Mary, Mary, How does your dewpoint vary?

Today we are modifying the old nursery rhyme which inquires about Mary's garden  to consider typical fluctuations in dewpoint temperature.  Since the dewpoint depends on the total atmospheric pressure, and that changes from day-to-day, the dewpoint will change (for a given amount of moisture in the air) as the barometric pressure changes. We'll also look at how dewpoint might change with altitude.

Heat from Breath, part 2

In the last post we considered heat lost through breathing and determined that breathing air at 35 °F results in a heat loss of about 2 Watts for every liter/min of air flow.  Of course, this depends strongly on the temperature and relative humidity of the ambient air.  In this post, we’ll consider both the effect of the condition of the ambient air, and also how much of the heat is latent (due to moisture) and how much is sensible (due to temperature).

Buoyancy

I used to read to my kids from a popular children's book which featured a curious monkey who got into trouble by grabbing a large bunch of helium balloons from a balloon salesman.  According to the storyline, the monkey was carried out over the city hanging from the balloons.  Now, I'm sure the illustrator for the book was deeply interested in thermodynamics (isn't everyone?) but may have been in too much hurry to calculate the appropriate number of balloons.  Or maybe artistic aesthetics overruled thermodynamic accuracy.  In any case, today we'll go back and look at the situation in more depth.

Moisture and Altitude

Psychrometric charts are very useful for determining properties of moist air, but it is important to remember that each chart is prepared for a given atmospheric pressure.  Of course, it is easy to find charts created for specific elevations, but it is sometimes hard to visualize the variations in properties with elevation.  In this post, we’ll look at the effect of elevation on the moisture content of air.

Altitude vs Temperature

People who ski, hike, run, or do other vigorous exercise at a higher altitude than usual typically feel some effect of the elevation.  It is common to feel difficulty getting enough breath.  Conversely, sometimes people who travel down from a higher altitude feel like they can run faster and longer than usual.  The cause, of course is that the lower density air at higher altitude provides less oxygen.  At a given altitude, though, the density of air also varies with the temperature.  In this post, we’ll look at the size of that effect.

The Standard Atmosphere

Most people don’t need to worry too much about the temperature and pressure of the atmosphere, as long as there is some of it there to breathe.  However, pilots, airplane designers, meteorologists, and many other people need to know how the temperature and pressure typically change with altitude.  Of course, on any given day at a particular location, conditions might be different than the “typical” condition.  Still, the typical conditions are useful enough that there are several definitions of the Standard Atmosphere describing conditions clear out into what is usually considered outer space.