This viral entity hardly needs an introduction – it’s headline news all across the world.
Looking ahead, there is a general expectation that in the Northern Hemisphere, the spring and summer seasons will supress the spread of the virus, buying us time to develop a vaccine, among other measures.
However, such seasonality is not a guarantee. Every virus has its own degree of sensitivity to climate factors – and not just ones that vary with the seasons.
“Every virus has its own degree of sensitivity to climate factors – and not just ones that vary with the seasons.”
The COVID-19 characteristics are still a matter of urgent research. So, it would be irresponsible of me to try and predict how it, specifically, will be affected. Instead, I have looked how climate factors affect the spread of viruses in general. This can give us some clues as to how COVID-19 might behave in the weeks and months ahead.
Viral Flights Like It Dry and Cold
A laboratory study conducted in 2007 – one of the first on environmental conditions and viral transmission – found that influenza spreads through Guinea pigs most slowly in very humid conditions.
Related to this, high temperatures also reduced the transmission rate. The warmer air is, the more moisture it can potentially suspend within it.
In the ‘real world’, studies have found similar results for human populations, when allowing for external factors such as the tendency for people to cluster together (taking shelter) during cold or very wet conditions.
This link between atmospheric moisture and transmission rates is down to how it affects the ability of viruses to travel via suspension in the air.
Viruses can hitch a ride on particulates or moisture droplets such as those expelled when an infected person coughs or sneezes. While travelling, any droplets of moisture they encounter will stick to their ride, adding weight to it. Sooner or later, it becomes too heavy to remain suspended and falls to the surface.
The more water droplets there are, the sooner an airborne virus crashes to the surface.
What About 26°C?
You may be wondering whether high temperatures can directly kill off a virus. While this is certainly possible, the required conditions are usually (it varies between viruses) more extreme than is usually seen in even the hottest of Earth’s climate zones.
Contrary to one claim you might have seen going around, there’s no evidence that COVID-19 is destroyed when temperatures reach 26-27°C. After all, this viral infection has already spread through some populations residing in tropical climates, where such high temperatures occur most of the time.
“…there’s no evidence that COVID-19 is destroyed when temperatures reach 26-27°C”
High temperatures are of some direct help, though. Cold air, with its low moisture content, can impair the function of the nasal mucus that we rely heavily upon to trap and expel potential invading bodies. The drier mucus is, the less effectively it works.
In a perfect hygiene environment, the air temperature and moisture content – as mapped below for March to May – would be the deciding factor in the spread rate of a viral infection like COVID-19.
Global coverage map showing the long-term average precipitable water at the surface (i.e. the moisture content of the air within one metre elevation above each square metre of surface) for the months March to May.
Human Hygiene: Naturally Imperfect
Our reality is far from a perfect hygiene environment (which, let’s face it, would be very unnatural). Even at times like these, when we’re extra-conscious of our day-to-day habits, it’s difficult to entirely prevent transmission of viruses or bacteria via contact, either directly with each other or via other surfaces.
When it comes to indirect transmission between beings, this tends to be most problematic in indoor environments. The reason might well be shining through the window as you read this.
Yes… the sun sustains most of the life on Earth, yet also has the capacity to destroy it on the molecular level. In fact, it can very nearly sterilise surfaces that are exposed to it often.
This is down to the ultraviolet (UV) radiation that forms part of sunlight. You’ll likely be familiar with what it can do to human skin when overexposed (sunburn!). Viruses have even less natural protection, making sunlight an excellent natural constraint on their numbers.
“The sun… can very nearly sterilise surfaces that are exposed to it often”
Just how effective it is at any given location depends on the strength of received radiation. This is no simple matter: it varies with position relative to the equator, the time of year, how clear the skies are of clouds and particulates, how much UV-absorbing ozone there is in the atmosphere and the terrain elevation.
Below is a broad overview of the UV situation, divided into four ‘zones’ that circumnavigate the world (with the two hemispheres approximately mirroring one another around an equator dividing the tropics). Please note that this is my own interpretation, intended only to give you a general idea.
You can also check out a map of global mean solar irradiance here (full credit to 3TIER).
Generally, the sun is least helpful for those of us living north of Seattle to New York (USA) in N. America, Paris (France) to Kyiv (Ukraine) in Europe and Saratov (Russia) to Sapporo (Japan) in Asia.
For the most sterile outdoor environments, the driest or most elevated subtropical regions of our planet are generally best. Africa does very well in this regard, as does the northern half of Australia. However, as detailed earlier, there is likely to be greater airborne spread of viruses in these places.
No Guarantee of a Seasonal Decline in COVID-19
Odds are this virus will prove at least somewhat susceptible to destruction by UV radiation and being ‘weighed down’ by atmospheric moisture.
In my opinion, it will be very unfortunate if the rate of spread shows no overall response in the tropics and Northern Hemisphere subtropics during the coming months.
Further north, there is still great familiarity with viruses being ‘seasonal’ (e.g. flu), but we can’t be sure that COVID-19 will be as susceptible to stronger sun and warmer, moister atmosphere. For now, all we can do is take sensible precautions and stay informed on the situation.
Meanwhile, non-tropical regions of the Southern Hemisphere must prepare to combat the virus during their coldest, weakest-sun months of the year. Successful mitigation measures there can in turn reduce the strain on the Northern Hemisphere next Nov-Mar.
It will be a tough time for so many of us, in all walks of life, but together, we can get through this.