Cloudy tonight or no?
Until recently, I’ve used The Weather Channel’s hourly forecasts on weather.com to decide if I should bother packing up the car for an observing session. Then, as sunset got closer, I’d check it again and then usually look at one of their interactive cloud maps to see if there were any surprises coming our way from the west.
But I’ve known for sometime that there was something out there called a Clear Sky Chart designed specifically as a weather forecast for astronomers.
This past weekend was a Kroes observing weekend for us here in Brown County, and at last week’s meeting, Tony mentioned the Clear Sky Chart weather predictions for the observing weekend. That got me thinking that I really needed to take a serious look at this tool.
Formerly called Clear Sky Clocks, the charts are weather forecasts created just for astronomers. Charts are available for more than 4,000 locations, and each provides data for a nine-mile radius. It forecasts cloud cover, transparency, seeing and light pollution from the moon for the next 48 hours.
Listed across the top is local time, reading from left to right in military time. The four rows below that concern Sky conditions, with values for cloud cover, transparency, seeing and darkness (which includes interference from the moon). The bottom three Ground rows display weather conditions at the surface.
Each block of color represents a value, and a legend below the chart explains what each color value is. You can also click on any block to display a full map and more detail about each row’s value (the blocks might be in your local time, but the map pages display in GMT). As I understand it though, if you can find a line of blocks in the first four rows that are blue, that’s when you should be able to get out and observe.
But be sure to look at the bottom three rows, too. The strength of the wind will determine your own comfort as well as the steadiness of your equipment. And humidity can mean fog, and can also affect your optics depending upon whether you’re sitting on the top of a hill or down in a valley. And temperature tells you how much layering you’ll need.
Because the maps are available for free, many clubs display local Clear Sky Charts, but if not, here’s the website where you can learn more and also search for a map near you:
If you’re new to the chart, you’ll find plenty more information about the charts and how to read them. And FYI, for this last weekend, weather.com was off both nights, but the Clear Sky Clock was right on target both nights.