Post #904: Dryer air arrives in Virginia

Source:  National weather service (, downloaded 12/2/2020.

I’ve been tracking the relative humidity in my house for roughly the past month.  November had relatively few days with truly dry outdoor air.  Air that was both cold, and had low relative humidity.

Unsurprisingly, I’ve had to run my indoor humidifier infrequently, so far this year.

Today, and looking forward to the forecast for the next two weeks, that appears to be changing.

As shown above, today is cool and dry.  That 31% outdoor relative humidity (at 47F) would result in a roughly 15% indoor relative humidity at 68F, absent any other inputs of water vapor into the indoor air.  Today my humidifier is running constantly but my indoor humidity is slowly dropping anyway.

For the next two weeks, it looks like a typical day will be 45F with 55% outdoor relative humidity.  Absent any other inputs, those conditions will translate to an indoor relative humidity of 25% at 68F.  That’s a long enough period that houses and other indoor locations should be fairly well dried out by the end of it.  Absent a humidifier, indoor air in this region will be quite dry by mid-December.

If you wonder why I’m tracking this, refer to Post #894.  I heard from a few readers who purchased humidifiers after reading that.  If you have one, but haven’t unboxed it and set it up, now would be a good time to do that, I think.

Post #879: Preparing for a hard winter, 7: It’s not the heat, it’s the (lack of) humidity.

Source:  Underlying data are from Johns Hopkins University, via the NY Times Github COVID-19 data repository.  I have once again had to expand the vertical scale of this graph since the last time I published it.  Three days ago.

This post has turned into quite a treatise.  Let me cut to the chase.  Here’s my hypothesis, regarding the last two waves of the COVID-19 pandemic in the US.

It’s not the heat, it’s the (lack of) humidity.

  • Hot weather + no indoor mask use = second wave of US pandemic, centering on southern states.
  • Cold weather + no indoor mask use = third wave of US pandemic, centering on northern states.

In short, I think that dry indoor air plus no mask use is going to be a toxic combination this winter.  And I’m afraid we’re seeing an acid test of that where the cold, arid winter climate of the US high plains and eastern slope Rockies intersects Republican anti-mask sentiment. Continue reading Post #879: Preparing for a hard winter, 7: It’s not the heat, it’s the (lack of) humidity.