Most thermal images are single band images and by default are displayed as greyscale images. Lighters or brighter areas indicate areas that are warmer, while darker areas are cooler. Single band thermal images can also be displayed in pseudo-color to better display the variation in temperature. Thermal imagery can be used for a variety of application including estimating soil moisture, mapping soil types, determining rock and mineral types, wildland fire management and identifying leaks or emissions. Multiband color composites can also be created if multiple wavelengths of thermal emission are recorded. An example of this is the TIMS image shown on the previous page.
Time of Day
Thermal imagery can be acquired during the day or night but can produce very different results because of a variety of factors. Some of these factors are thermal conductivity, thermal capacity and thermal inertia. Thermal conductivity is the property of a material to conduct heat or a measure of the rate at which heat can pass through a material. For example heat passes through metals much faster than rocks. Thermal capacity is a measure of how well a material can store heat, water has a very high thermal capacity. Thermal inertia measures how quickly a material responds to temperature changes. Based on these factors different materials warm and cool at different rates during the day and night. This gives rise to a diurnal cycle of temperature changes for features at the Earth's surface. The diurnal cycle encompasses 24 hours. Beginning at sunrise, the Earth begins to receive mainly short wavelength energy from the Sun. From approximately 6:00 am to 8:00 pm, the terrain intercepts the incoming short wavelength energy and reflects much of it back into the atmosphere. Some of this energy is is absorbed and then emitted as long-wave, thermal infrared radiation. Emitted thermal radiation reaches its peak during the day and usually lags two to four hours after the midday peak of incoming shortwave radiation, owing to the time it takes to heat the soil. Daytime imagery can contain thermal “shadows” in area that are shaded from direct sunlight. Slopes may receive differential heating depending on their orientation in relation to the sun (aspect) . In the above daytime image of the Las Vegas area the topography and topographic shadows are clearly visible.
The above graph shows the diurnal radiant temperature variation for rocks and soils compared to water. Water has relatively little temperature variation throughout the day. Dry soils and rocks on the other hand heat up more and at a quicker rate during the day. They also tend to cool more at night compared to water. Around dawn and sunset the curves for water and soils intersect. This point is known as the thermal crossover, which indicate times where there is no difference in the radiant temperature of materials.
Water generally appears cooler than its surrounding in the daytime thermal images and warmer in nighttime imaging. The actual kinetic temperature of the water has not changed significantly but the surrounding areas have cooled. Trees generally appear cooler than their surroundings during the day and warmer at night. Paved areas appear relatively warm during the day and night. Pavement heats up quickly and to higher temperatures than the surrounding areas during the day. Paved areas also lose heat relatively slowly at night so they are relatively warmer than surrounding features.
Things to Consider
Thermal infrared sensors can be difficult to calibrate. Changes in atmospheric moisture and varying emissivities of surface materials can make it difficult to accurately calibrate thermal data. Thermal IR imagery is difficult to interpret and process because there is absorption of thermal radiation by moisture in the atmosphere. Most applications of thermal remote sensing are qualitative, meaning they are not employed to determine the absolute surface temperature but instead to study relative differences in radiant temperature. Thermal imagery works well to compare the relative temperatures of objects or features in a scene.
It is important to note that thermal sensors detect radiation from the surface of objects. Therefore this radiation might not be indicative of the internal temperature of an object. For example the surface of a water body might be much warmer than the water temperature several feet deep, but a thermal sensor would only record the surface temperature.
There are also topographic effects to consider. For example in the northern hemisphere, north facing slopes will receive less incoming shortwave solar radiation from the sun and will therefore be cooler. Clouds and fog will usually mask the thermal radiation from surface features. Clouds and fog are generally cooler and will appears darker. Clouds will also produce thermal cloud shadows, where ares underneath clouds are cooler than the surrounding areas.
Explore: Supplemental Thermal Imagery Activity
Explore Landsat 8 Thermal Imagery. Download thermal.zip and extract the files. Open all of the .dat files in ENVI. There are three Landsat files of Humboldt County acquired by Landsat 8 in October of 2015. The three files consist of a Landsat reflective image (Bands 1-7) and two thermal images. The first image was obtained during the day at the same time as the reflective image, the other image is a thermal image captured the following night. Compare the differences between the two thermal images. Use the Crosshairs or Cursor Value tools to examine the differences in temperatures of different land features. Also explorer how the temperatures change between daytime and nighttime. Note that the data values for the thermal images will be the radiant temperatures in Kelvin.