How do you know when itís going to rain?

There must have been times when you see a big dark cloud move overhead and you say to yourself "It's going to pour" and nothing happens. Not all "dark" clouds bring rain. There are certain signs that Mother Nature gives that normally precede a rain storm. There are some obvious indicators that rain is falling from an approaching cloud. Thunder and lightning is one of them. There are more subtle signs that rain is coming. When a cloud is producing moderate to heavy rainfall, the bottom of the cloud will appear smoother for a lack of a better word. In other words, you won't see the outline of the individual clouds as well since the rain is obscuring the clouds themselves. On certain days, when you have good visibility overall and there are isolated rain showers or thundershowers, you will actually see the rain falling from the cloud. It will appear as a veil of gray extending from the base of the cumulonimbus cloud. Some folks claim that they can smell the rain coming. Now, we all know that when it first begins to rain, there is a certain smell that the ground emits. However, to smell rain before it begins is something that I'm not sure about. I think that folks are detecting the higher humidity levels that often transmit smells better. Another indication of rain, especially in the summer, is when the wind switches direction and increases rapidly. Heavy rain associated with thunderstorms cools the air through evaporational cooling and the cool air rushes downward as it is heavier than the warm air surrounding the storm. The air then rushes outward from the storm. The highest winds in a thunderstorm normally blow in the direction that the storm is moving. In other words, if you are southeast of an approaching thunderstorm that is moving southeastward, the northwesterly winds that precede the storm should be the strongest winds associated with the storm. Cloud patterns also can indicate that a steady rain is approaching. Sometimes the clouds appear like a herring bone pattern. This is usually a sign that rain or snow will occur within 24 to 36 hours. I will update this weather fact if I can think of more "signs of rain". If you can think of any that I haven't mentioned, please e-mail me.

Can Contrails from Jetliners Indicate a Change in the Weather?

Contrails can actually indicate that a change in the weather is on its way, especially during the cooler months of the year. How? Well, let's say that the weather is cold and dry and there is a deep blue sky. A polar or arctic air mass moved into your area recently and you begin to notice the contrails from passing aircraft. This is a sign that the upper levels of the atmosphere have started to moisten up a bit to allow for the contrail to persist for an extended period. What this means is that warmer air will soon return and/or precipitation is on its way. Since warm, moist air is much lighter than cold dry air, it rises above the cooler denser air. Therefore, the upper atmosphere is first affected by the approaching warm, moist air. This additional moisture content in the air in the upper atmosphere causes the contrail to last several minutes and sometimes the sky can be crisscrossed by numerous contrails. Many times, a general increase of high cloudiness begins after the first contrails are visible. Cirrus clouds are usually the first on the scene. Then, the cloud cover lowers and thickens and precipitation develops. Other times, contrails signify that warmer weather is on its way with no precipitation.

Dave