It’s probably a stretch to include weather forecasts under the category of telescope accessories, but for darn sure, I do use weather forecasts as much as I do any of my telescope accessories. I like to plan, ahead, as to when I can setup & use a telescope or my astronomy binoculars and weather forecasts allow me to do just that. Weather forecasts, for an astronomer like me, are for more than checking to see if I will have clear skies, too. That’s a place to start, of course, but if the forecast is for clear skies, I still dig for a little more information.
After checking for clear skies, the next thing I check in a weather forecast is the moon phase and moonrise and moonset times. The presence or absence of the moon in the sky determines what kind of astronomy I will do and what telescope I will choose for the evening. Working with or around the moon is a big part of any observing plan.
Next thing I check is for humidity/dew point/dew and so on. I live in an area where dew forms, quickly, at night and where the relative humidity tends to be high. High humidity means less sky transparency and less than optimum conditions for hunting the faint stuff with my Dobsonian telescope, but dew is a much more serious headache and, more than once, dew has shut down an observing session with the telescopes or binoculars before I was ready to call it quits at the telescope eyepiece. When the weather forecast calls for fog after midnight, as it often does in the arm months of the year, I know I have a fight on my hand.
The last thing I check in the forecast is the wind. It’s not often a factor where I now live, but there was a time, when I lived on the prairie, that wind made using a telescope all but impossible, even though the sky was clear and the humidity low.
Will I be doing any astronomy, tonight? I better check the forecast.