Warm circulation

  • Global circulation, including the Hadley cell. Image courtesy of NASA.

    The Intertropical Convergence Zone marked by convective clouds and heavy showers/thunderstorms on 20 August 2010. Image courtesy of NOAA.

    Forecast pressure pattern for midday on Sunday, showing the Azores High displaced towards the British Isles. Image copyright MeteoGroup.

    Forecast for midday Monday of winds at the 500 hectopascal (hPa) level (an altitude of about 5,500m). The strongest are in red and lightest in black, so this shows the jet stream looping north of the British Isles with an upper high centred over southern England. The high at the surface will be similarly positioned. Image copyright Meteogroup.

  • Warm circulation
    08.03.2012 15:51


    A very mild weekend is coming up and, given that winds will be light and that potential daytime warming is becoming greater as the northern hemisphere gradually tilts towards the sun, it could feel distinctly warm in any sunshine.

    That is not to say that it won’t still feel chilly overnight, but that sort of situation is not unusual in early spring when the ground is still a bit cold and chills the air just above it on still nights.

    The vernal equinox is fast approaching, and after March 20th the days will be longer than the nights; and as the apparent position of the sun shifts northwards the Earth’s atmosphere tends to follow.

    The average position of the polar jet stream creeps northwards, for example, and the Intertropical Convergence Zone (ITCZ), the region known by mariners as “The Doldrums”, edges northwards from the equator towards the Tropic of Cancer.

    The ITCZ marks the southern boundary on of one of three distinct “cells” that divide the atmosphere in both the northern and southern hemispheres.

    This one, the closest to the equator, is known as the Hadley cell, named after the 17th to 18th century amateur meteorologist George Hadley who discovered the atmospheric structure behind the Trade Winds, which are part of the Hadley cell’s circulation.

    At the ITCZ air rises strongly in response to heating at the surface, helping to produce the downpours and thunderstorms that nourish the equatorial rain forests.

    The air rises through the troposphere, moving northwards then turning right / eastwards in the northern hemisphere (left in the southern hemisphere) because of the Coriolis force induced by the earth’s rotation. It cools and then descends again somewhere around 30 degrees north.

    This descending air dries and warms due to compression, creating belts of high pressure that girdle the globe in the subtropics, typified by high temperatures and scant rainfall – regions, of course, where many of the world’s deserts are found. At sea these are called the Horse Latitudes.

    The circulation of the Hadley cell is completed when air at the surface moves out of the southern edges of these high pressure zones back towards the equator. The Coriolis force again turns the wind to the right in the northern hemisphere, and this is what creates the Northeast Trade Winds that intrigued Hadley in the first place.

    One semi-permanent area of high pressure in the northern hemisphere is the Azores High (sometimes knows as the Bermuda High, especially in the USA, because of its extension in that direction).

    A strong Azores High can give the British Isles and Europe warm and dry weather in the springtime and hot, arid conditions in the summer. This depends on the jet stream looping far north of Europe in a way that it has rarely done during the summers of the last couple of years.

    This allows the Azores High to build more strongly farther northwards, occasionally re-centring over the European mainland or near the British Isles.

    This is the sort of pattern we are starting to see at the moment, and why we can probably expect above normal temperatures well into March with, unfortunately for those areas already in or close to drought, below average rainfall.

    It remains to be seen whether this will be a periodic pattern through the spring; given the current atmospheric drivers it may be but it’s long period to be looking at attempting to forecast. Naturally, then, the prospects for summer are even more thickly veiled.

    By: Stephen Davenport