The start of spring in the United States is defined by two distinct systems: one based on the Earth’s orbit (astronomical) and one based on fixed calendar dates (meteorological). Both definitions accurately track the transition from winter, depending on whether one focuses on astronomy or climatology. Understanding both perspectives clarifies why a single, definitive start date for spring does not exist.
Defining Spring by the Vernal Equinox
The traditional start of spring is determined by astronomy, marked by the vernal equinox. This event occurs when the sun appears to cross the celestial equator, moving from the Southern into the Northern Hemisphere. At this precise moment, the Earth’s axis is neither tilted toward nor away from the sun.
For observers in the Northern Hemisphere, the vernal equinox typically falls on March 19th, 20th, or 21st. This milestone signifies that daytime and nighttime are of nearly equal length across the globe. After the equinox, the Northern Hemisphere receives more direct sunlight, causing days to progressively grow longer until the summer solstice. Astronomical spring lasts until the summer solstice, which occurs around June 20th or 21st.
The Fixed Calendar Approach to Spring
Weather scientists and climatologists use the meteorological season definition for spring. This system uses fixed calendar months to divide the year into four three-month seasons, simplifying the calculation of seasonal statistics. For the United States, meteorological spring encompasses March, April, and May.
Under this fixed calendar approach, spring consistently begins on March 1st and ends on May 31st every year. This aligns the seasons with the annual temperature cycle, making temperature records easier to compare when based on whole, consistent months. This method provides a reliable framework for comparing weather patterns without the annual variability of the astronomical equinox.
Why the Start Date Is Not Always the Same
The astronomical start date of spring is not fixed to a single calendar day because of how the Gregorian calendar interacts with the Earth’s orbit. A tropical year—the time it takes for Earth to orbit the sun—is approximately 365.25 days. Since the calendar year is only 365 days long, the equinox occurs roughly six hours later each subsequent year.
This six-hour drift is largely corrected every four years by inserting a leap day on February 29th. The leap day effectively “resets” the date of the equinox back to an earlier calendar day, allowing the date to cycle between March 19th, 20th, and 21st. Due to these time shifts and the U.S.’s wide range of time zones, the vernal equinox has not occurred on March 21st in the continental U.S. during the 21st century.