SIOUX CITY, Iowa (KCAU) — Time to get the swimsuits out, stock up on sunscreen, and start planning those days at the lake, or even just at your local pool. Love it or hate it, summer is almost here!

Meteorologically, summer begins in just over three weeks on June 1st. However, officially, summer begins with the summer solstice which will take place on June 21st this year.

Why are there two different start dates for summer though?

The answer is fairly simple.

In the meteorology field, we keep lots of records, including rainfall and temperature data. This includes snowfall totals, rainfall totals, overall precipitation totals, daily high and low temperatures. This information is then used when we look at climatology, for example average monthly snowfall and temperatures. We use that climatology derived from recorded data for a variety of things, like when looking at drought conditions. We are able to compare a certain year’s precipitation to the current year. Or we could use all the data from multiple years to look at the average over a specified period of time and compare it to what’s currently happening and use that information to forecast what might happen (ex: will drought conditions worsen, improve, or stay the same?), based on that information.

However, if we recorded seasonal data based on astronomical seasons, there would be a degree of variability from year to year since the date of solstices (summer and winter) and equinoxes (spring and fall) shift dates back and forth every year.

To keep things more consistent in the interest of meteorological record keeping, we instead break the seasons into 4 different 3-month periods: December, January & February (Winter); March, April, & May (Spring); June, July, & August (Summer); and September, October & November (Fall). These groupings are based on annual temperature cycles, with winter, of course, being the coldest period and summer being the hottest. With these seasons being the span of three entire calendar months, instead of varying periods of time, since astronomical seasons span from either: solstice to equinox (summer/winter) or equinox to solstice (spring/fall), and the date of each equinox and solstice varies from year to year, the length of each meteorological season remains consistent, between 90 days for winter (91 in a leap year) and fall and 92 days for spring and summer.

As a result of the minimal yearly variation in meteorological seasons, this makes it easier for record-keeping purposes since there are precise start and end dates for each seasonal period from year to year. In turn, this makes it much easier to calculate seasonal statistics since the seasons aren’t beginning and ending in the middle of months, so the entire month is within a single season instead of, for example, the first half of June being considered spring and the second half being considered summer. This allows meteorologists and climatologists to simply use entire data sets from equinox/solstice months when looking at information like seasonal temperature averages and ranges or average seasonal precipitation.

While meteorological seasons line up with calendar months, astronomical seasons are based on the natural rotation of the sun. The vernal (spring) and autumnal equinoxes take place when the sun passes directly over the equator and has no tilt towards the sun, resulting in equal lengths of daytime and nighttime. The summer and winter solstices take place when the Earth’s tilt towards the sun is at its maximum. After the vernal equinox in the northern hemisphere, day length increases through the summer solstice, which marks the longest day of the year, with the sun directly overhead in the Tropic of Cancer located at latitude 23.5° north. After the summer solstice, day length begins to decrease again, with equal day and night length again on the autumnal equinox. Day length continues to decrease until the winter solstice, which marks the shortest day of the year with the sun directly overhead in the Tropic of Capricorn at 23.6° south.

The equinoxes mark the first days of spring and fall, this year the vernal equinox took place on Monday, March 20th at exactly 3:24 PM CST and the autumnal equinox will occur on Friday, September 22nd at 12:50 CST.

Likewise, the solstices mark the first days of summer and winter. The summer solstice this year will occur on Wednesday, June 21st at exactly 8:58 AM CST and the winter solstice will take place Thursday, December 21st at exactly 9:27 PM CST.

For more information on seasons, both astronomical and meteorological, visit: https://www.ncei.noaa.gov/news/meteorological-versus-astronomical-seasons