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Background Information on Computer Forecast Weather Map

The atmosphere is a 3-dimensional entity. Changes in the weather patterns at the surface and higher up in the atmosphere are intimately related. As such, in the formulation of forecasting strategies, surface and upper-air weather charts must be taken as a whole if the evolutionary weather patterns are to be properly understood. The computer model forecast weather maps (hereafter "weather maps" for short) produced by the Hong Kong Observatory cover East Asia and the western Pacific, and through the depth of the troposphere from surface to 12,000 m aloft. The horizontal resolution is 10 km. Provision of such weather maps will enable the public to gain more insight into the driving mechanisms behind the weather affecting Hong Kong and its neighbouring regions in the next two to three days. It will also provide special users, such as air transport industry and aviation enthusiasts, with a complete set of prognostic information for reference. For better understanding by the users, the following paragraphs will explain some of the important and perhaps unfamiliar meteorological terms.

 

Classification of Weather Maps

Weather maps can be classified basically into two types, namely "surface" and "upper-air". The term "surface" generally refers to the surface of the earth, implying that different points on a surface map, e.g. a point over the ocean or a point on a mountain ridge, may represent drastically different altitudes. Therefore, users have to bear in mind the representative altitudes when interpreting such maps. At different altitudes, surface pressure is referenced commonly with respect to the mean sea level for easy comparison of various weather systems.

For "upper-air", weather maps are usually drawn for isobaric (i.e. equal pressure) surfaces following the practice of conventional upper-air observations and analysis. Air density and atmospheric pressure generally decrease as altitude increases. As such, pressure scale could be taken to represent altitude. The following table lists four common isobaric surfaces and their corresponding altitudes:

Atmospheric Pressure Approximate Altitude
200 hPa 12,000m
500 hPa 5,600m
700 hPa 3,000m
850 hPa 1,500m
925 hPa 1,000m

 

Streamline

Streamline charts are usually drawn for upper air to display the spatial distribution of flow directions at a particular instance of time. As indicated by an arrow head, the flow direction at any point along a streamline is parallel to the wind direction at that point. The following diagrams illustrate typical flow patterns of two common circulation systems in the northern hemisphere: (left) in-flowing streamlines with anti-clockwise rotation, known as cyclone; (right) out-flowing streamlines with clockwise rotation, called anti-cyclone (and vice versa for southern hemisphere) .

cyclone anti-cyclone
cyclone anti-cyclone

 

Cyclones and anti-cyclones in the upper air are often associated with low and high pressure areas on the surface. Low-level cyclones usually imply horizontally confluent flow and vertically rising air resulting in inclement weather. Low-level anti-cyclones bring in fine and stable weather due to horizontally diffluent flow and vertically sinking air.

 

Vorticity

Vorticity is a physical quantity measuring the rotational sense and speed of a fluid flow. The vorticities of cyclonic and anticyclonic circulations take on positive and negative values respectively. The more positive the vorticity, the stronger the cyclonic flow is. On a 850-hPa vorticity map, overlaid with streamlines, regions of strong positive vorticity are highlighted with colours to demarcate higher chance of unsettled weather.

 

Relative Humidity

Upper-air relative humidity maps reflect the spatial distribution of moisture content in the atmosphere. Since moisture is a pre-requisite for cloud formation, it is more likely to find clouds at places where relative humidity is high. Under favourable meteorological conditions, precipitation may also occur. From a relative humidity map at 700 hPa, one could estimate the depth and distribution of clouds in the mid to lower atmosphere.

 

Geopotential Height

Geopotential, or gravitational potential relative to mean sea level, is the energy an air parcel may potentially acquire by virtue of its position in the gravitational field of the earth. In the atmosphere, geopotential varies with altitudes and across latitudes. On an isobaric surface, higher geopotential implies higher potential energy and vice versa. Just like streams flow according to the layout of terrain, air will flow from high to low geopotential regions. Usually, areas of relatively high or low geopotential correspond with areas of high pressure or low pressure respectively. For the sake of atmospheric analysis, geopotential is commonly converted (namely, divided by the standard acceleration due to gravity) to geopotential height and expressed in units of geopotential metre. To a good approximation, geopotential height of, say, an isobaric surface is equal to the altitude of the surface. From 500-hPa geopotential height maps, one can depict important weather systems on the large scale, e.g. subtropical ridge or long-wave troughs in the westerlies. These large scale systems not only dictate how cold or warm air intrusion proceeds, but also steer the movements of tropical cyclones and other upper-air disturbances.

 

Jet Stream

Jet stream is an elongated belt of high speed air, typically found in upper tropospheric westerly flow round the globe. A jet stream may stretch over tens of thousands of kilometres in length from west to east and a few hundred kilometres in width from north to south. In contrast, its depth is only a few kilometres. Within the core of a jet streak, flow speed may reach 200 to 300 km per hour with possibility of multiple maxima. Consequently, strong wind shears exist on the periphery of a jet stream, both vertically and horizontally. In long-haul air travel, pilots may actually choose to ride the jet streak to save fuel and reduce flight time. On a 200-hPa map, regions with wind speed exceeding 150 km per hour are highlighted with colours to mark the location of jet stream.