouds of the ITCZ are generally north of the equator, covering south and southeast Asia, Africa north of the equator, and Central America. There are few clouds to be found over South America, Africa south of the equator and Australia.

In the January image the position of the clouds of the ITCZ are displaced to the south, relative to the summer image. India and southeast Asia are now essentially cloud free while northern Australia is covered with clouds. In Africa the mass of clouds is displaced to the south. In South America most of Brazil is cloud covered while Central America is getting sunshine.

There are many other differences between these two images of weather in summer and winter. Very obvious is the great difference in temperature of the lands of the Northern Hemisphere between the two images. There are also significant differences in terms of the types of clouds in the middle latitudes.

Now look at the nature of the clouds in the middle latitudes compared to those in the ITCZ, near the equator. In the ITCZ, the clouds are splotchy and of no particular shape. In the middle latitudes of both hemispheres we see clouds that are long streaks of quite uniform shades of gray. In the middle latitudes we have warm fronts and cold fronts which produce this type of cloud. The frontal type of weather does not exist near the equator.

Here are two portions of a composite map from May 13, 1999. This image is presented to show the differences in the nature of the weather systems in the middle latitudes with the weather of the ITCZ. On the left map black lines have been drawn around cloud systems in the middle latitudes. The same map on the right has not been annotated.

Common to the middle latitudes are large storm systems consisting of warm and cold fronts. These storms appear on the maps as lines of clouds. Sometimes there are waves in the lines. Sometimes the lines of clouds curve into spirals. Look at the lines of clouds annotated on the map on the left.

The storm systems in the southern hemisphere are better developed because there is little land to break up the storms.

The text has some good satellite images of the linear cloud patterns of the middle latitudes on page 153, 229, 310, 316, 324, 337 and 377. Of course these are better than average examples which is why they were chosen for inclusion in the text.

Along the ITCZ the clouds are patchy compared to the linear clouds of the middle latitudes. The weather of the ITCZ consists largely of clusters of thunderstorms. These show up on the satellite imagery as patches of white and gray, with white being the higher cloud tops.