Monitoring the weather

Satellites (continued)

While satellite images represent only part of the information needed for a forecast, you can build a better appreciation of what's happening in the atmosphere by learning typical features, and where possible, comparing your identifications with features in the corresponding weather map.

(below right) VIS ('Visible' imagery, at top) and infrared (IR) images taken from the Japanese geostationary meteorological satellite at 12.30pm on 22 September 1992. The VIS picture appears more detailed because areas of low cloud are much more distinct. On IR pictures areas of low cloud often blend into the ocean or land surface because of the relatively small difference between the temperatures of the cloud tops and that of underlying surfaces. West of Western Australia, the VIS picture shows detail of the low cumulus clouds while the IR image has only a grey smudge, not as easily distinguishable from the ocean surface. Similar differences are obvious east of Queensland and elsewhere. VIS pictures are superior in some respects, including resolution, but IR has the major advantage of providing night images.

Some tips:

• Many significant cloud formations or weather features can be identified by characteristic patterns shown on the preceding page and on the next page.
• The satellite images are generally not photographs. The most commonly seen images are 'heat pictures' showing temperatures recorded by infrared sensors that display cold areas (especially clouds) in light colours, and hot regions (land and sea) in dark tones. Image clarity varies with the quality of satellite reception, and the technology by which you view it.
• Features with similar temperatures can confuse even skilled interpreters. At some times of day it can be difficult to distinguish between the ground and low cloud, fog and low cloud, or sea and low cloud. High clouds may obscure important features. (Meteorologists resolve some of these difficulties by comparisons with images from the visible part of the spectrum.)
• Watching loop sequences on TV, or comparing successive still images, will let you track the birth, growth and decay of weather systems.
• The false-colour images sometimes seen on TV are specifically enhanced to give meteorologists easier identification of subtle temperature differentiations, and hence the location and strength of weather features, compared to the lesser contrast of grey scale imagery.

TIME NOTE: some presentations of satellite imagery carry Universal Time ('Z' time) which is 10 or 11 hours behind Australian Eastern Standard Time, depending on daylight saving.