Invisible clouds

[acey]

The satellite image right now seems to show my region and most of UK completely clear - but my sky is completely cloudy. Perhaps I'm sitting under one little spot of cloudy weather, but I assume that the real explanation is that the cloud is low - the satellite shows cold, high cloud as a bright image, but low cloud and mist that is near ground temperature doesn't show up much. The forecast was for clear weather too, and it started out that way, until I got the scope set up.

Moral of the story: treat satellite images with care. But always give it a go, if there seems to be any chance at all.

Now I'm off to bed. Which presumably means the clouds will magically disappear.

[JamesF]

I find the IR images quite difficult to interpret at times, probably for exactly the reasons you suggest. Mind you, there have been a few days when sat24 shows the sky above me to be clear in visible light when I can't actually see a thing for cloud.

James

[Paul M]

These particular images are infrared. It's the only way of seeing clouds at night!

So you are looking at temperature. They are logically negative images. Cold areas or bright warm areas are dark.

This time of day/year the land is about the same temperature as the sea so land masses don't show well. Very high cloud shows up as bright as it is very cold. Very low cloud, such as is spoiling the sky tonight is just about the same temperature as the land and sea so it doesn't show up!

The brightest and therefore coldest clouds are often thin, high cirrus. What might look like a solid overcast on IR imagery might be quite transparent from the ground. Or it might be the tops of towering cumulo nimbus (thunder storm) clouds. Experience helps the user differentiate what sort of cloud is represented but as with this low cloud we currently have, if it doesn't register in the image you just don't know it's there.

I usually look at animated images. Sometime you can make out subtle contrast variations that move between frames giving away the presence of low cloud.

[acey]

Funnily enough I didn't go to bed, and when I looked out again at 2a.m. the sky was clear. So I observed until dawn, though there was frequent recurrence of ground mist, my optics dewed, and when I tried using a hairdryer I found my battery pack had run out of, er, steam. So it was a funny old night, but I still managed to see some stuff, and finished up with very nice views of the Orion Nebula and Jupiter as the first light of dawn appeared. Think I should maybe get a cloud detector.

[wxsatuser]

Thats just one of 12 channels.

If you look real hard you can make out the lower cloud.

It's a bit like imaging stars the detail is in the processing.

The various channels can be combined to give very good cloud readouts.

Most you will see don't have any or much processing at all.

The resolution on that image is about 3km, similar to NOAA APT, not brilliant for

local forecasting.

[LukeSkywatcher]

The white "fluffy" clouds in the image are low thick rain clouds. The grey stuff clinging to coastlines and inland are high thin clouds. The blackest areas on the map are clear skies. Thats what the experts say about such IR images.