Vortex street off Cape Verde Islands

From the CIMSS blog: visible satellite movie of dust plume off west Africa and a vortex street  off Cape Verde.  [Satellite: Met-9 Feb 7, 2012]

120207_met9_vis_dust_anim.gif (GIF Image, 1280 × 960 pixels) – Scaled (89%).

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estimating the size of cloud particles from 36,000km away

It is possible to estimate the size of cloud particles from satellites. This is a useful tool in forecasting severe or hazardous weather.

Many of us have experienced large thunder storms in the summer, which sometimes lead to flooding.  The development of these storms depends on the processes in the cloud. How can we see those processes from space?

A weather satellite measure the amount of energy that gets to the satellite from the Earth. That energy will be sunlight that has reflected of the Earth and the thermal energy emitted by the Earth.  Energy at 3.9micros is sensitive to the size of the particles that the energy is reflected from. Small particles give more reflection than larger particles. This is not true for energy at say 0.6microns. This is because cloud droplets are about 4microns or bigger, the similarity in wavelength of the energy and size of the particle increases the effect and so make it measurable from space.

Combining particle size information means we can make statements about the processes inside a cloud. For example, ice particles are much larger than water particles, if we see a cloud top that has very small particles and is very cold the we can assume that these particles are super cooled water. This would be a icing hazard for aircraft.

Information from EUMETSAT’s geostationary satellite can be combined to produce a cloud micro-physics image. In this image blue colors are used for the cloud temperature (cold is blue), red for the thickness of the cloud (thick is red), and green colors for the size of particles (small particles are green).