Heat and temperature are two distinct, but closely related, concepts. Heat is a measure of the quantity of energy contained in a substance. It is the total amount of molecular vibration (energy) in a material. Temperature, on the other hand, is the average energy of its molecules. Temperature is a measure of how fast molecules are moving within a substance. It is an indicator of the level at which the heat energy exists.
Heat Units
Heat is measured in several ways, discussed briefly below.
British Thermal Unit
A British thermal unit (BTU) is the amount of heat energy required to raise the temperature of 1 lb of water (measured at 60°F at sea level) by 1° F. Common materials that burn store a standard amount of heat energy per pound. This information is valuable to firefighters when they are calculating the amount of water required during fire extinguishing operations and to fire protection engineers when they are designing and installing fire extinguishment systems and equipment.
One BTU is equal to 252 calories (metric heat unit), 3.96 large calories (kilogram calorie), or 1,055 joules (mechanical heat unit). Below is a list of some common combustibles and their equated latent heat of combustion: 
Calorie
A calorie is the amount of heat energy required to raise the temperature of 1 gram of water (measured at 15 degrees Celsius [°C] at sea level) by 1° C. One calorie is equivalent to 4.184 joules.
Joule
The joule is the heat energy unit in the International System of Units (SI). It is the amount of heat energy provided by 1 watt flowing for 1 second.
Temperature Units
Temperature units can be used to compare the difference in heat energy levels between two materials. Temperature is measured by monitoring how much an object expands from its size at a given starting point (the freezing point of water, for example) and defining a unit of measurement (1 degree). All temperatures are then multiples of that defined unit of measurement.
Fahrenheit Degree
The Fahrenheit (F) degree is named for the German scientist Daniel Gabriel Fahrenheit, who invented the thermometer at the beginning of the eighteenth century. There are 180 increment degrees between the temperature of melting ice (32 degrees) and the boiling of water (212 degrees) on the Fahrenheit temperature scale. 1°F is equal to 5/9 degrees Celsius.
To convert (approximately) a temperature on the Fahrenheit scale to the Celsius or Centigrade scale, you first subtract 32 degrees from the Fahrenheit temperature and then multiply by 5/9.
For example, if a person's body temperature is 98.6°F, its temperature in Celsius is
Celsius Degree
The Celsius (C) degree is a metric unit of temperature measurement. It is named for the Swedish professor Anders Celsius, who invented the Centigrade temperature scale in the 1720s using the freezing point of water as 0 degrees and the boiling point of water as 100 degrees. This unit is approved by the SI.
To convert (approximately) normal body temperature on the Centigrade scale to the Fahrenheit scale, first multiply the Celsius temperature by 1.8, or 9/5, and then add 32.
Rankine Degree
The Rankine (R) degree is a traditional unit of absolute temperature. The temperature units for Rankine and Fahrenheit are equal (1 degree Rankine represents the same temperature difference as 1 degree Fahrenheit), but the zero points differ. The zero point on the Rankine scale is set at absolute zero, which is –457.6 degrees, the hypothetical point at which all molecular movement ceases. The unit is named for British physicist and engineer William Rankine (1820–1872).
To convert degree units from the Rankine scale to the Fahrenheit scale and the Fahrenheit scale to the Rankine scale use the following formulas:
F = R – 457 and R = F + 457
Kelvin Degree
The Kelvin degree (K) is equal to the Celsius degree, but the Kelvin scale has its zero point set at absolute zero, which is –273.1. This unit is approved by the SI. The Kelvin degree is named for British inventor and scientist William Thompson, who was knighted by Queen Victoria in 1866 and named Baron Kelvin of Largs in 1892.
To convert degree units from the Kelvin scale to the Centigrade scale and the Centigrade scale to the Kelvin scale, use the following formulas:
C = K – 273 and K = C + 273
Heat Transfer
Heat can be transferred to other materials through conduction, convection, radiation, and direct flame contact.
Conduction
Conduction is the transfer of heat energy through a medium (usually a solid). Heat causes molecules within the material to move at a faster rate and transmit their energy to neighboring molecules. The heat of conduction can also be transferred from one material to another via direct contact in the same fashion as internal molecular movement. The amount of heat transferred and rate of travel is dependent on the thermal conductivity of the material. Dense materials (metals) are good conductors of heat energy. Fibrous materials (wood, paper, cloth) and air are poor conductors. In a fire situation, heat can be conducted via steel columns and girders to abutting wood floor joists causing them to smolder and eventually ignite.
Convection
Convection is the transfer of heat energy through a circulating medium (liquids and gases). During firefighting operations, hot air expands and rises, as do the products of incomplete combustion. Fire spread by convection is mostly in an upward and outward direction through corridors, stairwells, and shafts from floor to floor via hot air currents.
Radiation
Radiation is the transfer of heat via infrared or ultraviolet waves or rays. These heat waves travel in a straight line through space at the speed of light in all directions and are not affected by the wind. Objects exposed to radiated heat will absorb and reflect a certain amount of heat energy, depending on certain factors. The darker and duller the object, the more heat it will absorb and the greater chance it will reach its ignition temperature and burst into flames. Light-colored, shiny objects tend to reflect radiated heat, absorb less energy, and are less likely to reach their ignition temperature. Radiated heat waves will travel through space until they are absorbed by an opaque object. These waves will pass through air, glass, transparent plastics, and water. Large amounts of radiated heat can travel large distances (50–100 feet) to ignite nearby buildings and structures.
Direct Flame Contact
Direct flame contact is the transfer of heat energy via direct flame impingement or auto-exposure, such as occurs with a flame traveling upward and outward from a roof, window, or doorway to a neighboring building or exposure.
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