P - R

P

Permafrost A layer of soil beneath the earth's surface that remains frozen throughout the year.

Polar climates Climates in which the mean temperature of the warmest month is below 10ºC; climates that are too cold to support the growth of trees. 
Population A group of individuals of a particular species occupying a particular area at a specific time is known as population.
Members of a population share a common gene pool and are capable of interbreeding among themselves to produce fertile offsprings.

Psychrometer An instrument used to measure the water vapor content of the air. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.

Pyranometer An instrument that measures the amount of radiation.       

R 

Rain Precipitation in the form of liquid water drops that have diameters greater than that of drizzle.

Rain gage A device-usually a cylindrical container-for measuring rain-fall.

Relative humidity The ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at the particular temperature and pressure. The ratio of the air's actual vapor pressure to its saturation vapor pressure.