Weatherwise, there are plenty of differences between today, May 21 and two months from now, July 21. The most obvious is that it's cooler now - averages this time of year are highs of 75^o and lows of 55^o. Those numbers will go up to 87^o and 68^o in two months.

But May 21 and July 21 share something in common. Both days are just about exactly one month away from the summer solstice - June 21, the first day of summer. Therefore, based solely on the sun's position in the sky, you can't tell the two days apart. That's because on both days, the sun not only rises at the same time and in the same place, it also follows the same path across the sky, and then sets at the same time and same place.

This begs the question: why, then, is July 21 so much warmer, on average, than May 21? Well, it's all a matter of accumulating energy. As of today, the sun is still rebounding from its relatively low position in the sky during the preceding colder months. There hasn't been much time to really heat things up. But by July 21, two more months of relatively intense sunshine will be in the pocket, and it shows by the higher average temperature.

This kind of comparison isn't unique to late May and late July - it works for any pair of days that are both the same number of days away from the summer solstice.