Predicting clear skies

[Eh]

Is there an easy way to predict if the night sky will be clear by using observations during the day? Obviously a very cloudy, rainy day will lead to a similar situation at night, unsuitable for stargazing. But it's not always so obvious. During the day, the sky can be filled with clouds one moment and be completely clear the next. Can weather be used as an indicator here? For example, if it's very hot are clear skies more or less likely? I'm ignoring the usual pollution for simplicity.

[selfAdjoint]

The best way is to check the weather on TV or through the web. They can do better than rules of thumb.

[turbo]

It is handy to consult the weather channel, your local forcasts, etc, but in terms of usefulness, you need to define your observing goals, and develop some experience with your local conditions.

You may find that the very clear nights that occur shortly after a cold front has passed yield very "twinkly" skys that are best used to try to glimpse very dim but diffuse objects. If you want to see very small planetary details, etc, you will often be better off choosing a night between frontal systems when the air is steady, although sometimes not as clear.

Good luck!

[check]

If you're looking for a resource, this one's probably the best: http://66.78.6.237/~cleardar/

[chroot]

Even better, use the clear sky clock. It's fairly accurate. You might also want to consider using aviation weather forecasts, which are more precise and more frequently updated.

http://cleardarksky.com/csk/

- Warren

[Eh]

Of course I could just check for a weather update or look at some of the other resources provided. But I'm also curious as to what conditions are usually involved in the process.

[selfAdjoint]

Weather is caused by differences in air pressure and the thermodynamics of the water vapor/liquid/ice in the atmosphere. A uniform force, the coriolis force acts to trun differences in pressure into basically circular wind patterns.

When water droplets evaporate they cool their surroundings; cool air tends to sink. When water vapor condenses into droplets it warms its surroundings; warm air tends to rise. Large scale air masses meet at near singularities called fronts; fronts can travel over long distances changing shape and intensity as they go. Fronts lift the air ahead of them and cause cooling and condensation which causes heating and rising which makes layers of clouds. Most of our weather comes from these facts.

Modern computerized atmospheric models can do tonight's weather pretty well. For three or four days they are useful but a long way from perfect. The usual weekly forecast is about 50-50 right.