HOUSTON – Ahhhh, the first day of fall. When the leaves change color, temperatures cool and we can order a pumpkin spiced latte that warms our hands as well as our hearts. But not in Houston. We’re looking at a high in Houston of 96° Saturday. This prompts the question, when does fall start in Houston?

The autumnal equinox:

It’s important we understand what the first day of fall really means. The autumnal equinox has nothing to do with temperatures. It has everything to do with the Earth/sun relationship.

Saturday morning Sept. 23, at 1:50 a.m. the sun’s direct rays will hit the equator. This is an instant in time which occurs worldwide when the sun is directly over the equator. This is fall equinox. The word equinox comes from Latin and means “equal night.” In Houston we don’t get a perfect 12 hours of daylight and 12 hours of night on this day, but it’s close. Sunrise is at 7:10 and sunset is 7:16. What is cool about tomorrow is the sun rises east and sets due west.

The sun is not a point in the sky, it’s a disk. The scientific definition for the time of sunrise, as defined by the United States Naval Observatory, is when the top edge of the Sun is first visible (at sea level). Similarly, the definition of sunset is when the top edge drops below the horizon. This difference gives us 2-3 minutes more sunlight each day. The atmosphere also refracts or bends sunlight, making the Sun appear on the horizon while it is still physically below the horizon.

The equilux:

The equilux is the day closest to equal day and night. That day is Sept. 27, when the sun is above the horizon for 11 hours, 59 minutes and 31 seconds.

We will get cooler:

It has been a brutal summer. From record highs, record morning warmth and triple digit days. What fall does bring is shorter days. In one and two months our daylight will drop by an hour. Not as much sun will translate to cooler temperatures. And we are also at the time of year we get our first real cold front. A front that will drop temperatures to the 70s and lower the humidity. It’s not on the 10-day forecast yet, but it will get here sometime in October.