What is its sensitivity to temperature, humidity, solar radiation and other factors?
There is some scientific literature on this topic and more is being submitted. I will summarize a few of them below, but let me give you the bottom line: it appears that warm temperatures and high humidity is bad for the virus (or similar viruses). Also ultraviolet radiation from the sun is problematic for the virus. Thus, in many populated locations in the northern hemisphere, the virus will experience a less favorable environment.
A recent submission to the journal SSRN by Sajadi et al. (Temperature and latitude analysis to predict potential spread and seasonality for COVID-19 ) suggests for the coronavirus there is
“significant community spread in cities and regions only along a narrow east west distribution roughly along the 30-50 N corridor at consistently similar weather patterns (5-11C and low specific and absolute humidity).”
According to the Sajadi et al paper, the regions of major community spread has been characterized by average winter (November-March) temperatures of 5-11C (39-52F) and relative humidities of (47-79%), which they suggest is consistent with the optimal values of viral spread on surfaces for similar coronaviruses (4C and 20-80%, found in another paper, Casonova et al (2010).
These are exactly the kind of temperature/relative humidity values experienced in Seattle the past several months (see plots below for SeaTac Airport).
Sajadi et al. goes on to note that there has been a lack of significant community transmission in regions that are warm and humid. One of their compelling figures shows the location of major regions of community spread of COVID-19 (black circles) compared to near-surface temperatures (at 1000 hPa)–see below.
From Sajadi et al. 2020
Major coronavirus outbreaks have avoided the tropics and the summer (southern) hemisphere and seem to prefer a certain temperature range within roughly 20F above freezing. Really cold environments do not seem favorable.
Their claim is supported by a recent figure which shows the number of cases around the world (see below),with few cases in the warm, humid tropics and the southern hemisphere.
Image acquired here:https://experience.arcgis.com/experience/685d0ace521648f8a5beeeee1b9125cd
And even in the U.S., a plot of the cases (courtesy of the NY Times) shows lots of cases over the Northeast and Northwest, where temperatures/humidities have been favorable, but far fewer cases in the southeast U.S.
Compare this occurrence map with the temperatures for the past 30 days (courtesy of the NOAA Climatic Data Center). The light brown colors are in the favorable range: the major metro areas in those colors (Seattle, New York) are the ones being hit hardest. The warmest areas in the southeast have had relatively few cases.
Another major challenge for viruses like coronavirus is ultraviolet (UV) radiation from the sun, which has been demonstrated to inactivate them (one paper is here). Solar radiation (and thus UV radiation) will increase rapidly across the U.S. during the next few months as we approach the summer solstice and skies progressively clear as the midlatitude storm track moves northward.
Many coronaviruses display a notable seasonal cycle (less in summer), which is consistent with the above (check here for a good description of the seasonal issues), but there are still many unknowns.
All said and done, there is good reason to expect that the summer, which will bring much warmer temperatures, much more ultraviolet radiation, a greater tendency for folks to be outside, and very humid conditions over the eastern U.S., will aid in reducing the current coronavirus outbreak. Let’s hope so.