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Peaks and valleys

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Posted: Jul 8, 2014 11:12 AM by Meteorologist Josh Eachus
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Topics: storms, ridge, trough, showers, summer

Forecasters have announced to the point of redundancy that heat and humidity are two of the primary ingredients for shower and thunderstorm development. Living in South Louisiana, one must wonder then why it doesn't rain every single day from June through September.

Beyond heat and humidity, there is an overlying rule however, a much more subtle process that dictates whether or not showers and thunderstorms develop. As an ever evolving fluid, the atmosphere is always trending toward chaos. Thus when the atmosphere achieves a state of balance, it inevitably breaks down and the subsequent changes spark active weather.

From weather 101, a difference in temperature and moisture content from the surface to the upper levels can also be referred to as instability. As long as warmer, more humid air lies beneath relatively cooler and drier air, the atmosphere is unstable and air will have a tendency to rise. This is a process noted within your own home, as upper floors tend to be warmer than those beneath. Warmer air, rising into cooler air condenses, forming clouds and, if enough instability or external processes are at work, eventually cloud droplets will grow too large and fall as rain.

That being said, with warm and humid air at the surface, there are a few mechanisms that can create showers and storms.

1. With the presence of a frontal boundary, surface air is forced upward. This can happen even if a stable layer in the lower levels of the atmosphere prevents air from rising on its own. Think of a front like a wedge, nudging air into the sky.

2. An "upper-level disturbance" (also known as a wave or upper-low) can create a swirl in the upper level winds that acts as a scoop, pulling existing unstable air upward.

3. On a very hot and steamy day, the typical orientation of the atmosphere in which air cools with height, clouds, showers and storms can sometimes bubble up on their own if the temperature change with height is dramatic enough.

During summer in South Louisiana, with all active weather patterns typically left off to the north, fronts are rare, upper-level disturbances are few and the air is so warm that the lower levels of the atmosphere remain stable. So how does the region ever get rain during the summer?

We count on the previously noted break down in atmospheric balance to moisten our lawns and gardens. Such breakdowns are the peaks and valleys in the upper wind pattern, or ridges and troughs.

A ridge is a pocket of higher air pressure and thus warmer air aloft. Due a decreased temperature change with height, even on the hottest day, the atmosphere is capped and air cannot efficiently rise. With less rising air, we see fewer showers and have fewer clouds and as a result see more sunshine. Ridges lead to clearer, hotter and drier days. Ridges are like a lidded bottle of soda, even when shaken (unstable), with the cap on, nothing happens.

A trough is a pocket of lower air pressure and thus cooler air aloft. With a much steeper change in pressure from surface to sky, air can rise more freely. With more rising air, clouds are numerous and shower and storm development is common. Troughs lead to a stormier pattern with slightly cooler temperatures. Troughs are like a shaken (unstable) bottle of soda, with no lid on to stop them from bubbling up.

You can get forecasts from Meteorologist Josh Eachus weekdays on 2une-In from 5-7am and News 2 at Noon from 12-1pm. Additionally, you can get the fastest and latest forecasts and weather news by checking in with wbrz.com/weather, liking Josh on Facebook and following him on Twitter.

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