San Antonio TX USDA Zones 1890 - 2024

[ChrisA]

I've been curious to look back at history to see how zones have changed in San Antonio going back to the beginning of consistent weather records (1886), using a 30-years of data to form an average lowest temperature for that period of time.  The data indicates that San Antonio is not new to a barely 9A zone designation, but has waffled on one side of the 20F mark during that period of time. So 9A is not new, nor, as many people have stated numerous times on this forum, is it indicative of the ability to grow anything more tender in this area than it has been able to support in the past 135 years.

I'm interested to know what your thoughts are on this.  

 

And the current period of only 14 seasons:

 

[ChrisA]

If we look at the zone data using 10-year averages the current decade, season to date, is appreciably colder than any other decade going back 135 years.

[Ben G.]

I have read and watched quite a bit over the last ten years about climate patterns in the US for the last 150 years or so. This isn't terribly surprising based on all I have learned. It seems that for the US as a whole, the first half of the 20th century was pretty warm. (The 30s actually being the warmest decade by raw temperature data) Followed by cooling from the 60s through the 80s. Then we had a warmer period again from the 90s until recently. I am not even sure if the US (as a whole) is experiencing colder winters, or if it is just us poor suckers in the central US.

From what I have read there are many possible natural and man-made reasons for these changes. I personally don't see anything too unusual or scary about the current cooler winter trend. I don't like it, but it may just be another decades long cycle. On the other hand...we could resume a warmer trend any time. I don't know enough to guess really. From what I have gathered, even the best qualified among us are just guessing too...with more background knowledge and credibility than I have.

[richtrav]

It’s not really fair to compare the past 4 winters to other periods that are artificially cut into decades. I would compare it to what we know were cold periods, say from the late 70s to 1985. I suspect that period would come out colder than the past 4-5 winters. Also one thing to consider is how high the maxima were in these cold outbreaks, they were generally worse in the 1980s, I believe in Dec 1990 there was one afternoon where the high was only 24 and in Feb 1989 I think there were 3 consecutive days of highs in the 20s even though it never dropped into the teens. Only 2021 can compete with that in our latest spate of cold outbreaks. 

It’s a mistake to think that winter minima are completely random, obviously there are larger patterns going on here that come and go over longer periods (usually 3 to 10 years, 12 years in the case of Florida from 1977 to 1989). That was noticed as early as the early 1800s by people in New Orleans, they noted that that the period around the turn of the 19th century had had more severe and frequent cold outbreaks than previous decades. People have their own pet theories as to why this happens. 

[ChrisA]

I see your point regarding artificially cut periods and only looking at seasonal low temps, but that is how the USDA zone maps are calculated, which is the subject of this post.  Yes, those metrics do not tell the whole story, like # of days below freezing, daytime highs, solar insolation, etc.  Those details however do not concern the zone map designations.

[richtrav]

Too lazy to enter all the numbers, but what is the 100 year average?

[ChrisA]

8 hours ago, richtrav said:

Too lazy to enter all the numbers, but what is the 100 year average?

Good Morning! The 100-year average (1924-2024) is 20.6F, for all years the average is 20.5F.

[07vette]

Where did you get the data from? I was going to try to make one of these for austin.