Infrared Satellite Imagery

Some of the incoming radiation from the sun is absorbed by the Earth, and re-radiated at longer wavelengths. Much of this infrared radiation is absorbed by clouds and water vapour in the troposphere, and is then re-radiated. Satellite radiometers measure the radiance of the emitting surface. Radiance is the rate of emission of electromagnetic energy per unit area of a surface, and it depends on the temperature of the surface. A simple way of thinking about this is the difference between white hot and red hot — the object that is white hot has a higher temperature.

When we are looking at infrared satellite images, what we are really looking at is the temperature of the emitting surface. The surface could be cloud, land or ocean, but important is cloud top temperatures. Note that it’s the temperature of the cloud tops that will be measured.  Because the temperature of the atmosphere at different levels is known, the tops of the clouds can be calculated. No information about cloud bases can be determined from infrared satellite images.

The images are usually coloured to make them easier to use, with darker colours usually indicating warmer cloud tops and brighter colours indicating cold cloud tops. 

On MetFlight, one image is available per hour. These images are available 24 hours per day, because the Earth continues to emit long wave radiation at night.

Let's look at an example

In the image on the right, we can see there is an area of colder cloud tops (around -40°C) over the south of the South Island, but there is no information about the depth of the cloud layer. We can also see an area of relatively warm cloud tops along the South Island West Coast, and what appears to be a cloud free area off the Canterbury coast. One of the major limitations of infrared satellite imagery is that if the cloud tops are a similar temperature to the land or sea, they will not be easy to distinguish. A particular example of this to be aware of is when there is fog or stratus, because this will be a relatively thin layer, with a temperature very close to the surface below (sea or land). At night, when fog and stratus are more common, its presence cannot be determined from infrared images alone, however, once the first visible satellite image is available, the areas will become obvious.

Invercargill

Above mean sea level

TCu

Towering Cumulus

agl

above ground level

Cb

Cumulonimbus

Flight Information Service communications

ATS

Air Traffic Service

Royal New Zealand Air Force

in vicinity of aerodrome Thunderstorm

ATC

Air Traffic Control

VC

in vicinity of aerodrome

Probabilityxx%

TIL

Until

FM

From

Becoming

Temporarily

Routine air report from aircraft in flight

FIR

Flight Information Region

Internet Flight Information Service

VFR

Visual flight rules

Automatic Terminal Information Service

BWR

Basic weather report

New Zealand Flight Information Region

Aerodrome special routine meteorological report

Automatic aerodrome routine meteorological report

Aerodrome routine meteorological report

TAF

Aerodrome forecast

Area forecast

Significant meteorological information