The beginning of hurricane season
Hurricanes are large tropical storms – similar storms in different parts of the world are known as hurricanes, typhoons, or cyclones. They develop over warm water and have very high wind and heavy rainfall. Hurricanes also cause sea surges, which can lead to serious flooding as the storm reaches the coastline.
- Tropical cyclones with winds over 119km/h are known as hurricanes in the Eastern Pacific and North Atlantic
- The same type of storm is known as a typhoon in the Northwest Pacific and as a cyclone or tropical cyclone in the South Pacific or Indian Oceans
- The Eastern Pacific enters hurricane season each year on 15 May, while the season starts on 1 June in the Atlantic
Tropical cyclones originate as low pressure systems over tropical or sub-tropical waters. They start as clusters of thunderstorms with wind at low levels circulating anti-clockwise in the Northern Hemisphere or clockwise in the Southern Hemisphere. The storm graduates from being called a tropical depression to tropical storm to hurricane depending on the sustained strength of the wind.
Satellites have had a huge impact on the identification and forecasting of these storms. Before satellites, ships and coastal regions had limited advance warning to allow them to avoid or prepare for storms, while now storms can be tracked in visible images – often for many days – before they strike land.
Forecasters are able to use satellite imagery over areas where convection takes places, such as west of Africa, to spot the initial formation of a storm before it has even become a Tropical Depression. Dr. Mark Higgins of EUMETSAT explains the development of one storm, which became Hurricane Isabel, over the Atlantic:
Hurricane forecasters use both visible and infrared satellite imagery to track the motion and cloud patterns of hurricanes, and infrared to monitor cloud-top temperatures.
Information from satellites is important, long before a storm has even formed. Weather models that predict the development and path of the storms use atmospheric information gathered by satellites.
Crucial information about the ocean environment is also collected from space. The ASCAT instrument on Metop measures the speed and direction of the wind over the ocean. This is important for monitoring the formation and development of the storms and is used to pinpoint the storm centre.
Forecasters also pay attention to the sea surface temperature. Warm water will allow a cyclone to pick up energy and intensify, while travelling over cooler water will weaken it. The temperature of the sea surface is monitored from satellites. For a cyclone to form, the sea surface temperature needs to be at least 26 °C. Information about the storm can also be gathered from buoys in the sea, and from aircraft.
Long before the hurricane season starts, forecasters at centres like the European Centre for Medium range Weather Forecasting and the US National Hurricane Center make predictions about how many storms are likely to form. These forecasts are made using computer models, which take into account all of the measurements available about the atmosphere and the ocean. These satellite observations and observations from the surface are used to simulate what might happen.
Occasionally, storms also develop outside of hurricane season. If you are interested in following developing storms in detail, there are lots of resources online, including Stormtracker from the UK Met Office, the WMO’s Severe Weather Information Centre, and the hurricane blog by the US National Oceanic and Atmospheric Administration.