Professional Pilot, October 2016
Average position of the ITCZ in January blue and July red The ITCZ is arguably the most dominant weather maker of the tropics and something all pilots should be familiar with is more radiation coming in from the sun than is leaving from the earth and its atmosphere The radiation surplus in the tropics combines with the radiation defi cit in the polar regions to create a heat pump that circulates the atmosphere between the 2 But it is not quite that simple If the earth didnt spin air would heat and rise in the tropics fl ow along the stratospheric boundary to the poles where it would cool descend and fl ow back to the tropics as a cool north surface breeze However because the earth spins the fl ow curves from its path and forms a transition zone between 235 and 665 that turns the single cell fl ow into a 3 cell system At the boundary of the tropics strong high pressures form and the air descends Some of the air fl ows 34 PROFESSIONAL PILOT October 2016 out at the surface to return towards the equator while the rest of it fl ows poleward This region of descending air is the subtropics and those strong highs ensure that this is also where most of the worlds hot and dry deserts are found This smaller circulation moving air between the equator and the subtropics is known as the Hadley Cell after the 18th century scientist who discovered it as the cause of the trade winds that powered sailing vessels across the oceans at the time Intertropical Convergence Zone At the other end of the Hadley Cell circulation the region where the sun is directly overhead at noon is where the winds returning from each hemispheres subtropics converge to be reheated fi lled with moisture and once again rise through the troposphere This zone of converging surface air is called the ITCZ and this is where a majority of the worlds thunderstorms and lightning can be found Because of the surplus of available heat energy the warmed troposphere is able to expand upward to around 60000 ft MSL placing the storm free stratosphere out of reach for just about all but a few very high fl ying aircraft Because the ITCZ relies on the most intense direct sunlight it migrates north and south across the equator with the seasons When the sun is over the northern hemisphere from March to September the ITCZ resides between about 5 and 15 North Conversely from September to March the ITCZ moves toward Global map of thunderstorm occurrence Most of the worlds thunderstorms occur in the tropics specifi cally in the region around the equator where the trade winds converge and over land where daytime heating can enhance dynamics and instability
You must have JavaScript enabled to view digital editions.
