Oh, how I already miss the dry and pleasant weather we had just a week ago. The humidity has returned and on Tuesday, we might see some home-grown showers and thunderstorms erupt. But it begs the question - why doesn't it rain every day it's humid?

To first answer that, we have to understand that there are two ways that we get rain - one is from the collision of air masses, such as when a cold front moves through. That's what happened last Friday when the rain fell sideways in the pre-dawn hours. The other way is from the process known as convection. That's basically when warm, humid air rises.

How does the air rise? Well, one way is to heat it. As it rises and encounters cool air above, it condenses and forms a cloud. It rises similar to a hot air balloon; the air inside the balloon is warmer than the air outside the balloon so it ascends. The cooler it is outside the balloon, the higher the balloon will rise.

The reason we don't always have rain in humid weather is that sometimes, the air above the ground is nearly the same temperature as the ground, or slightly warmer. When this occurs, we say there is a cap or lib on the atmosphere. However, if the ground heats up sufficiently, eventually the air above the ground will be less warm and the lid will be removed. Then clouds can rise up, expand and eventually produce rain.

The trick for our area is to determine what the lid is and how much heating will push the lid up. To do that, we rely on a network of weather balloons released only twice a day to give us a picture of the atmosphere. Then we can estimate how much heating we'll need to "lift the lid". It's not an exact science, however, because we can't exactly measure the condition of the atmosphere above the ground. If more balloons were released over a greater area, we might be able to better predict this.

So my best advice on warm, muggy days is to keep the umbrella handy and keep a close weather eye to the sky. When dark, low hanging clouds are around, they're usually a sign that cloud has grown high up in the sky and is about to let the rain fall, probably on you!

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