There is great variation between the pressure distribution pattern aloft and at sea-level. The cellular pattern of pressure distribution seen at the earth’s surface disappears with increasing elevation.
In the middle of the troposphere at 700 millibar level the pressure-cells grow much weaker. In the northern hemisphere in January the pressure is characterized by a gradual decrease from the equator pole-ward at an elevation of about 3000 meters.
It has also been observed that at this level the pressure gradient is maximum in the middle-latitude zones.
Besides, at an elevation of about 3000 meters the pressure distribution pattern in the month of July, the middle of summer, is altogether different from that of January, and the pressure gradient is relatively less steep than that in January when all the pressure belts migrate towards the north.
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In July, there is an extensive area of high pressure in the low latitudes at the level referred to above. However, between latitudes 20° and 30°N and S there is a slight increase in air pressure at the same level.