by Tom Ross, Meteorologist
So, even though we are farther away from the sun in July, we aren’t any cooler than when we are closer to the sun in January. That is because of the 23½ degree tilt of the earth’s axis, which allows us in the Northern Hemisphere to receive the most solar radiation during our summer months. As we head through our warmest three months of the year on average (June, July and August), many folks have concerns about the prospect of droughts, hurricanes and hot weather. This month, we will focus a bit on the heat and talk about precipitation as well.
One measure of a warm summer is the number of days the temperature reaches or exceeds 90 degrees F. In any given year, the number of days that can happen is about seven days or so. Looking at the data collected from the Asheville airport records, we have had some years with no 90-degree days; the last time that occurred was back in 2009. In fact, we have had four years since 2000 with no 90-degrees days. In contrast, the year with the most 90 degree or higher days was 1952, with 32. The longest consecutive string of 90 or higher days was back in 1977, with 16 days from July 6th through July 21st. Our most recent string of 90 or higher days was back in 1993 from July 3rd through July 12th.
The graph in the accompanying chart (below left) shows the highly variable pattern of number of 90-degree days by year. We will see what the rest of summer will bring in regard to 90-degree temps.
One of the factors that can promote these heat waves of 90-degree-plus days is the lack of beneficial rainfall and afternoon thunderstorms. It’s possible that we could get into a pattern where the atmosphere is “capped,” meaning the atmosphere is warm and stable and there is a lack of daily showers and thunderstorms. This allows the solar radiation to really warm us up, with little moisture and cloudiness to take the edge off the heat.
In contrast, if you think it is really hot here — try Death Valley in July, where the daily average high is 116 degrees F and the nighttime low is about 88 degrees F.
In terms of moisture, we have not had prolonged periods of thunderstorm activity through at least mid-June, which has led to a yearly deficit of precipitation of around 4 or 5 inches. This situation can turn around quickly, but the trend has been for drier and warmer conditions, and that may very likely be our weather trend for the remainder of the summer.
Next month, we will talk a bit more about the Hurricane Season of 2016. We have already had several storms this year, and it could be quite active.
Meteorologist Tom Ross managed NOAA’s Climate Database Modernization Program during his 25-year career at the National Climatic Data Center (NCDC) in Asheville. He was a senior weather forecaster at Accu Weather in Pennsylvania. Tom teaches classes on weather and climate at various local venues.