Keeping Cool in the Shadow of Old Italians

In the high desert here north of LA, this time of year we ride very early in the morning when the air is still hopefully cool, but it's also rather humid, and DEAD CALM. The dead calm is a welcome change from incessant winds, usually in the afternoon this time of year, but all day long during spring, fall and to a lessor extent, winter.  

The calm does have a downside though, and it can be very uncomfortable, or in extreme cases maybe even life-threatening. This because if you climb a long hill or otherwise really exert yourself, making a sustained push into Zone 5, when you get to the top of the hill or end of the Strava Segment and pull over to catch your breath and hydrate you can actually get even HOTTER. 

With the high morning humidity in the 60-70% range, stopping means no wind, and probably no shade either. Even in the shade though, the lack of any breeze means your body's evaporation cooling system doesn't work, and your core temp can soar. I've been more aware of this lately bc I now have a short-haired dog I walk ever morning, and dogs don't sweat, they cool themselves by panting and evaporating moisture off their nose and mouth, and by standing broadside to any wind. 

 

There is often a solution to this still air however, and it was discovered by the Italian physicist Giovanni Battista Venturi and published in 1796.

The Venturi Effect states that when the flow of air is constricted two things happen.

1. The air speeds up
2. The pressure of the air decreases by a commensurate amount

We're obviously interested in the first, but the second effect is what makes the carburetor in a car operate as the low pressure draws gasoline into the air-stream, effectively creating an optimal fuel air mixture. So how do you get a restricted airflow while on a ride? Usually by getting between two buildings, fences, or occasionally a large stand of trees like Cyprus Trees that grow straight up like walls.

It's really striking how much of a difference this can make, especially since energy = mass * velocity squared. That extra velocity means a lot more work is getting done and those beads of sweat will soon be gone and your discomfort with it. 

As a pilot I feel compelled to point out the Venturi Effect is also responsible for the fog that forms on the tops of wings when aircraft pull high Gs. This is because the wing's topside is curved, while its bottom side is flat, so the air is accelerated along the topside, lowering the pressure and with it the temp of the air. When the air temp drops, especially in humid conditions, water-vapor becomes visible.  

There, now you have something to contemplate while out riding, and a way to stay cool in still & humid conditions, plus, doesn't this image just look cool? BTW, wanna guess why your CO2 cartridge gets so cold when you use it? :)