Science of the summer solstice

ST. JOSEPH, Mo. (News-Press NOW) -- It’s official, St. Joseph— summer has arrived! The first day of summer, also known as the summer solstice, will occur on Friday, June 20, 2025. This marks the point when the Earth’s Northern hemisphere is tilted most directly toward the sun making it the longest day and shortest night of the year, the opposite will occur in the Southern hemisphere.

On June 20, St. Joseph will enjoy 15 hours and 2 minutes of daylight, with the sun rising at 5:49 a.m. and setting at 8:52 p.m. Starting June 21, the days will gradually begin to shorten. By the time the winter solstice arrives on Dec. 21, 2025, daylight will shrink to just 9 hours and 24 minutes. So soak up the sunshine while you can—summer is here, but the daylight clock is already ticking.

Longer days are a positive for many, especially those with a love for the outdoors. More solar energy also leads to higher temperatures, hence the relationship between astronomical and meteorological seasons. While duration and intensity of sunlight ultimately drive seasonal temperature changes, there is actually a lag between the longest day of the year and the hottest time of the year climatologically. 

For example, the average high temperature for St. Joseph is around 86 degrees during the summer solstice, about one month later, the average high peaks at 88 degrees in mid to late July. Of course, every year is different when it comes to the actual hottest day due to a multitude of factors such as cloud cover and regional weather patterns.

As if right on time with the start of astronomical summer, the first heat wave of 2025 will roll across America’s heartland this weekend. High temperatures will likely remain below 100 degrees locally, although mid 90s are in the forecast for several days in a row. Combined with humid air, this first stretch of high heat will be enough to pose risks for vulnerable populations, especially following relatively mild temperatures so far this season.