Written By Chris Webster, Meteorologist
In my previous blog post I wrote about how the winds high in the sky differ from the winds we usually experience on the surface of the Earth.
We can get an appreciation of the winds aloft by looking at a loop of images from a geostationary satellite. The Japan Meteorological Agency launched the satellite we use most in New Zealand, and it orbits the earth at an altitude of 35,000 kilometres above Papua New Guinea. It's specially configured to move in an orbit synchronized with the rotation of the Earth, hence the name geostationary (meaning Earth-stationary).
Take a look at this loop of satellite pictures for the week 28 July to 4 August 2008:
The images are similar to what you see on the Maps & Rain Radar page (by selecting Satellite Imagery), TV or newspaper, but with added colour. Firstly, you must be aware that the satellite is "seeing" in the infra-red, like wearing special night-goggles to look at the Earth. We've then applied a colour enhancement to highlight what's happening. With this particular enhancement, cold surfaces are shown as light grey or white and warm surfaces are a darker grey or black; very cold surfaces, typically clouds high in the sky, are coloured red (-50 C) or green (-60 C); there are even a few patches of blue at -70 C, indicating extremely cold cloud that is high above the Earth.
We can learn so much by looking at a loop such as this!
First let's look at the area across central Australia. Over the dark background (the warmer Australian landmass) there are bits of white and red cloud high up above the land, caught up in a very strong westerly jet-stream of air that's racing towards the Tasman Sea and New Zealand. A weather balloon flight at a height of 10 km above Wagga Wagga (New South Wales) reported a wind of 356 km/hr around the time of the middle of the loop (1 August 2008). Winds over Sydney peaked at 302 km/hr. You almost never get winds that strong near the Earth's surface.
Over the Top End of Australia there are filaments of high cirrus that originated near the equator and are moving towards the jet-stream. A bit farther down to the south there are the comma-shaped clouds of lows, plus intervening highs, that we see on weather maps trundling through our patch of the Earth, the mid-latitudes.
If you look at the loop several times, you'll notice patches of dark grey cloud (near the ground or sea) swirling around in quite different directions (and speeds) from the white or coloured cloud higher up.
As an interesting aside, notice how the Australian landmass seems to pulse from dark to light each day? This happens as the land warms from the sun in the daytime (dark shading) and cools at night (light grey shading). The effect is strongest over the southern states.
The jet-stream isn't apparent at the very top and bottom of the images, and this is quite typical. Near the equator and near the poles the winds aloft are not as strong as in-between.
I think this loop gives us insight into why the wind is different high in the sky. I'll continue this thread in a later post.