entified counties with populations most vulnerable to extreme temperatures. Vulnerability was split into four main categories represented in Figure 3-4. The darker the color the higher the vulnerability to heat.
Local temperature trends
To understand the local effects of extreme temperatures, we examined daily temperature data from 38 NOAA weather stations across Montana. We determined the maximum and minimum temperature trends in summer months (June, July and August) at each station. The temperature records at individual stations may not coincide or reveal the same patterns as the Montana climate region results indicated in the MCA, which were averaged (Whitlock et al. 2017). Our intent was to examine recent temperatures and their trends at a scale where human health may be monitored and impacted in the future. Our findings are:
- Summer maximum temperature.—Fifteen out of the 38 stations across Montana indicate an increasing trend in summer (June, July, August) maximum temperatures over the last 30 yr. The easternmost stations show no trend in maximum temperatures. No stations have a negative trend in maximum summer temperatures (i.e., cooling) over the last 30 yr (1990-2019).
- Summer minimum temperature.—From 1990-2019 minimum summer night temperature has an increasing trend at 15 of the 38 stations across Montana, and a decreasing trend at four stations. All of the decreasing trends in summer night temperature (i.e., cooling) are in western Montana. Five of the 15 stations with increasing summer maximum temperature also show increasing trends in summer night temperature; three are in urban centers. In other words, stations that show a trend towards increasing temperature during the day also experience a trend towards warming night conditions. Warm nights represent a burden on the human thermoregulatory system, particularly for the elderly and those who rely on natural ventilation (e.g., open windows) to reduce body temperature (Mills et al. 2015).
The number of continuous days and nights at high temperature results in cumulative, negative heat impacts on people. In a study that looked at large cities at sea level, human mortality increased when extremely hot nights followed hot days (Xu and Tong 2017). While a different scenario than faced by Montanans, increased human mortality has the potential to become a concern here given projected increased extreme temperatures.
- Winter minimum temperature.—Winter (December, January, February) minimum temperatures indicate an increasing trend (i.e., winter progressively getting warmer) at only two of 38 stations, both in southwestern Montana, and four stations indicate decreasing trends in winter minimum temperature from 1990-2019. Over the longer period of 1970-2019 (50 yr), 11 out of 35 stations show winters warming while no stations indicate decreasing winter temperature. Thus, the time span for analysis influences the trend, as do the months included in a season. For example, when winter is expanded to include November and March in a different study (not published), 13 out of 15 weather stations show winters getting warmer.
Increasing temperature trends over the last 30 yr at individual weather stations are not limited to the larger cities in Montana where heat islands might exist. Small rural towns are also vulnerable to increasing heat and, based on limited healthcare facilities and other factors (see Section 4), may include citizens who are already vulnerable to extreme conditions. The number of summer days with maximum temperatures over 90oF (32oC) is projected to increase 11-33 days by mid century (Whitlock et al. 2017).
Degraded air quality, or air pollution, refers to myriad substances, both human-made and naturally occurring, including tobacco smoke, carbon dioxide, nitrous oxides, sulfur dioxide, particulate matter, ground-level ozone, and volatile organic compounds. Outdoor air pollution, some forms of which are also global warming gases, is estimated to contribute to 64,200 premature deaths in the US annually (Watts et al. 2019). Outdoor air pollution is expect