My instructor brought on an interesting question today during ground school:

Why are we more hypoxic at lower altitudes during the night?

Anyone care to give it a try?

This is either a case of not understanding what "hypoxia" means, or your instructor has got the facts mixed up...

Hypoxia is a physical condition which occurs when part or all of the body is deprived of adequate oxygen. For pilots, hypoxia usually results from the ambient pressure being too low for the lungs to take what oxygen there is and put it into the bloodstream.

We are not "more hypoxic at lower altitudes at night", because pressure is pressure regardless of day or night.
Hypoxia is not a problem for folks walking around on the surface (near sea level) at night, right? So... why would "lower altitudes" have less oxygen at night?


What you and/or your instructor might be  thinking of is reduced night vision due to increase in  altitude. The reduced oxygen level affects night vision much more than day vision.

According to the American Optometric Association:

The effect of altitudinal hypoxia on night vision is primarily one of an elevation of the rod and cone threshold. Although decreased cone function is clearly demonstrated by the loss of color vision at hypoxic altitudes, the decrement in central VA is usually insignificant. However, scotopic night vision at altitude can be significantly reduced. Scotopic vision has been reported to decrease by 5% at 3,500 feet, 20% at 10,000 feet, and 35% at 13,000 feet, if supplemental oxygen is not provided. Thus, the use of oxygen, even at low pressure altitudes, can be very important at night.

The rest of the article explains more:

http://www.aoa.org/x5352.xml

As far as breathing goes, pressure is pressure, whether it's dark or light outside. If anything, night temps being cooler (in general) allow for a lower lapse rate in ambient pressure (the colder it is, the denser the air is).

SubZer0 wrote on Jan 12^th, 2010 at 12:19am:


First I've heard of it...

Copied from FAA AIM dated February 14, 2008. http://www.faa.gov/air_traffic/publications/ATPubs/AIM/AIM_Basic_2-14-08.pdf

Just to clarify this thread, what sort of aircraft are we talking about here. Pressurised, unpressurised, commercial or private??

Matt

Hagar and Mitch hit the target with their replies. Hagar, I took that information from the same place

Matt, in this case, we are talking about an unpressurized GA aircraft, such as a C-152 in my case.

There is some basis for this, although it is not every night if my memory serves right (been 8 years since that class).  I can not remember the exact reason, but hypoxia can occur at lower altitudes at night.  We were usually advised to avoid flight above 8000MSL at night at our 141 school for just such reason.

C, that is very interesting. My teacher now wants to know WHY are eyes more susceptible to the effects of hypoxia at night.