Changing Chesapeake: Tropical Nights


Figure 1 – Not the Tropical Nights we are discussing.

Over the past couple months, we’ve described the Climate Change Indices Growing Season Length and Frost Days. It’s pretty obvious from their names what these indices describe.  This week, we’re going to focus on another index that isn’t so obvious – Tropical Nights. Googling “Tropical Nights” doesn’t help much because you are more likely to get links to night clubs, social events, and cheesy pictures like the one to the right than information about climate change.  Tropical Nights actually sounds kind of nice as it conjures up thoughts of island beaches.  In actuality , it is a good indicator of warming trends over the long term.

Tropical Nights, abbreviated TR, refers to the number of days in a year where the minimum temperature does not go below 68°F (20°C). We typically see the daily minimum temperatures at night and 68°F is a typical temperature we might see at night in the tropics, hence the name Tropical Nights. Or so I think – I can’t really find much information on the etymology of Tropical Nights relative to Climate Change Indices.

Figure 2 - Tropical Nights Index calculated from Northern and Southern NCDC stations located around Chesapeake Ba

Figure 2 – Tropical Nights Index calculated from Northern and Southern NCDC stations located around Chesapeake Bay.

As with the other Climate Indices we’ve discussed, Kari has analyzed the long term data set from 18 National Climatic Data Center (NCDC) stations located around Chesapeake Bay for the TR index. The data were separated into Northern stations and Southern stations and then averaged (Figure 2) as shown by the blue and red lines on the graph.  The thicker lines represent the rolling 21 year averages of the same data. Comparing the early 1900s to the early 2000s, there are about 30 more Tropical Nights throughout the Chesapeake region. And 4 out of 5 years with the largest number of TR occurred within the last decade (2005, 2010, 2012, and 2014).

Increasing night time temperatures are not well understood.  Obviously the sun has gone down so the nights are not so much heating up as much as they aren’t cooling off. Theories as to lack of cooling include increases in cloudiness and urban-caused atmospheric aerosols (which keep heat from from being radiated back out to space at night) to changes in atmospheric circulation and increased humidity (humid air retains more heat). Warmer night time temperatures have implications for life here in the Chesapeake Bay Region. Crop yields for corn, soy beans, and rice are shown to be negatively correlated with increases in night time temperatures.  There are serious health implications as well. According to the National Weather Service, excessive heat events are the number 1 cause of weather related deaths. And more warm nights means people are more likely to run their air conditioners. This means more electricity needs to be produced, generally from coal fired power plants which sends more CO2 into the atmosphere. Tropical Nights may sound pleasant, but more of them in the Bay region is definitely not good news.

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