Self-contained breathing apparatus pressure changes with ambient temperature. As temperature decreases, the pressure inside the cylinder decreases. The relationship between the temperature and the pressure of a gas is defined by the formula "PV" = "nRT". (See Universal gas constant.) What is particularly important to understand from the formula is that the temperature is in kelvins, not degrees Fahrenheit. Consider the freezing point of water at 32 °F (0 °C, 273.15 K) and compare it to 96 °F (35.6 °C or 308.71 K; normal human body temperature is 37 °C). While 96 is arithmetically three times 32, the difference in temperature from a
Brightness temperature black body; formula_7 is Planck's constant; formula_1 is frequency; formula_9 is the speed of light; and formula_10 is Boltzmann's constant. For a grey body the spectral radiance is a portion of the black body radiance, determined by the emissivity formula_11. That makes the reciprocal of the brightness temperature: At low frequency and high temperatures, when formula_13, we can use the Rayleigh–Jeans law: so that the brightness temperature can be simply written as: In general, the brightness temperature is a function of formula_1, and only in the case of blackbody radiation it is the same at all frequencies. The brightness temperature