Temperature

Air temperature can basically be described as how cold or how warm something feels from contact with another object.  Temperature is usually measured with a thermometer.  What thermometers actually measure is the average kinetic energy of air molecules that surround it.  The molecules of air are in constant motion.  The energy of molecular motion is in the form of heat which is measured as temperature.  Air molecules in cold air move slowly compared to molecules in warmer air.  Kinetic energy  in air molecules is directly related to the velocity of the molecules.  So, colder air has less kinetic energy than warmer air.  When air molecules collide with a thermometer, kinetic energy is transferred from the air molecules to the glass and in turn to the mercury inside the thermometer.  As a result the mercury molecules inside the thermometer begin to move faster and further apart, pushing the mercury up the thermometer's scale.  In colder air, the energy from the air molecules collide with the thermometer transferring less kinetic energy as compared to warmer air.  In turn the mercury molecules move slower in colder air and the mercury inside of the thermometer contracts. 
 
Temperature and Kinetic Energy (image) 

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