Living in the middle latitudes, a key to meteorological forecasting is in the hands of what happens high in the atmosphere, thousands of meters up. Since it affects us a lot here in Youngstown, and will over the next week, I thought I’d do a little explaining about the two types of upper level air flows that impact our weather at the surface.
First off, why do we care what happens above earth’s surface? In the upper atmosphere, meteorologists look at the temperature of the air below a given surface. These “heights” determine if a trough or ridge is in place, and the patterns of these troughs and ridges are the two types of air flows that determine weather systems.
As you move up in the atmosphere, air pressure decreases. An average surface pressure is around 1013 millibars. In meteorology, we use many different pressure levels higher up in the atmosphere to determine the aforementioned troughs and ridges. Let’s just take an easy example and use the 500 millibar surface because that’s where about half of the atmosphere is above you and half the atmosphere is below you. However, the location of this 500 mb line differs in the atmosphere depending on the temperature below it. Since cooler air is more dense than warm air, the height of the air is lower because it takes up less room. Because cool air rises, troughs form from the rising air. On the contrary, ridges form where heights are higher due to warmer less dense air.
Now that I’ve confused the heck out of you, let’s put it into easier terms. What meteorologists love to see is a zonal flow, because it’s quieter. A zonal flow means that the upper level winds are blowing parallel to latitude lines. This creates a lack of troughs and ridges, thus allowing for a quieter surface pattern. An example of this is shown below:
On the contrary, when there is a meridional flow present, alternating troughs and ridges create multiple storm systems, and thus lead to a pattern of stormy and calm weather. Also a shift of colder and warmer temperatures are present. An example of this would be:
As you can see here, two big dips (or troughs) are affecting the US – one in the Rocky Mountains and the other in the northeast. These troughs create ridges as well. At the surface, the troughs represent storm systems and the ridge, calm weather.
How it affects you: These troughs and ridges in a meridional flow move from west to east and thus create for alternating weather in one specific location (i.e Youngstown will be clear a few days followed by cloudiness and precipitation for a few days). In the zonal flow, the lack of ridges and troughs create a calmer weather pattern. Obviously weather changes constantly and these patterns do not stay constant for a long period of time. This post was to elaborate on why our weather does change a lot, but many other factors also affect a weather forecast. However, this is one major culprit to weather forecasting and so I thought it may be interesting for you all to read since our weather really does change in the blink of an eye here in Youngstown!
I would like to thank www.theweatherprediction.com for the background information for this post. Images in this post are copyrighted from RAP Weather Forecasting global forecast weather models.