Satellite Imagery and Cloud Observation

Exam Tip: When interpreting visible imagery, be cautious of "false positives." Bright patches are not always thick clouds; they can also be caused by snow, bright land surfaces, or sunglint (the reflection of the sun off the water).

Thermal Infrared Imagery and Cloud-Top Temperature

When the sun goes down, meteorologists switch to Infrared (IR) imagery. Instead of reflected light, these sensors detect thermal infrared radiation emitted by objects on Earth. This makes IR imagery functional during both day and night.

The primary metric used in IR imagery is brightness temperature. Because temperature generally decreases with height in the troposphere, there is a direct relationship between the temperature of a cloud and its altitude:

• Colder cloud tops indicate higher altitudes.
• Warmer cloud tops indicate lower altitudes.

This is a critical tool for identifying deep convection. For instance, a cumulonimbus cloud—a deep convective cloud associated with thunderstorms—will show very cold brightness temperatures because its tops reach high into the atmosphere.

The Challenge of Thin Cirrus

One significant limitation of IR imagery involves thin cirrus clouds. Because cirrus clouds are composed of ice crystals and are often quite thin, they may allow thermal radiation from warmer surfaces (like the ground or lower clouds) to pass through them. In these cases, the measured brightness temperature might be warmer than the actual physical temperature of the cloud top.

Enhanced Infrared Colors

To make these temperature differences easier to see, meteorologists use enhanced infrared imagery. This applies color scales to represent specific temperature ranges. For example, deep convective towers might be highlighted in bright colors to stand out against warmer, lower-level clouds.

Warning: Color assignments are subjective display choices made by the software developers. A specific color (like red) does not always represent the same temperature across different satellite products. Always consult the product legend to ensure accurate interpretation.

Water-Vapor Imagery: Observing the Atmosphere Aloft

While visible and IR imagery focus heavily on clouds, water-vapor imagery is used to detect moisture patterns in broad atmospheric layers high above the surface. By utilizing specific thermal bands, this imagery allows meteorologists to trace atmospheric circulation and identify large-scale weather patterns, even in areas where no clouds are present.

It is a common mistake to assume that dark regions in water-vapor imagery indicate dry air at the surface. In reality, dark shades often indicate warmer emission coming from deeper, drier layers of the atmosphere. The air at the surface could still be quite humid even if the water-vapor imagery appears "dark".

Summary of Selection Criteria

Choosing the right imagery depends entirely on the meteorological question being asked: