Interpreting the Gradient Level Wind Analysis

The Gradient Level Wind Analysis is a snapshot of the airflow near the surface of the earth. The arrowed lines are called streamlines and represent the direction of the wind. The dashed lines are called isotachs, and connect points of equal wind speed. The standard isotach intervals are 15 and 30 knots (28 km/h and 56 km/h) - knots is the preferred unit for these charts as they complement the Bureau's marine services.

The gradient level lies about 1000 metres above the earth's surface, and is the level most representative of the air flow in the lower atmosphere immediately above the layer affected by surface friction. This level is free of local wind and topographic effects (such as sea breezes, downslope winds etc).

Streamline charts are much more useful than isobaric pressure (MSLP) charts for showing the weather patterns over tropical areas. While MSLP charts are good for estimating wind direction and strength over mid and high latitudes, in the tropics pressure gradients are weak and often don't give a good indication of the prevailing winds. Meteorologists overcome this difficulty by drawing charts of the actual wind flow. The surface wind may be estimated by decreasing the gradient level wind speed by approximately 20% over the ocean, 40% over land and assuming a direction deviation of about 10-30 degrees. If looking along the direction of the wind, the deviation is to the right if low pressure is on your right (or if high pressure is on your left).

On streamline charts, low pressure systems (including tropical cyclones) appear as inflowing circulations - clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. High pressure systems appear as outflowing circulations, with direction of rotation opposite to that of the lows. Near the equator, when the wind changes direction as it flows from one hemisphere into the other, closed eddies may appear; these are indicated by an "E" and are not associated with high or low pressure (and are often associated with clear weather).

Lows are indicated by an L symbol, highs by H , accompanied with the value of central pressure in hectoPascals. Tropical cyclones are particularly intense low pressure systems, identified by the cyclone symbol, together with information on the name, maximum wind speed (knots) , central pressure (hPa) and current direction of movement (speed in knots). In the northwest Pacific, tropical cyclones are called Tropical Storms, and the more intense systems are called Typhoons. In Australia, these systems are called Tropical Cyclones and Severe Tropical Cyclones.

The broad streams of air flowing toward the equator from the midlatitude highs are called the trade winds: southeast winds in the southern hemisphere and northeast winds in the north; these wind streams tend to be strongest in the winter hemisphere when high pressure systems are more intense.

In the summer hemisphere, persistent winds tend to flow into the near-equatorial area from the opposite hemisphere, and are frequently associated with widespread cloudiness and heavy rain. These winds are referred to as the northwest monsoon in the southern hemisphere (December-March) and the southwest monsoon in the northern hemisphere (June-September). The monsoon flow is on the equatorward side of an area of low pressure called the monsoon trough, and tropical cyclones often develop from lows located in this trough.