marine meteorology

marine meteorology was founded by the American naval officer Lieutenant Matthew Fontaine Maury, when he convened an international meeting in Brussels in 1853 to establish standards for the reporting of weather at sea and around coastlines. One result was the setting up of the International Meteorological Organization, which has now become the World Meteorological Organization. Thanks to Maury's efforts in standardization, and recent major advances in observational methods—especially through remote sensing, the development of theoretical understanding, and increasingly powerful computers—today we benefit from high standards in weather forecasting. For example, ‘sea winds’ is an orbiting microwave radar device that is used to predict big storms. It measures wind, rain, and even ice on the world's oceans, and data from it is supplemented by readings from ocean radio transmitting buoys. It is particularly useful for surfers looking for outsize waves.

Our weather is generated by the disparity in the amounts of radiant energy received from the sun at polar and tropical latitudes. In the tropics, as the warmed air rises, it is replaced by air coming from the poles. At the poles the cold air is continually sinking. This generates a basic pattern of convection, in which winds in the lower atmosphere blow towards the equator, while those in the upper atmosphere blow towards the poles. Once the air begins to move latitudinally, Coriolis force comes into play and rotates its direction. So the basic pattern of airflow is broken up globally into a series of circulation cells.

Weather is latitudinally zoned. Near the equator are the doldrums, the trade winds blow between the latitudes of 10° and 20°, and then around 30° are the horse latitudes, which are high-pressure zones. At temperate latitudes cyclonic and anticyclonic pressure systems generate more variable weather patterns. At 60° is a zone of low-pressure systems, and finally over the poles high pressure dominates. The contrast in the high and low pressure systems between these high latitude regions fluctuates, and leads to decadal cycles in climate. In the North Atlantic this cycle is known as the North Atlantic Oscillation. In the tropics there are also significant variations in the geographical location of features like the Intertropical Convergence Zone (ITCZ), which irregularly generates El Niño events.

The water cycle plays a central role in determining weather patterns. The evaporation of water vapour from the ocean's surface cools the sea surface temperature. In the atmosphere, water vapour is the third most abundant gas, although only about 0.001% of all the water on earth is in the atmosphere at any one time. The warmer the air the more water vapour it can contain. Conversely, if air is cooled some of the water condenses into cloud and mist. When the air rises, the atmospheric pressure it is subjected to falls, so it expands and cools, and clouds are formed. Where there are coastal hills and mountains, onshore winds are forced up by the topography so a line of clouds forms along the coast. In the days of sail, clouds on the horizon were a useful indication of the proximity of land. Clouds affect the heat balance of the ocean, as they insulate the surface of the ocean surface from much of the sun's radiant heat. Winds accelerate the rate at which heat is exchanged across the ocean's surface; the fiercer the wind the faster heat is exchanged. Convection resulting from rapid heat exchange in the tropics can generate tropical storms.

In the atmosphere there are air masses, which are analogous to the water masses in the ocean. Boundaries between different air masses are also called fronts. The main front between the cold polar air masses and the warm subtropical air masses is thrown into a series of long meandering waves that slowly travel around the globe. These meanders are regularly perturbed to form large-scale eddies about 1,000 kilometres (625 mls.) across that are low-pressure or high-pressure systems; the former are depressions and the latter anticyclones, both familiar features of weather forecasts. Within a depression the boundaries between different air masses are either warm or cold fronts depending on which air mass is advancing. The cooler air mass pushes under a warmer one, so along the fronts air rises. The approach of a cold front is heralded by the cloud formations—changing from high-level cirrus, to lower cirro- and altocumulus clouds (mackerel sky), and then to lower-level stratocumulus clouds. These heavier, lower clouds along the fronts often bring rain. Clouds and rain tend to be heavier with the passage of a cold front because the gradient of the front between the air masses is steeper. Winds associated with depressions blow anticlockwise (veering) in the northern hemisphere and parallel to the isobars (lines of equal atmospheric pressure) rather than across them. So as a depression passes through the wind direction reverses.

Good forecasting is very much dependent on verification provided by the 10,000 mariners who participate in the Voluntary Observing Ship Programme by sending in regular weather reports while at sea. Their data help continually to update the theoretically derived forecasts.

See also Fitzroy, Robert.

Bibliography

Hamblyn, R. , The Invention of Clouds: How an Amateur Meteorologist Forged the Language of the Skies (2001).http://gmao.gsfc.nasa.gov/VLM/marine.htmlwww.wmo.ch/index-en.htmlwww.doc.mmu.ac.uk/aric/eae/index.htmlwww.atmosphere.mpg.de/enid/1442

M. V. Angel