Temperature

Temperature is a measure of molecule agitation in a substance, which is represented as the hotness of a body. Therefore, temperature can be thought of as a measure of the hotness of a body.

Factors determine the temperature at the earth’s surface (i.e., why is it hot in the tropics and cold at the poles)?

The temperature at the earth’s surface and the lower atmosphere where most of the weather is found depends on two factors:

1. How much heat energy in the form of short-wave energy (solar radiation) reaches the earth’s surface. This depends on the following factors:

a. Latitude. The more directly the sun’s rays hit the earth’s surface, the greater is the quantity of heat energy transferred and therefore the greater is its temperature. The sun is directly overhead in the tropics, and this is where the intensity of insulation is greatest.

b. Season. The earth’s axis of rotation is tilted with reference to its orbit. This tilting creates the same effect as a change of latitude and is the primary cause of the different seasons and associated temperature changes throughout the year.

c. Time. The time of day determines the amount of the sun’s heating on the earth’s surface. The highest temperature of the day is not experienced until later in the afternoon, at approximately 1500 hours during the summer months.

2. The energy absorption (and retention) capacity of the surface.

a. Absorption. The type of surface determines how much heat energy is absorbed, i.e., its reflective quality.

b. Specific heat capacity of the surface.

Difference between Celsius and Fahrenheit

Celsius and Fahrenheit are both scales used for measuring temperature. The Celsius scale (°C) divides the temperature between the boiling point and the freezing point of water into 100 degrees. That is, the boiling point of water is 100°C, and the freezing point of water is 0°C.

Note: The Celsius scale is the most commonly used form of temperature measurement. The Fahrenheit scale (°F), is based on water boiling at 212°F and freezing at 32°F.

What is the formula to convert Celsius and Fahrenheit?

The formula to convert Celsius to Fahrenheit is °F = 9/5 (x°C + 32)
The formula to convert Fahrenheit to Celsius is °C = 5/9 (x°F – 32)

OAT

OAT is the ambient outside air temperature.
Describe SAT. SAT is the ambient static air temperature. This is commonly used as a different name for outside air temperature (OAT).

TAT

TAT is the total air temperature indicated on the air temperature instrument; it is a product of the static air temperature (SAT) and the adiabatic compression (ram) rise in temperature experienced on the temperature probe.

Note: Therefore, TAT is a higher temperature than outside air temperature (OAT) whenever there is an airflow into the temperature probe, which is sometimes referred to as a heating error when you need to calculate the actual OAT. This heating error can remain after a flight due to the residual heat left in the probe, which is why you sometimes have a difference in OAT and TAT shortly after a flight.

How does a change in air temperature affect an aircraft’s flight level?

An air temperature that differs from the international standard atmosphere (ISA) temperature will result in a different actual flight level (i.e., height above the ground) than the pressure level read by the altimeter. A higher than ISA air temperature makes the air less dense and lighter in weight, causing the density altitude to differ from the pressure altitude. This results in the actual flight level being higher than the pressure level read by the altimeter. However, a lower than ISA air temperature makes the air denser and heavier in weight, causing the density altitude to differ from the pressure altitude. This results in the actual flight level being lower than the pressure level read by the altimeter. Therefore, when flying from a high to low (temperature), beware below, because your actual flight level (and therefore ground clearance) is lower than indicated by your altimeter. In other words, your altimeter overreads. This high-to-low mnemonic applies equally to pressure values as it does to temperature.

How does air temperature affect relative humidity?

The amount of water vapor a parcel of air can hold depends on its temperature. That is, warm air is able to hold more water (in a vapor or liquid state) than colder air. In other words, cooler air supports less water vapor.

What is dewpoint?

The dewpoint is the temperature at which a parcel of air becomes saturated. That is, its capacity to hold water vapor is equal to that which it is actually holding, or in other words, its relative humidity is 100 percent.

Note: Dewpoint is also sometimes called the saturated temperature. The higher the moisture content in the air, the higher is its dewpoint temperature.

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