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Storms in the desert

Imagine... you are walking down the road, extremely tired, following a sports session after school. A huge sandstorm comes your way, carrying with it massive trees, innocent animals and even vehicles.

What would you do? Run for your life? Well... you don't need to panic, because the cruel disasters we know as dust storms and sandstorms do not occur in Sri Lanka. But, there is no doubt that you will be interested in knowing more about them.

The best way to explain what a dust storm is, is to describe some of its causes. Essentially, a dust storm is caused by strong winds that travel over dry, arid (parched) land with little vegetation, and pick up tiny particles such as sand, dust, and other debris.

Land becomes susceptible (vulnerable) to erosion from high winds when the ground becomes bare. This can be caused by poor land management concerned either with agriculture or overgrazing. The dust and debris carried by the wind form a large cloud or wall of dust that sweeps across the landscape with the wind.

Dust storms can be caused by many meteorological events which can be broken up into two categories, the first one being large-scale, non-convective events and the second, convective (related to heating) events. A meteorological event can be classified into one of these two categories by the type of changes in the weather, associated with the event, the predictability (ability to be foretold) of the event, the duration of the event, and when the event occurs.

Large-scale, non-convective events are often characterised by sustained high winds at the surface that are often associated with wind storms.

Dust storms resulting from an event in this category can be predicted anywhere from 24 to 36 hours ahead of time, due to the easy identification of the recognisable weather patterns that often occur with this type of event.

These dust storms can last from three to four hours, or up to two or three days, depending on the situation. Dust storms of this type often occur in the late winter to early spring, when there is extreme pressure.

They often worsen in the late morning and are more intense (strong) in the late afternoon. In the case of a dust storm that lasts more than a day, the intensity has a tendency to weaken through the night. A convective event would be something like a thunderstorm outflow or a micro burst. They are far less predictable than the large-scale non-convective events.

Dust storms caused by convective events are predictable up to three hours ahead of time. However, for a specific location, they are only identifiable minutes before they hit. The duration of these dust storms varies with the mechanism that causes them.

A dust storm caused by a micro burst may last only a few seconds, while one caused by a macro burst can last a few minutes, and a dust storm caused by a wake depression can last for hours. The phenomena that are classified as convective events occur during the late afternoon, in the spring and summer.

Large-scale, non-convective events, such as the passage of fronts (boundary between warm and cold air masses) and troughs (region of low atmospheric pressure) or the down-mixing of upper level winds are responsible for two thirds of dust storms.

Convective events are responsible for less dust storms, but the ones they are responsible for are the most dramatic clouds where visibility is lowest.

Dust storms take place mostly on the Great Plains of North America, Arabia, the Eastern Mediterranean region, Cyprus and many other places such as the Gobi Desert of Mongolia, the Taklamakan Desert of northwest China and the Sahara Desert of northern Africa.

Severe dust storms can reduce visibility to zero, making travel impossible, and can blow away valuable topsoil, while depositing soil in places where it may not be wanted. Drought and wind contribute to the emergence of dust storms, as do poor farming and grazing practices.

In desert regions, at certain times of the year, sandstorms become more frequent, because the strong heating of the air over the desert causes the lower atmosphere to become unstable. This instability brings higher winds in the middle troposphere (layer of air extending upwards from the Earth's surface) downward, producing stronger winds at the surface.

The dust picked up in such a storm can be carried thousands of kilometres: Sahara dust storms influence plankton growth in the western Atlantic Ocean and, according to some scientists, are an important source of scarce minerals for the plants of the Amazon rainforest. Dust storms can often be observed from satellite photos, the use of which is highly valuable in forecasting; of particular use are the NASA imaging devices.

Dust storms are also known to occur on a massive scale on the planet Mars. Storms on Mars last longer, and cover larger areas, than on Earth; some of these storms cover the entire planet and last for hundreds of days.

Local, regional and global weather patterns can be strongly influenced by accumulations of dust in the atmosphere.

Dust storms remove large quantities of surface sediments and topsoil with nutrients and seeds: in the 1930s, drought and dust storms created the 'Dust Bowl', greatly reducing agricultural production on the North American prairies at that time. Wind-borne dust are known hazards to human health. Dust storms are also an important source of nutrients for soils in desert margin areas.

- Compiled by Chamitha Kuruppu

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