In short, +5°F.
In detail, official temperature records for Houston date back to 1883 from multiple “official” weather stations. These locations have included:
(1) Houston Downtown (various locations) (1883-1990);
(2) Houston Hobby Airport (KHOU) (1930-2018); and
(3) Houston Intercontinental Airport (KIAH) (1969-2018)
KIAH is currently and has been the official weather station for the city of Houston since around the time the airport opened in 1969. Here is the history of record keeping in Houston from the National Weather Service:
“The history of the Houston office can be traced as far back as 1881, when a cotton station (in an unknown location in Harris County) was used as a base for taking weather observations. As the city of Houston grew, the need for a Weather Bureau Office resulted in the establishment of the Houston office downtown on September 16, 1909. Between 1909 and 1968, the Houston Weather Bureau office occupied three separate locations in the downtown area – the Stewart Building (at Preston and Fannin Streets), the Shell Builiding (at Texas and Fannin Streets) and its last location downtown at the Federal Building (at Franklin and Fannin Streets). The expanding needs of the old Weather Bureau forced a move out of the downtown area. In 1970, the National Weather Service moved the Houston area office to Alvin (southwest of Houston). Around that same time, as a part of the move out of downtown, the weather observations part of operations moved to the its now current site at Houston Intercontinental Airport in northern Harris County.”
Former local meteorologist Anthony Yanez blogged about this very topic in 2009, writing…
“The choice to have Bush Intercontinental as the official recording station came from the aviation community. There was pressure to have observations at the airports as the airline industry grew. Since human observers were required to take the readings, it did not make financial sense to keep multiple observation sites in place, so downtown office observation sites were moved to the airports. The switch was not driven by climate parameters but rather by aviation interests.”
So, what is the lowest temperature ever recorded in Houston? The lowest temperature ever officially recorded in Houston was +5°F set twice: once in Downtown Houston (1930); and once again at Houston Hobby Airport (1940). Here is a table with the lowest temperatures ever officially recorded in Houston:
Interesting to note that Houston has never seen below-zero temperatures in recorded history. In fact, temperatures have only been below +10°F on four days since 1883 (note: the two readings of +7°F and +9°F in December 1989 occurred on the same day at two different locations).
Billy Forney III 
[…] While there isn’t a real strong trend in this data, an interesting observation can be made about the dispersion of temperatures around the 10-period moving average (shown as the green line in the graph above). In the early 1900s, the dispersion appears quite large, but beginning in the 1990s, the dispersion around the moving average became significantly less. Why this is the case is not easily understood from the data, but some possible explanations are possible. For example, perhaps better recording devices and techniques were used in later years. Or, because the official reporting sites in the early 1900s were located at various locations in downtown Houston, it’s possible that small differences in the microclimates caused wider variations in temperatures. Taking this possibility further, if you look at the graph starting around 1970 (midway between 1960 and 1980 in the graph), the dispersion around the moving average seems to be much less variable after 1970 than before. This could be because the official reporting site was moved to its current location at Houston’s Intercontinental Airport. I mentioned this in a recent blogpost. […]
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