Hurricane season in slow motion

  • Radar image showing precipitation over the Atlantic (credit: NOAA)

    Sea temperatures in degree Celsius (East Atlantic)

    A satellite image of hurricane Katrina (credit: NOAA)

  • Hurricane season in slow motion
    29.08.2013 13:56


    Eight years ago the most destructive hurricane to make landfall in the US, hurricane Katrina, wreaked havoc in New Orleans. As we approach the end of August 2013 there have yet to be any named hurricanes in the Atlantic despite the National Oceanic and Atmospheric Administration centre (NOAA) forecasting an active hurricane season. If NOAA’s latest predictions are correct the Atlantic is still in store for 6 to 9 hurricanes (3 to 5 of them major) before the end of November.

    Hurricanes have an extensive life cycle which begins in central Africa. Their formation is associated with intense thunderstorms which form in this part of the world. These storms occasionally drift into West Africa and eventually into the eastern Atlantic. If the atmospheric conditions are right, then these clusters of thunderstorms will form into a Tropical Storm. If conditions continue to be favourable then a hurricane will form.

    For hurricanes to form the sea temperature needs to be above 27C to several metres depth. Warm, moist air is drawn into these clusters of thunderstorms which can then eventually form a circulation. The atmospheric conditions high up in the troposphere are also essential for hurricane formation. Wind speed and direction need to be relatively stable throughout the atmosphere. If there is too much variation with height, the deep convection needed to form a hurricane is thwarted.

    So, the question remains as to why the Atlantic has spawned so few hurricanes this year. Sea temperatures in the eastern Atlantic are above average for the time of year. A number of intense thunderstorms have formed over parts of West Africa. Given these forcing factors, why the lack of activity? Meteorologists believe it might, in part, be explained by Saharan dust at high levels in the atmosphere. This very dry air effectively prevents extensive cloud formation, weakening the system. Another poignant factor is associated with a change in wind direction with height. At present this is a significant factor in inhibiting development.

    If NOAAs predictions are to come true, the rest of the hurricane season could well be rather intense. Weather forecasters and those vulnerable in the Caribbean, Mexico and the United States will be keeping a keen eye on the developing situation over the Atlantic in the coming weeks.

    By: James Wilby