Onions are often defined by the hours of daylight required for the plant to grow bulbs.  These are know as short day, intermediate, and long day onions. Short day onions begin to develop bulbs when the day length increases to a length of 10 to 12 hours.  These onions are best grown in the American South, and generally sweeter in taste.  Their high sugar and water content make them best used for cooking and immediate use, not storage.  Long-day onions require 14 to 16 hours of sunlight to grow bulbs and are best planted in the American North.  With a low sugar , and high sulphur content, these onions are best for storage, or immediate use in the kitchen.  Growing the wrong day length onion in the incorrect area will result in onions that don't grow to their full potential, so purchasing the proper day length is the first tip to success.  

If you were to draw an imaginary line across the map of the country from San Francisco to the tip of South Carolina: gardeners north of the line should plant long day, while gardeners south of the line should plant short day in winter for a spring/summer harvest, while intermediate works well for north and south.

Example of a short day onion ~ Gabriella. Great sweet flavor, does not store well.

Examples of a long day onion ~ Walla Walla, Rossa Di Milano, excellent keepers.