Satellite and Radar imagery are useful tools to know what is happening right now with the weather. They are real observations that complement the weather forecast.
This article will cover:
I. Satellite
Satellites are in space, and they give us a completely different viewpoint than humans have from the ground. Instead of looking at clouds from below, satellites give us a view from above.
Most satellite images available online come from geostationary satellites. Geostationary satellites stay fixed above the same point above the Earth at all times. Having a "fixed" viewpoint in the sky allows them to see the same image at all times, which is perfect for making time animations and seeing how clouds move in the sky.
Five geostationary satellites cover the globe except at the very high latitudes close to the poles.
source: https://ncics.org/
Here are different types of images created by those geostationary satellites.
1) Visible
This satellite image is the same as what you would see with your own eyes if you were inside the satellite looking through the window. This image tells you where the clouds are located and makes it easy to locate large weather systems, such as big low-pressure systems or hurricanes.
The whiter the cloud, the thicker is the cloud typically. So a thick cloud like the cumulonimbus will be completely white, and a thin cloud like the cirrus will be grey.
The image below, since it is visible like the human eye, is only available during the day and not at night, which is quite a limitation.
Source: https://learningweather.psu.edu/
Visible Image of Hurricane Katrina (2015)
Source: https://www.researchgate.net/
2) Infra-Red
Infra-Red satellites allow us to measure the temperature of the obstacle first hit by the beam sent by the satellite. This technology is the same as infrared night goggles that give you night-vision to see warm bodies and objects at night.
If it is cloudy, the wavelength from the satellite first hits the top of the cloud and is reflected back to the satellite. The temperature of the emission at the top of the cloud can be calculated, which indicates the height of the top of the cloud. Therefore:
A white colour indicates a cloud whose top is very high (eg, cirrus, cirrostratus, cumulonimbus)
A less white colour indicates a mid-level cloud (eg, alto stratus, alto cumulus)
A grey colour indicates a low-level cloud (eg, cumulus)
Below is an infra-red satellite image from PredictWind - the whiter the colour, the higher the top of the cloud.
3) Enhanced Infra-Red
Like any image, satellite images can be processed to alter the colours to make it easier to read. A common feature is to colour the cold areas corresponding to clouds whose top is high (eg, cumulonimbus, cirrus, etc). Those coloured areas usually represent high/deep convective areas that extend very high in the atmosphere and could be associated with areas of strong weather (strong rain, etc.). See the image below:
A fixed satellite image is useful, but the animation of those images in time gives the best understanding of the clouds and how they move, see the animation below:
II. Radar
A rain radar is different from a weather satellite. First, the rain radar is located on land and not in space. Second, it measures the amount of precipitating water. So,
Weather satellites indicate the cloud cover.
Rain radar indicate the amount of precipitation within the cloud.
The radar sends a microwave. If there is water, it gets reflected back to the satellite, and the amount of precipitating water can be deduced based on the reflectivity. The unit is decibel (dBZ). The warmer the colour, the more intense the rain.
Like satellite images, rain radar images can be animated, thus showing how rain cells move. It is then possible to estimate where the rain cells are heading and the likelihood of being impacted.
III. Combined Satellite and Radar image by PredictWind
As mentioned, satellite and radar images are complementary. PredictWind uses image processing to merge both images and create the hybrid image, as shown below.
The white & grey areas are clouds from the Infra-red satellite.
The coloured areas are rain from the rain radar.
IV. Analysis of satellite and radar images
Example 1: Satellite image of an Atlantic depression
Below a visible image of a low pressure in the Northern Hemisphere.
This image can be used to identify the centre of the low, the cold & warm, the occluded front, the warm air section, as drawn below.
Example 2: Strong thunderstorm
Rain radar allows for identifying areas of strong thunderstorms and can give an early warning of an extreme event in the short term.
For example, in the Mediterranean Sea at the end of the summer, thunderstorms can develop quickly into extreme weather events when the water is still warm
.
Cells with intense colour and also shapes in an arc/bow are potential:
Example 3: Fog
Fog is a special type of cloud. This cloud is next to the ground at a very low altitude, and the cloud has a similar temperature to the ground.
During the daytime, fog is visible on the visible satellite images and not visible on the infra-red image since the cloud has the same temperature as the nearby cloud.
Therefore, you can identify the areas with fog by comparing visible and IR images.
Example 4: Inter-Tropical Convergence Zone (doldrums)
In conjunction with a rain radar onboard, satellite images can give insights on where the doldrums are and where it is best to cross.
