Another heatwave has come and gone for New York City and naturally many wonder how much climate change has had a role in enhancing the heat waves we have experienced.
So I decided to dive into some data and look at the heat waves of our past, many I have listed above, to see if there were any notable changes from the start of the 20th century to now.
My process included finding the most well know heat waves. Data before 1950 can be a bit difficult to find, especially for dew points. The dew point data pre 1950 was calculated via reports from the National Weather Service where the relative humidity was only used and then I recalculated the dew point. The temperatures and dew points in the chart above is an average high temperature and dew point for the period of the heatwave. A heatwave in this study is the same as the AMS definition of 3 days straight of 90°F or higher. Along with temperature and dew point analysis, I also studied the 500 MB patterns of each event.
Initially, there was an attempt to perform the same analysis for the Philadelphia metropolitan area, but data for pre-1945 was still not available at the time of this writing but will be reviewed in the future in a separate analysis.
There were two clear trends that I noticed over the past 119 years and that is there has been little change in New York City in terms of the average high temperature of the region’s heatwaves. If anything, the most extreme temperatures came from the 1930s to 1950s when the average heatwave high temperature was found to be around 98°F. Since then, the average heatwave high temperature has declined to around 95°F. The second trend really tells us a lot though about the warming atmosphere overall. The dew point trends from 1900 to 2019 is clearly showing a rising trend.
Before 1970, the average dew point range during heatwaves ranged from 69°F to 73°F then from 1970 on through now, the dew points in heatwaves have averaged from 72° to 76°. This shift in the dew points would be a far more reasonable indicator of a warmer planet than any “extreme” temperatures.
A couple of points that needed to be considered. The disaster of the Dust Bowl has to be considered when discussing this analysis as the July 1936 Heatwave which featured the hottest temperature in New York City, 106°F. Based on NOAA data and various reports, this air mass was a hot, dry air mass born out of a combination of downsloping winds off the Rocky Mountains, poor agricultural practices in the Plains, a prolonged drought in a positive AMO regime, and an active solar cycle which limited high latitude blocking regimes. These combinations supported a man influenced deadly heatwave for the eastern two-thirds of the nation and also helped skew the data in enhancing high temperatures and degrading the state of dew points. This is why I went back to 1900 (data beyond that point is further unreliable for dew point data) to get a full scope of moisture in the atmosphere around New York City observation stations.
There was no change at all in 500 MB pattern evolution of the pattern nor the building of 500 MB heights in a heatwave event. The highest 500 MB heights were found in the 1930’s where 585 dm to 595 dm. Otherwise, in all over decades from 1900 to 2019, the 500 MB heights of heatwaves typically ranged from 590 dm to 595 dm.
HYPOTHESIS FOR FINDINGS:
So now we see the data, but why are we seeing these trends? Here are my thoughts.
First, from a global perspective, the Earth is warming. With a warmer atmosphere overall, the amount of moisture the atmosphere as a whole can hold increases. This factor allows for tropical air masses to hold more moisture when those air masses move up the East coast of the United States.
Second, from a local perspective, one has to consider the Urban Heat Island effect in contributing to two factors of a heatwave. The first is that the retained heat can establish a heatwave event faster and extend the event by an additional twenty-four hours. The second is that the low-level moisture is being trapped and builds up in the atmosphere until forcibly removed from New York City. The reduction of vegetation and more areas of pavement acts as a feedback mechanism to build more heat which supports more humidity. Thus the rise in dew points.
The heatwaves for New York City are not becoming hotter, but they are more humid. If you take out the influences from the Dust Bowl in the 1930’s, the change in the average temperature of a heatwave change very little. However, there is a clear increase in the dewpoints, even when you remove the influence of the Dust Bowl from the 1930’s. While the increase in the dew points may not seem significant, remember the difference in between dew points around 70°F to 73°F would mean the heat index would increase by 5°F, which is a significant increase strain on the human body.
In future articles, I will tackle the many causes of climate change. Detail the chemistry and physics of the carbon dioxide molecule, and much more. The goal of these discussions is to take out the hysteria and misinformation from both sides of what has become, sadly, a highly politicized discussion.