Until now, the nights have been longer than the days, but on the equinoxes, there is equality between day and night. On these two days, the sun is out for exactly 12 hours everywhere on earth, said Donald Wuebbles, head of the Department of Atmospheric Sciences. The northern hemisphere's vernal equinox is the same day as the southern hemisphere's autumnal equinox, so Tuesday was the first day of fall in the southern hemisphere. Walter Robinson, associate professor in atmospheric sciences, said even though the sun shines the same amount of light on the earth's atmosphere on both the vernal and autumnal equinoxes, the earth takes time to warm up from the winter, so the temperature is not the same on both days. This explains why the weather does not necessarily cooperate with the calendar right away.
Spring is also the time for the most thunderstorm activity, especially in the Midwest.
The differences in temperature between the south and north in the United States are great, creating high potential for storm activity. This provides ideal conditions for breeding big storms so spring is our severe weather season, a time for tornadoes and the like.