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Margaret Orr: Have we had cold fronts push through in July in NOLA's history?


Hard to believe, but we’ve had three cold fronts in the month of July!

Sure, it’s not a pull out the sweater kind of cold front; after all, the average low is 76 and the average high is 93 at Audubon Park.


It’s also hard to look back in history to learn how often cold fronts move through in July.  That’s because all the important information is not saved, like hourly wind direction, speed and dew points. So let’s first look at this month to figure out when the cold fronts moved through.

One cold front brought us cooler weather for July 3.

It’s hard to tell if you look at the lows and highs from Audubon for that day. The low was 75 and the high 95, but the low is down from 81 the day before. If you look at Slidell, there is a more definite drop. The low was 77 on July 2 and 69 on the July 3.

The second cold front is easier to spot. There was 0.44 inches of rain at Audubon on July 15, with a low of 78 and a high of 96. The next day the low was 73 and the high 91. Rain is often an indicator of a front, but typical summertime storms can also cause the temperatures to drop. 

The third cold front is really hard to spot if you are just looking at climate data. We had a low of 80 Tuesday morning and a high of 90 at Audubon. A low of 80 is not a big indicator of a cold front! In fact, it’s higher than the day before, but the high temp is down from 99 on Monday. No rain was reported. 

The reason the temp was so warm after the cold front passage Tuesday morning is that the wind was out of the north. This time of the year the lake water is very warm, in the upper 80s. The north wind over the lake picked up the warmth from the lake and carried it to the south shore. So actually, the warmer low reported at Audubon can be an indicator that we had a north wind. This time of the year our winds are usually off of the Gulf and very warm and humid.

Dew points and wind direction are not saved in climate data. Dew points are the measure of moisture in the air. This time of the year dew points run in the 70s, sometimes near 80, because the wind is mostly out of the south. Warm moist air blows in from the Gulf. When a cold front moves through, the wind usually shifts to the northwest and cooler drier air moves into the area. When dew points drop from the 70s into the 60s, it’s an indicator that the air is drier. When the wind shifts from the southeast to the northwest, it’s an indicator that a cold front is moving through. It can also be caused by a thunderstorm.

I can look at the hourly data now and see that at 1 a.m. Tuesday the temp at the airport was 84, the dew point 77 and the wind north at 6 mph. By 3 a.m. the dew point dropped to 70, and by 4 a.m. 69. Obviously the cold front started moving through early morning.

This kind of information is not available looking at past years. I chose to look at Audubon because that climate data is available back to 1893!

Record lows are an obvious indicator of a cold front this time of the year.  But remember cold fronts are not always obvious.

I found three cold fronts in 1918. July 2, 1918, 0.23 inches of rain fell. The next day the low dropped to 61. Lows stayed in the 60s through July 7. The high actually climbed to 95 on July 6. Some may ask, "How could there be a cold front in the area with a high like that?"

Dry air heats more efficiently than moist air. There is a larger range of temperatures with dry air. The low that morning was 63, and the high 95. That’s a 32-degree difference.

The next cold front was July 14, 1918, 0.02 inches of rain fell the previous day. Lows then dropped into the upper 60s, with highs in the low to mid 90s.

The next cold front looks like it was July 25, 1918, when 0.09 inches of rain fell.  The low was 74 and the high 94.  The next day the low was 72 and the high 90, and the next two days the low fell to 70.

So looking back in history, we do not have all the details that we have today. If you want to look at the climate data, check out 1930 and 1953. Both years appear to have three cold fronts.
