The Scales of Atmospheric Motion

The Scales of Atmospheric Motion The atmosphere is always in motion. Each of its swirls and circulations is known to us by name- a gust of wind, a thunderstorm, or a hurricane- but those names tell us nothing about its size. For that, we have weather scales. Weather scales group weather phenomena according to their size (the horizontal distance they span) and how long of a lifespan they have. In order from largest to smallest, these scales include the planetary, synoptic, and mesoscale. Planetary Scale Weather Planetary or global scale weather features are the largest and longest-lived. As their name suggests, they generally span tens of thousands of kilometers in size, extending from one end of the globe to another. They last weeks or longer. Examples of planetary-scale phenomena include: Semi-permanent pressure centers (the Aleutian Low, Bermuda High, Polar Vortex)The westerlies and trade winds Synoptic or Large Scale Weather Spanning somewhat smaller, yet large distances of a few hundred to several thousand kilometers, are synoptic scale weather systems. Synoptic scale weather features include those having lifetimes of a few days to a week or more, such as: Air massesHigh pressure systemsLow pressure systemsMid-latitude and extratropical cyclones (cyclones that occur outside of the tropics)Tropical cyclones, hurricanes, typhoons. Derived from the Greek word which means seen together, synoptic can also mean an overall view. Synoptic meteorology, then, deals with viewing a variety of large scale weather variables over a wide area at a common time. Doing this gives you a comprehensive and nearly instantaneous picture of the state of the atmosphere. If youre thinking this sounds an awful lot like a weather map, youre right! Weather maps are synoptic. Synoptic meteorology uses weather maps to analyze and predict large-scale weather patterns. So each time you watch your local weather forecast, you are seeing synoptic scale meteorology! Synoptic times displayed on weather maps are known as Z time or UTC. Mesoscale Meteorology Weather phenomena that are small in size- too small to be shown on a weather map- are referred to as mesoscale. Mesoscale events range from a few kilometers to several hundred kilometers in size. They last a day or less, and impact areas on a regional and local scale and include events such as: ThunderstormsTornadoesWeather frontsSea and land breezes Mesoscale meteorology deals with the study of these things and how the topography of a region modifies weather conditions to create mesoscale weather features. Mesoscale meteorology can be further divided into microscale events. Even smaller than mesoscale weather events are microscale events, which are smaller than 1 kilometer in size and very short-lived, lasting minutes only. Microscale events, which include things like turbulence and dust devils, dont do much to our daily weather.