Temperature, Wind, Humidity and Comfort

Comfort is very important in any built space. It is governed by the surrounding environment.

For humans, comfort usually means:
  1. Temperature –  ~21 °C to ~24 °C   (According to the West Midlands Public Health Observatory, UK)
  2. Humidity –  25–60% Relative Humidity
  3. Wind speed –  about 1m/s for every 3 °C rise in temperature
What does all this mean?
There are two ways of measuring temperature; Dry Bulb Temperature (DBT) and Wet Bulb Temperature (WBT). The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. DBT is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. The wet-bulb temperature is the temperature a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel.
Relative humidity (abbreviated RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. Relative humidity depends on temperature and the pressure of the system of interest. Absolute humidity is the total mass of water vapor present in a given volume of air. It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C. The absolute humidity changes as air temperature or pressure changes. Specific humidity (or moisture content) is the ratio of water vapor mass ( mv ) to the air parcel’s total (i.e., including dry) mass ( ma ) and is sometimes referred to as the humidity ratio. Specific humidity is approximately equal to the “mixing ratio”, which is defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel.
Wind speed is, well, wind speed! The relation I gave above came from a graph like this one:

Wind direction also matters a lot in providing proper ventilation. This diagram shows how wind flows on the surface of the Earth. The main reason for wind flow is non-uniform heating of the atmosphere by the Sun.

Also, a psychometric chart is a graph of the thermodynamic parameters of moist air at a constant pressure, often equated to an elevation relative to sea level.

This can also be used to determine comfort levels in specific conditions.

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