Understanding Meteorological Scales

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Meteorologists look at many different features of the atmosphere in their work. These can range from short term, small scale phenomena to large scale, long term features. Everything in the atmosphere is inter-related and one cannot seperate the various features into discrete 'packages' to any great extent in order to forecast what is going to be happening in the capricious fluid that we live in.

The atmosphere has many levels of motion in it's particles from the random movement of molecules to the global zonal circulations which involve the entire atmosphere. In order to group the atmospheric phenomena that occur around us, we use a scheme suggested by radar meteorologist M.G.H. Ligda (1951) which divides the atmospheric features into groups of like space and/or time. The four groups that are most discussed are:

Scale Micro Scale Meso Scale Synoptic Scale Planetary Scale

Space Scale 1 km or less 1-100 km 100 - 5000 km > 5000 km

Time Scale 10,000 seconds or less 10,000 seconds to 1 day 1 day to 2 weeks > 2 weeks

Some of the phenomena that are found in each of the scales are:

Microscale Mesoscale Synoptic Scale Planetary Scale

Turbulence, Tonadoes and Waterspouts Thunderstorms, Land/Sea breezes, Valley effects Hurricanes, Mid-latitude highs, lows and fronts Mid-tropical tropospheric westerly circulation

Microscale weather is of a very small scale usually covering an area with a diameter of 1mm to a few hundred metres and lasting a short time. This means that most of the microscale weather is created by changes in the atmosphere close to, or in contact with the earth's surface. Generally, microscale weather is not normally discussed by weather briefers or forecasters, but is more looked at by researchers. The exceptions are microbursts, turbulence, tornadoes and waterspouts which DO have a direct and possibly deadly effect on aviators.

Mesoscale weather is significant to aviators and briefers and are known as 'local weather effects'. This means that the outflow winds experienced along the west coast, the valley effect of the valley around Whitehorse and the streamers generated along the eastern shore of the great lakes all fall into this category. All of the above may have a great impact on aviation safety and these are always included in an aviation weather briefing; and are used by meteorologists in the creation of short term weather forecasts.

Synoptic scale weather is what the public is most used to seeing on the nighly weather report. The highs, lows and fronts that create or control the large scale weather patterns that we live in and fly through are generally understood to have the greatest effect on 'what the weather will be like'. This may or may not be the case, but a forecaster or briefer can look at the patters in the atmosphere and, when combined with the knowledge of the mesoscale weather can usually predict in the short to medium term what an aviator can expect at destination.

Planetary weather, while significant in forecasting, is generally not of great concern to the general aviation public with the possible exception of the jetstream.

These scales are used in the study and forcasting of weather, and as mentioned above, cannot be seperated but must be considered as a whole. When a forecaster sits down to make his predictions, he goes through each of the scales and determines what effect each type of phenomena, feature of system will have on the local weather moving from the planetary scale to the microscale.

Understanding Air Masses
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