www.physics.ubc.ca/~renyuan

Relative humidity is a percentage of water vapour per area at a specific temperature. Relative humidity is calculated at all times of the year, but the humidex is usually only given in the warmer months from May to September.

Relative humidity can be calculated using various methods and instruments. It is a calculation to find out how many grams of water vapour can be held at a given temperature. Usually, the warmer the air, the higher its capacity for holding water vapour. Each specific temperature has a holding limit for water, and the actual amount of water held in the air at the time of measurement can be represented as a percentage. For example, if a parcel of air at 20ºC has a limit or capacity of 16 grams of water vapour and 16 grams of water vapour is measured, the air's holding capacity is at its maximum and the relative humidity (RH) of the air is said to be 100%. Take the same air with 8 grams of water vapour and your RH is 50%.

P.O.P. stands for "Probability of Precipitation." It does not predict when, where or how much precipitation will occur. When a weather service issues a forecast calling for rain, it is usually followed by a probability.

For example: "P.O.P. 60%." This expression means there is a 60% chance that any random place in the forecast area, such as your home, will receive measurable rainfall. Measurable rainfall means at least 0.2 mm of rain or the water equivalent of snow.

Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. The intensity of rain is based on rate of fall. "Very light" means that the scattered drops do not completely wet a surface. "Light" means it is greater than a trace and up to 2.5 mm an hour. "Moderate" means the rate of fall is between 2.8 mm to 7.6 mm per hour. "Heavy" means over 7.6 mm per hour.