Troposphere

Weather occurs in a band of the atmosphere called the troposphere extending about 16 km (10 miles) above the earth's surface at the equator, thinning to half at the poles. The motion and interaction of low-pressure and high-pressure air masses is the result of the nature and produces the weather. To find the origins of these large areas of low and high pressure air, we look at what drives the weather - the heat from the sun.

The sun shines on the earth’s surface and doing so heats it resulting in the air above it heating up, expanding and then rising. The rising warmed air creates a partial vacuum (an area of low pressure) and as it rises, surrounding higher-pressure air flows in.

With the sun not shining at night time, the earth's surface cools and so does the air above it which contracts and sinks. This denser, heavier high-pressure air flows outward and displaces the surrounding lighter low-pressure air.

The sun's effect is greater at the tropics than at the poles, creating semi permanent bands of high and low-pressure air masses at different latitudes around the earth. This results in a region of high pressure at each pole, and a band of low pressure around the equator.

The warmed air rising from equatorial regions spreads to the north and south, and in the process, cools and sinks again over the Horse Latitudes, forming bands of high pressure air masses (the subtropical highs) at about 30°N and 30°S.

Between these bands of high pressure and those of the polar high pressure regions there are further bands of low pressure, at about 6o°N and 6o°S. These pressure bands are sustained by the global and local heating effects of the sun.

Coriolis Effect

Air returning to the equatorial zone from the adjacent high pressure bands is deflected to the west by the Coriolis Effect (the result of the earth's rotation) to become the Northeast Trades wind in the Northern and the Southeast Trades wind in the Southern Hemisphere. From these high pressure bands, air flowing to higher latitudes become the Prevailing Westerlies of the Northern and Southern Hemispheres, while cold, dense air at the poles is deflected by to the west by the rotation of the earth creating the Polar Easterlies. The four categories of air mass, characteristically different and classified according to origin are:

• Continental Polar air (cold and dry, originating over land near the poles),
• Continental Tropical (warm and dry, originating over land in the tropics),
• Maritime Polar (cold and wet, formed over seas near the poles) and ;
• Maritime Tropical (warm and wet, formed over tropical seas).

An air mass is warm or cold, wet or dry, but when formed, it retains those characteristics even though it makes contact with different air masses. Different air masses rarely mix and it is this feature that creates the weather systems, mainly at the edge of converging masses of air. Air flowing out of the high pressure zones in the Northern Hemisphere is deflected to the right by the Coriolis Effect, resulting in slow moving high-pressure air masses (anticyclones) rotating in a clockwise direction. High pressure air flow in the Southern Hemisphere is deflected to the left, and these southern anticyclones rotate in a counter clockwise direction bringing clear, settled weather.

Established sea-based low pressure areas in northern and southern latitudes are over water warmer than the adjacent polar icecaps. The Coriolis Effect redirects air flowing into these low pressure areas producing a vortex spiral (counter clockwise in the Northern Hemisphere, clockwise in the Southern) which is evident in the extreme such as hurricanes, cyclones and typhoons. Unsettled, moist and windy conditions characterize low pressure systems.

Weather Fronts

The rotation of pockets of warm and cold air opposing each other intermeshes and at the meeting point between high and low pressure air masses, conflict occurs. The air masses intermingle either gradually losing their identity or combining to produce destructive weather.

Warm, moist, low-pressure air rises, cools, deposits its excess moisture and descends for most of the time, but it can act as a catalyst to produce violent weather conditions. A depression forms forcing the warm air to rise when an approaching cold high-pressure air mass convex leading edge slides under the concave edge of the lighter warm air of a low-pressure air mass.

This interface between the two, characterized by high, white cloud sloping away from the observer, is the often-volatile cold front, accompanied by thunderstorms and strong winds. Warm moist air overtakes the cool denser air climbing up and over the convex slope, creating clouds as it rises and cools. The interface and the cloud formations slope toward the observer, and the indication of an approaching warm front is the high cloud followed by a period of rain and poor visibility until the next air mass arrives.

When one wedge of cold air catches up with a preceding one and the warm air that was separating is squeezed upwards is an occluded front gives rise to strong winds and rain if the cold and warm air masses differ considerably. Weather Instruments

Cloud Types

Clouds are indicators of approaching weather systems and are classified into cloud types according to their shape colour and height. Cumulus or cumulo- are rounded, billowy clouds; Stratus or Strato- are layered clouds; Nimbus or Nimbo- are rain clouds; and Cirrus or Cirro- are ice crystal clouds. Alto- means high.

Clouds tell the skilled observer what is occurring with the weather

and what is to be expected as each family of clouds is created by particular condition of temperature, humidity and height.

Three main ways in which clouds form:

• Air is heated by radiation from a warmer area of land or sea and rises through convection currents;
• Air is forced to rise when it meets obstacles such as mountains; or
• Air rises over a mass of colder, denser air.

As the air rises it cools and when it reaches its dew point, clouds form. There are three main cloud layers:

• High cloud is usually above 6,000m (20,000ft) Cirrocumulus, Cirrostratus and Cirrus;
• Medium-level clouds are at 2,100-6,000m (7,000-20,000ft) Cumulonimbus, Altostratus, Altocumulus ; and
• Low cloud lies up to about 2,100m (7,000ft) Stratus, Nimbostratus, Stratocumulus and Cumulus

Maritime Weather Forecasts