How to Interpret Satellite Images
The images obtained by weather satellites are essential for any pilot, as they show the state of the atmosphere at a given moment. Therefore, they are extremely helpful for analyzing weather conditions and making forecasts.
When analyzing a satellite image, you should start with a simple INM-type image. In it, you should look for the cloud areas of interest: those that are currently affecting you or will affect you in the future. Sometimes these will be clouds associated with cold and warm fronts; other times, we’ll look for fog (low-level clouds). Other very important clouds include storm clouds or vertically developing clouds, etc.
A highly recommended exercise is to use a surface weather map from a time close to that shown in the image, with isobars, low-pressure systems, high-pressure systems, fronts, etc., plotted on it… (500 hPa map + IR + VISIBLE). The goal is to correlate the cloud structures with the meteorological features shown on these maps.
VISIBLE (VIS)
These images closely resemble what the human eye sees (the coloring is a digital interpretation). The visible light received by the satellite is sunlight reflected off the Earth's surface and clouds. They are taken only during the day.
In the VIS photo, the brightest (white) areas are the ones that reflect the most sunlight. In general, these photos show all the clouds that are present.
In the VIS image, the white areas (i.e., those that reflect sunlight) indicate the presence of dense, thick, and compact clouds. The gray areas (low reflectance) indicate the Earth's surface as well as thin clouds. The areas that appear black (no reflection) indicate oceans, seas, rivers, and lakes.
INFRARED (IR)
They show thermal radiation, or heat radiation, emitted by the Earth's surface and clouds (note, this is not reflected radiation). The IR photo is a representation of temperatures.
The hotter a cloud or surface is, the darker it appears in the image. IR photos don't require sunlight to be taken (unlike VIS photos). Therefore, they are taken 24 hours a day (day and night).
In the IR photos, the white areas (very cold temperatures, for example -20 to -60ºC) indicate high clouds, and in particular, cloud development and storm clouds (very high and cold).
The gray areas (intermediate temperatures, for example, from +10 to -20ºC) indicate low or medium clouds, and it is also the color that the surface of land and seas appears with at those temperatures.
The black areas (approximate temperatures of +10 to +30ºC). These are warm (heated by the Sun) and cloudless land surfaces, or warm ocean currents.
IR + VIS
This is an IR image in which temperature ranges have been enhanced through digital processing. Preset colors are applied to the image, thereby showing the temperatures of cloud tops on the visible image.
The objective is to outline the main cloud systems and highlight areas of storm clouds (the highest and coldest cloud tops) in relation to the terrain. This reveals various areas where precipitation is likely.
The reference colors (from the attached photo) are: blue, which corresponds to a temperature range of -30ºC (i.e., an altitude of 7 to 8 km); dark blue = -40°C (9 km), green = -50°C (11 km), and red = -60°C (13 km).
CONCLUSION
Let's take a look at the three attached photos, which were taken at the same time on Saturday, Nov. 16, 2024 (at 8:50 a.m. = 14:50 UTC) by the National Meteorological Service (INM).
The three images show two main cloud areas: one over western Puntarenas Province and another in southern Panama. In the VIS image, the cloud areas appear white and dense (they strongly reflect sunlight). In the IR image, they also appear white, but this is because they are high, cold clouds. In the IR-color image, two nuclei with blue edges stand out in the center of each cloud area, indicating the presence of scattered showers and thunderstorms.
