• Weather is the state of the atmosphere at a given time and place
• Climate represents average weather conditions for a place over a long time period (years)
• The distribution of solar radiation regulates the seasons
• The tilt of Earth's axis is the principal reason for variations in incoming solar radiation
• The intensity of incoming solar radiation decreases from the equator toward the poles
• The Sun is directly over the Tropic of Cancer during Summer in the Northern Hemisphere, and it lies above the Tropic of Capricorn during Winter

eather represents the state of the atmosphere at a given time and place. Characterizing weather requires that we measure conditions such as temperature, precipitation, air pressure, wind speed and direction, humidity. In contrast, climate is the average weather conditions for a site measured over a long time period (years). We must review weather data for several decades to thoroughly characterize a region’s climate. 

Maximum temperatures across the continental U.S. on January 1, 1996.  High temperatures ranged from the high 80's (Florida) to freezing (northern Maine). The daily temperatures represent weather conditions but the overall temperature distribution (cold in the north, warm in the south) reflects regional climate patterns. Click on the image to view an animation of maximum temperatures recorded for January 1 for a 10-year cycle (1987-1996). Image courtesy of the National Climatic Data Center.
Maximum Temperature	Precipitation	Maximum Humidity
Click on the images above to view animations of maximum temperature, precipiation, and maximum humidity for the continental U.S. on the first day of each month for 1995. Regional climate patterns result in warm humid conditions (high temperatures, high precipitation, high humidity) in the southeastern U.S. and hot, dry conditions (high temperatures, low precipitation, low humidity) in southwestern states.

Weather and climate are the result of a complex series of interactions between all elements of the earth system (hydrosphere, atmosphere, biosphere, solid earth) but are largely controlled by the interaction between the Earth and Sun. The distribution of solar radiation on the Earth's surface regulates the length and order of the seasons (more solar radiation in summer, less in winter) and drives many of the processes that contribute to weather (e.g. evaporation). We are all aware that temperatures are lower in winter and warmer in summer but few people know why. (What is your answer?)

Surely, there are few more basic scientific questions than: Why is it colder in winter than in summer? Yet even graduating seniors at a prestigious eastern University were unable to answer the question correctly (more than 90% got the it wrong). The most common explanation given was that Earth was closer to the Sun in summer and further away in winter - unfortunately, the exact opposite is true. Earth's orbit is a little uneven and the planet comes closest to the Sun during winter in the Northern Hemisphere (January 3) and is farthest away during Summer (July 4).

Earth is farthest from the Sun at its aphelion and closest during its perihelion.

	The Sun is overhead at the Tropic of Cancer on June 21 and at the Tropic of Capricorn on December 21. It is overhead at the equator during the Spring and Fall equinoxes.

The principal reason for the seasonal differences in climate around the globe is the tilt of Earth's axis. Earth rotates around an axis that is tilted 23.5 degrees to vertical. The Tropics of Cancer and Capricorn are located 23.5 degrees north and south of the equator respectively. Insolation is greatest when the Sun is directly above a location on Earth and decreases as the angle of the Sun's rays becomes more oblique. The axial tilt places the Sun directly overhead at the Tropic of Cancer in the Northern Hemisphere during the Summer solstice (June 21). Likewise, the Sun's rays strike the Northern Hemisphere more obliquely when the Sun lies over the Tropic of Cancer in the Southern Hemisphere during the Winter solstice (December 21). The Arctic and Antarctic Circles are located 66.5 degrees north and south of the equator (or 23.5 degrees south and north of the North and South Poles respectively).

Day and night would each last exactly 12 hours everywhere on the globe if Earth's axis was vertical. In contrast, the hours of daylight change at each point in the Northern Hemisphere from a maximum during the Summer solstice to a minimum on December 21 when the Sun is directly overhead at the Tropic of Capricorn. Day and night are split equally during the equinoxes. The length of each day increases traveling northward during Summer in the Northern Hemisphere and decreases southward in the Southern Hemisphere. Perpetual daylight (24 hours) occurs at the North Pole, while the South Pole is in darkness. This pattern is reversed during the Winter solstice when the South Pole is illuminated for 24 hours and the North Pole is in darkness.

The tilt of Earth's axis results in twenty-four hour daylight at the North Pole and almost complete daylight north of the Arctic Circle during summer in the Northern Hemisphere (left) and perpetual darkness during winter (right). The situation is reversed south of the Antarctic Circle.

Solar radiation strikes the earth more directly at the equator and tropics than in polar regions. More heat is therefore transferred to the earth in the tropics than at the poles. Global atmospheric circulation patterns represent the planet’s attempt to move warm air toward the poles and cold air toward the equatorial region. These patterns are complicated by Earth's rotation, dividing into three large convection cells in the Northern and Southern Hemispheres that control climate patterns. Global temperature and precipitation patterns are directly related to global atmospheric circulation patterns and differentiate climate regions across the globe.

Areas with consistent climates are grouped together in climate regions. Climate regions are differentiated based upon monthly temperatures, monthly precipitation, and precipitation values. Archeological, historical and geological records indicate that climate has changed during Earth's history. These climate records give indirect evidence of past climate change over both the long-term (millions of years) and short-term (hundreds or thousands of years).

The cause of long-term global climate changes (Causes of Climate Change) are processes that operate on a over intervals measured in millions of years. The most likely causes are associated with the changing locations of continents and oceans (plate tectonics) that in turn affect atmospheric and oceanic circulation patterns. Short-term climate fluctuations occur on cycles lasting thousands of years are related to variations in the earth’s orbit around the sun that cause the amount of insolation (incoming solar radiation) to vary with time.