19 Feb 2013

The wonders of detailed satellite imagery

As we carry on with everyday life on the ground, lots of satellites are busily circling the planet, taking snapshots of what is happening below.

Satellite imagery has a very important part to play in weather. It not only allows us to see what is happening right now, it also gives invaluable information that can be put into weather computer models to predict what is going to happen in the future.

As technology advances, the resolution of the images that satellites take increases, with a huge amount of detail captured in a single image.

When glancing at a satellite image, it’s easy to just see bits of cloud, but if you look closely, so much information can be gained – allowing an understanding of what is going on in the atmosphere above us.

To demonstrate this, I thought I’d take a high-resolution image of the UK captured yesterday by Nasa’s rapid response satellite and explain some of features on it.

Mountain snow

This image shows snow cover over the mountains of Scotland. Although it appears white like cloud, snow cover often looks whiter and brighter than many cloud types – especially when freshly fallen – thanks to its high albedo reflecting sunlight back up into the sky.

Also, you can see that the snow cover hugs the rugged terrain, with perfect dendritic patterns carved out by the snowless valleys below.

Lingering mist

If you look closely at this image of the Orkney Islands, you can just make out a slight haze close to Kirkwall. This isn’t cirrus cloud, but mist that has lingered into the early part of the afternoon following a foggy morning.

You can tell that this is mist rather than cirrus because it tends to follow the coastline, rather than randomly extend across a larger area.

Mountain wave clouds

At the top left of this image, you can see that the clouds are formed in discrete bands just off the south west of Scotland.

These are called mountain wave clouds because they form when air in the atmosphere hits a mountain and is caused to undulate on the other side – much like a car suspension would bounce up and down after hitting a speed bump in the road.

When the air undulates upwards, it cools and condenses, forming clouds, and when the air sinks, it warms and evaporates leaving no cloud.

In this particular case, the wind is blowing from the south east, so it has been the Pennines that has caused the air to undulate on this particular day.

Dissipating contrails

South east England is home to many busy airports, with planes leaving contrails across the sky. As these contrails disperse, they tend to spread out and leave discernible trails that can been easily spotted by satellites.

Cumulus cloud streets

On this image, you can see little puffs of cloud that form near the south coast of England, which grow larger as they head inland – pushed by (on this particular day) a south easterly wind.

These are called cumulus cloud streets as they form in distinct rows, similar to rows of streets parallel to each other.

They are caused by moist air from the sea being pushed inland which is subsequently forced to rise as the sun heats the ground. As the air rises, it cools and condenses, forming clouds.

As these cumulus clouds are blown further inland by the wind, they become bigger, as the image shows.

Frontal cloud

This final image shows a classic image of frontal cloud just to the south west of Cornwall. It is probably the easiest thing to spot on a satellite picture because it shows up as a thick band of white cloud covering a large area – often meaning that rain is looming.

So the next time you see a satellite image, have a closer look. You will be surprised what you can see if you look hard enough.

Images courtesy of Nasa’s Land Atmosphere Near-real time Capability for EOS (LANCE)

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