A traditional thermometer consists of mercury or another liquid in a glass tube and operates on the principle that the liquid expands more that the glass does when heated. New electronic devices include a thermocouple or thermister that send an electronic signal measuring temperature to the datalogger. These new probes can also be used to measure soil temperature. Commonly measured with a mercury barometer, a glass tube in which the height of a column of mercury fluctuates as the weight of the atmosphere changes. Changes in atmospheric pressure signal shifts in the weather.
Humidity refers to the amount of water vapour the air contains.
Cool air holds less water than warm air. A Psychrometer is an instrument used to measure relative humidity. It traditionally consists of two thermometers, one covered with a wet cloth. Evaporation cools this thermometer below the actual air temperature, recorded on the dry thermometer. Evaporation and cooling depends on how dry the air is at a given temperature.