July 15th, Day 27: Mitchell, SD to Sioux Falls, SD: 72 miles, 1099 ft. ascent

Today we had blessed relief from the Winds of South Dakota, to the extent that our average speed came back to 17mph, and we were ready for an Adult Beverage at lunch time, without feeling like we’d have to crawl into the bar let alone out of it.

There were no world famous tourist attractions between Mitchell and Sioux Falls, so we’ve just posted a few pleasing photos from our co-riders below with no particular theme.

But it did start getting humid today which reminded us we need to post on the concept of Dew Point. Right from the start of the ride, this has been something that people have regularly alluded to in the confident sort of voice that brooks no interrogation of what they actually mean. It turns out that most people’s knowledge is limited to  ‘High Dew Point Bad, Low Dew Point Good’, although Alastair seems to have a mental block on even this basic point. But we couldn’t find anyone who could explain what Dew Point actually is. Fortunately, in the age of Wikipedia, the answer is only a short few taps away – so long as you are lucky enough to be in a hotel where the internet vaguely works. So, here’s the basic answer: Dew Point is the temperature at which water vapour condenses out of the atmosphere and becomes water. For example, if the Dew Point is 25˚C (77˚F), then if the temperature of a surface is 25˚C (77˚F) then it will be wet even if it is sunshining not raining. The Dew Point can never be higher than the actual air temperature, because when the Dew Point reaches the ambient air temperature then you are surrounded by water condensing out of the air (more simply, it will be raining warm water).

Dew Point is highly correlated to humidity. There is a Mairs-Heggie Conjecture that Dew Point can be calculated by multiplying ambient temperature by humidity. So, if the air temperature is 25˚C (77˚F) and the humidity is 80% then the Dew Point is 20˚C (61.6˚F).

We could obviously check whether this conjecture is correct by doing a bit more googling, but on the other hand uninformed bullshitting has become so integral to our lifestyle over the past month that it seems a shame to spoil it all by just looking up the answer.

A corny picture of Mitchell, SD. Thanks to Marty Stabler.