Mid-level water clouds

Mid-level water clouds appear white in the HRV Cloud RGB images.

Water droplets and ice crystals strongly reflect short wave solar radiation in case of an opaque cloud, hence the strong intensity of the red and green colour beam. The blue colour contribution from the window channel IR10.8 has about the same intensity for mid-level clouds than the contribution from HRV. The warmer the cloud, the more yellowish its colour will be, the colder, the stronger the blue component.

For mid-level clouds, both channels (HRV and IR10.8) have about the same intensity, the resulting colour is a whitish grey. The HRV Cloud RGB image below shows mid-level cloudiness over north-western France.

HRV Cloud RGB from 17 July 2015, 12:00 UTC

Sometimes, the HRV channel of the HRV Cloud RGB is enhanced by a sun angle correction. This leads to higher intensities in the HRV reflectivities, especially at low sun angles during the winter months. As a consequence, warmer water clouds look more yellowish in the HRV Cloud RGB in winter then in summer.

Explanation of the white colour of mid-level clouds in the HRV Cloud RGB (see the recipe):

• Both channels (HRV and IR10.8) contribute with the same intensity to the HRV Cloud RGB, the resulting colour is white.