The amount of solar radiation reflected from land and sea surfaces, as well as the amount absorbed, depends partly on that portion of the spectral distribution of transmitted irradiant energy from the Sun that finally reaches these surfaces. In the Introduction, it was stated that this radiance rises rapidly to a peak at 0.48 µm, then trails off to near zero through wavelengths out to ~4.0 µm. That is confirmed in the plot shown below that also shows many of the principal water, carbon dioxide, and oxygen absorption bands.

A thermal sensor picks up radiant emitted energy from a surface target heated through radiation (solar insolation and sky radiance), convection (atmospheric circulation) and conduction (through the ground). Thus, most sensed heat from surfaces has its origin in solar illumination, that varies with both diurnal and seasonal changes as well as cloud cover, but there is also a small, nearly constant contribution from internal heat flux from the Earth's interior (much of this is due to thermal inputs from radioactive decay). Heat is transferred into and out of near surface layers owing to external heating by the thermal processes of conduction, convection, and radiation.

These properties include:

• Heat Capacity (C): The measure of the increase in thermal energy content (Q) per degree of temperature rise. It is given in cgs units of calories per cubic cm. per degree Centigrade, and its denotes the capacity of a material to store heat (recall from physics that a calorie [cal] is the quantity of heat needed to raise one gram of water by one degree Centigrade). Heat capacity is calculated as the ratio of the amount of heat energy, in calories, required to raise a given volume of a material by one degree Centigrade (at a standard temperature of 15° Centigrade.) to the amount needed to raise the same volume of water by one degree Centigrade. A related quantity, specific heat (c), is defined as (units: calories per gram per degree Centigrade) where ; this associates Heat Capacity to the thermal energy required to raise a mass of 1 g(ram) of water by 1 degree Centigrade.
• Thermal Conductivity (K): The rate at which heat will pass through a specific thickness of a substance, measured as the calories delivered in 1 second across a 1 centimeter square area through a thickness of 1 cm at a temperature gradient of 1 degree Centigrade (units: calories per centimeter per second per degree Centigrade)
• Thermal Inertia (P): The resistance of a material to temperature change, indicated by the time dependent variations in temperature during a full heating/cooling cycle (a 24-hour day for the Earth); defined as (k is a term, related to conductivity K, known as thermal diffusivity, in units of calories per centimeter squared per square root of degree Centigrade seconds ). P is a measure of the heat transfer rate across a boundary between two materials. e.g., air/soil. Because materials with high P possess a strong inertial resistance to temperature fluctuations at a surface boundary, they show less temperature variation per heating/cooling cycle than those with lower thermal inertia.

The interpretation of thermal data and images depicting temperature distribution over an area is not a simple matter. In many instances, efforts must be confined to looking for patterns of relative temperature differences rather than the absolute values because of the many complex factors that make quantitative determinations difficult, such as:

• Number and distribution of different material classes in the instantaneous field of view
• Variations in the angle of thermal insolation relative to sensor position
• Dependency of thermal response on composition, density and texture of the materials
• Emissivities of the surface materials
• Contributions from geothermal (internal) heat flux; usually small and local
• Topographic irregularities including elevation, slope angle, and aspect (surface direction relative to Sun's position)
• Rainfall history, soil-moisture content, and evaporative cooling effects near surface
• Vegetation canopy characteristics, including height, leaf geometry, plant shape
• Leaf temperatures as a function of evapotranspiration and plant stress
• Near surface (1 to 3 meters) air temperature; relative humidity; wind effects
• Temperature history of atmosphere above surface zone
• Cloud-cover history (during heating/cooling cycle)
• Absorption and re-emission of thermal radiation by aerosols, water vapor, air gases

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