Does Urban Heating Spark Thunder Potential?

    Are you from a place that receives a lot of thunderstorms? If so, you are probably familiar with the dangers they pose. But if not allow me to inform you:

Image from: https://kids.nationalgeographic.com/explore/science/lightning-/

Lightning is one of nature's biggest killers, and National Geographic states there are over 400 lightning strike victims annually. In order to decrease these fatalities it is important to understand the mechanisms thunderstorms, or "thunder days" (days of recorded thunder whether or not lightning is visible). One possible contributor is through urban heating. An urban heat island (or UHI) occurs when temperatures in a metropolitan area are higher than their surrounding rural counterparts. If there is a connection between this additional heating and thunder days, you can imagine how the expansion of cities could cause more storms to pop up.

So for this research I decided to see if a link exists between urbanization and thunder day frequency. To do this, I confirmed UHI evidence for three areas:

      - Huntsville, Alabama
      - San Jose, California
      - Minneapolis, Minnesota

These areas were chosen based on data availability and for a variety in location and elevation. The data starts in 1973, and the urban temperatures (red line) and rural temperatures (green line) were averaged for months of August, then plotted along a time series graph with thunder days (black line) to distinguish any patterns between the two. But even though warm temperatures are important for thunder development, UHI alone is not neccesarily enough to determine thunder day frequency. So i looked into other meteorological variables such as dew point, wind speed, and low visibility to see if they alos correlate positively or negatively with thunder days. The results and interpretations are as follows:

I found that all 3 cities contained a UHI for either most or all of the time series. Reasons for varying fluctuation could be due to increasing urbanization over time resulting in changes to vegetation cover. The most interesting finding was that dewpoint had the strongest correlation to thunder days of all the variables studied, not temperature. This suggests that moisture is the best indicator of thunder formation, at least from a localized standpoint.

This research was presented at the annual NASA MSFC Poster Session. I was ecstatic for it to win first place at the poster competition, and this was inspired me to continue interning and researching for projects in the future. Pictures of the event are below, along with a link to the poster. Note: Unfortunately the red and green temperature lines did not reach across the entire plots when converted online, so there is just a little data missing in the poster and in the results above. Same with the x-axis that shows the years 1973-2016.

Link to Poster

Additionally, this research won first place again as a combined effort of my co-worker's urban heat island projects. Along with my analysis of UHI on weather, Dawn White focused on health, and Sabrina Hodge focused on ozone. Unfortunately, I couldn't attend myself, but a link with more information about the event is provided below, along a couple photos Dawn (pictured below) provided. Big thank you to Dawn for showcasing our projects!

2016 Werner Von Braun Memorial Symposium

Frequently Asked Questions At The Poster Session

1) Why is there a peak in the year 1980? Was this an effect of El Nino of La Nina?

1980 was a very weak year for both El Nino and La Nina. Also, these events usually are ongoing for 5-7 years; since the peak is notable just for 1980 it is hard to believe these weather systems played a role. However, with additional research I was able to find out that year had an extraordinarily large high pressure system that sat over a large portion of the U.S from June to September. With high pressure ridges, the air normally dries out with adiabatic, dry compressed heating. It is speculated this is what made already-warm summertime temperatures even hotter for a drawn-out period of time, and this unusual nationwide phenomena of scorching temperatures caused more thunder days this year.

2) If global warming is evident, why is the temperature trend for cities going down or staying the same?

Mainly, this is because the areas I look into are too small to depict the global scale of the warming of the Earth. Global warming is defined as the gradual increase of averaged worldwide temperatures, so even if a few cities are cooler in comparison it wouldn't have much of an effect for the entire Earth.

3) If an area is prone to wet weather, wouldn't that increase the urban heat island?

Yes, rain and moisture amounts would cool off the area, and depending on the city expansion rate, this could decrease the UHI. However, rain amounts may not relate to thunder days. There could be days with just thunder/lightning but no rain, which is why the term is coined "thunder days" instead of thunderstorms.

4) Why is the dew point the key element in this research? Was this expected? Why or why not?

It was expected that dew point would play a big role in thunder day development because moisture is a big factor in their development. But because thunderstorms are so short-lived, meteorological variables like dew point and the others I used are usually left to short-term forecasting, and this study was implemented over a 43-year period. I think this is why even though dewpoint had the highest corelation, the values still were not anywhere near perfect (the highest being 32% in Minneapolis).

5) Why is dew point the variable with the highest correlation in Huntsville and San Jose, but visibility was the highest in Minneapolis?

In Minneapolis, dew point correlation ended up being the same value as in Huntsville at 31%, but I did find it suprising that it came in second next to visibility. To clarify, this variable refers to "low visibility" where less than 10 miles out can be seen. Normally, I would associate this with fog days, but I wondered if this was due to many heavy rain events in the Minneapolis area for this time period. It would be a separate study in itself to look into this to determine if this is the case, which are the plans moving forward in this project.

6) Future work states that you'll use humidity as another meteorological variable, but isn't that essentially the same thing as dew point?

While both give an idea of how much moisture is in the air, dew point is the only true measurement of atmospheric moisture. Dew point tells the temperature at which water condenses, while humidity gives a percent for how saturated the air is at a given temperature. So dew point is more reliable because it gives an actual temperature, but I think it'd still be a good check to see if humidity is correlating with thunder days the same way as dew point.