Weather Prospects for the Transit

The rarity of this event demands that special attention be given to the long-range weather prospects. Cloud cover maps show that the most promising skies for transit observations occur in two parallel low latitude bands straddling the equator. South of the equator, this band of minimal cloudiness runs from Brazil to southern Africa and Australia. In the northern hemisphere, the band runs across the southern United States, northern Africa and the Middle East, being interrupted across India and Southeast Asia by the onset of the southeast monsoon. The two regions are separated by the high cloudiness associated with the Intertropical Convergence Zone (ICZ) that runs approximately along the equator.



The low cloud zones are associated with the semi-permanent high pressure anticyclones found in June over the oceans at 35°N and at 25°S. These sub-tropical anticyclones are formed by the large scale Hadley circulation in the atmosphere that exchanges heat and humidity in equatorial regions with colder and drier air near the poles. Descending air in the center of these anticyclones dries the atmosphere through adiabatic warming and it is no accident that the world's great deserts are also found at these latitudes - among them, the Sahara, Kalahari, Atacama, and Arabian Deserts.

Away from the tropics, cloud cover increases rapidly with latitude, though the pattern is greatly modified by the presence of mountain chains that interrupt the flow of weather systems and the transport of moisture from oceans onto land. In North America, the Rocky Mountains block the flow from the Pacific into the continent and permit a zone of low cloudiness that stretches northward from Texas to the Canadian border. In northern Europe, Atlantic moisture flows unrestricted onto the land until being blocked by the Alps and Pyrenees, keeping Germany, France and England under high levels of cloudiness, while preserving the sunnier climates of Italy and Greece. Similar effects can be seen in southern Asia around Kazakhstan and Mongolia, although the favorable cloud patterns there are due as much to the absence of nearby moisture sources as the presence of mountains.

For those who are determined to see the transit in its entirety, the most favorable climatology lies in regions surrounding the Mediterranean, the Middle East, and portions of southern Africa. The Azores High, the closest anticyclone, extends a tentacle of high pressure down the length of the Mediterranean, suppressing cloud-bearing weather systems and helping to extend the reach of the Sahara's dry influence into Italy, Greece and the coasts of Turkey and Spain. Sunshine prospects are even better on the African coast. Clear skies are virtually guaranteed, but high temperatures may make observation of the transit a test of endurance at inland sites.

North America's deserts lie outside the transit's view, but the western plains from Oklahoma northward, offer the best chance for a tantalizing glimpse of the end of the transit at sunrise. The Appalachians and Florida are best avoided unless the forecast of the day calls for sunshine. The entire transit can be seen in the far north, where the sun maintains a continuous 24-hour presence, but cloud prospects border on awful. The global cloud map shows a minimum in cloudiness through parts of Alaska and Canada's Yukon Territory where mountains block some of the flow from the Pacific, but reaching the area will require an expedition down the Alaska Highway, or a flight to one of several high Arctic communities.

Meteorological statistics for the frequency of various amounts of cloud cover for a number of cities in the U. S., Canada, Europe, the Mediterranean and the Middle East can be found in the following tables.