The term comes from the greek word "klinein" meaning "to slope." It evolved into "klima" connoting a zone or region of the Earth as characterized by its atmospheric conditions.
When the sun at noon sits above the Tropic of Cancer or Tropic of Capricorn. The summer solstice has the longer period of daylight in the year and the winter solstice has the shortest period.
Sunlight falling upon the Earth warms the planet. The amount of sunlight however is not constant since the orientation of the earth to the sun changes.
Certain gases in the atmosphere trap the heat of the sun, causing the temperature of the Earth's atmosphere to become hotter than it would be otherwise.