Summer begins, astronomically, at around 6:10 pm CDT today, Wednesday, June 20th. This time marks a point in the earth’s orbit around the sun.
It is significant because the earth is tilted slightly as in spins, 23.5 degrees to be precise. Because it’s always tilted in the same direction relative to the plane of the earth’s orbit, the Northern Hemisphere (everything north of the equator) will be tilted at the sun (summer solstice) on one extreme and away from the sun (winter solstice) on the opposite side of the orbit.
Tilt defines the seasons. A sun which appears higher in the sky can direct more energy per square unit area than a sun with lower angle. Also, lower angles mean the sunlight has to pass through more atmosphere, leading to more energy loss.
This is the longest day of year and it also has the shortest shadows.
Weather-wise, not much happens!
I recorded a lecture this morning for my summer session students which you might find useful: