No 50s yet in Louisville, and that's about to set a record

As we look out our windows and wonder just how long our streak of sunny, warm weather will last... we are becoming increasingly confident that it WILL last long enough for us to hit a pretty striking record.

Oftentimes, once we get into the later summer season, the air dries out as weak cold fronts start to return. This drier air cools quicker and temps drop faster. Well, this year, that has been the case for most of us, but not so much in downtown Louisville.


Typically, Louisville sees a night in the 50s by the last days of August (though this has been trending later in the past ten years). For our suburbs and rural locations - those cooler nights arrived right on schedule. However, at our official observation station, Louisville International Airport, we have yet to see that happen.

As you can see by the graphic above, if we do not fall into the 50s by September 22nd we will set a record for the latest date past August 1 that we haven't dropped to this mark. The previous record was held on that date in 1921.


In doing some research about overnight low temps, we found a striking trend. The frequency of overnight lows of 75 degrees or greater in the summer months is increasing, while low temps below our coldest average of 27 degrees in the winter months have become less frequent. This trend line goes all the way back to 1945. See the charts below:


Without question, the planet is warming up. NOAA has stated that many recent months have been their respective hottest on record, including just this past August. The recent El Nino gets some credit for this, but there is more to it than that. Despite the ups and downs of sea surface temperatures in the Pacific that make up the El Nino and La Nina patterns, the global temperature trend is upward.

Meanwhile, it is well documented that Louisville has a major urban heat island problem. As I mentioned before, our rural and suburban areas have already seen lows in the 50s. Drive into the city in the evening after a sunny, dry day and you'll see your car's exterior thermometer go up the closer you get to downtown. Just this morning, the low was 63 in Louisville, while it was in the 50s just about 15 miles away. Because of the way the heat island works (by storing heat in the asphalt and concrete jungle) - the effects of it are felt more at night than in the daytime, thus affecting low temperature trends more than high temperatures.

The confluence of a record warm year on the planet and a strengthening urban heat island effect likely helped lead to Louisville's lack of a 50-degree night so far.

We'll see when that streak finally comes to an end! For now, stay cool out there.


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