Atmospheric Pressure

Atmospheric pressure is greatest at sea level, but even there it is variable.

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I watched the tire slowly filling until it was round and solid feeling again. I was impressed once more with the fact that air, something we can’t see, feel or taste, actually has a measurable mass and can change in density and pressure around us, just like air filling the tire.

Air pressure, or more accurately, atmospheric pressure, is gravity acting upon air molecules just as gravity keeps us anchored to the Earth and is key to life on this planet. Not only do we breathe oxygen and need it in concentrations high enough to support life which barometric pressure accomplishes, but barometric pressure also changes with the weather—or rather, the weather changes with fluctuations in barometric pressure. Evangelista Torricelli, one of the first to recognize atmospheric pressure in the 17th century, once said, "We live submerged at the bottom of an ocean of the element air."

Even though they are tiny, more than a million can fit end to end on the head of a pin, all air molecules have weight and this weight bears down upon the surface of the Earth. It is a classic example of the power of one when added to all others.

Average barometric pressure at sea-level is commonly cited as 14.7 pounds per square inch (PSI). When you think about it, that is a lot of force bearing down on us. However, this figure is just an average.

I really struggled to come up with an analogy for atmospheric pressure that would make sense. The best I could do was to think of a column of popped corn reaching far into the sky. The popcorn at the bottom would be very compacted, squished and crunched densely together under the weight of all the corn above. The higher up the column you would go, the popped corn would be fluffier and fluffier, but with fewer kernels in a given volume.

The really interesting thing is that this pressure is very inconsistent across the Earth’s surface. In general, there are more molecules by double at sea level compared to 18,000 feet above sea level. The actual weight of a given column of air depends not only on altitude, but also on temperature and wind. The bottom line for air pressure though is the actual number of molecules present. More molecules equals denser air equals higher pressure.

Air pressure changes with temperature—the colder the temperature, the more oxygen molecules the air can hold and the denser the air. Unequal heating of the air around the world causes changes in air pressure on a global scale. We often call these changes highs and low pressure systems. There are also daily fluctuations in pressure due to the heating and cooling cycle of the Earth and there are some latitudes with surprisingly stable atmospheric pressure. A high-pressure system usually indicates improving weather while a low-pressure system denotes an increasing chance of a storm.

Measuring air pressure requires an instrument called a barometer. In its simplest form, a barometer is a tube filled with mercury. Changes in air pressure cause the mercury to rise and fall and this can be measured. In the United States, we often talk about inches of mercury, but there are other terms and units of measure that complicate the information we receive. For instance, “An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.” So, while they all mean the same thing, inches, atmospheres, millibars and millimeters are different ways of measuring atmospheric pressure. There are other measuring systems as well, making this a confusing topic at times.

You may have a barometer in your home. They typically look sort of like a thermometer with a needle gauge. Mine is very basic: when the needle is right of center, it indicates fair weather or high pressure, and the further right the fairer the weather. Left of center indicates increasing low pressure and the likelihood of stormy weather. Some barometers will try and break this down further, but a lot depends upon temperature and current conditions.

I find the whole concept of atmospheric pressure a little fascinating. This unseen force, influenced by an unseen power (gravity), helps to create and maintain a world where we can live. Without either of them, life would not be possible on this planet.

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Help Idaho Wildlife

When we traveled across the state in October 2017, most of the vehicles we saw using the wildlife management areas did not have wildlife plates. Buying wildlife plates is a great way for non-hunters and hunters alike to support wildlife-based recreation like birding.

C'mon folks, let's help Idaho's wildlife by proudly buying and displaying a wildlife license plate on each of our vehicles! 

See below for information on Idaho plates. Most states have wildlife plates so if you live outside Idaho, check with your state's wildlife department or vehicle licensing division for availability of state wildlife plates where you live. 

And tell them that you heard about it from Nature-track.com!

Wildlife License Plates

Great news! as of 2024, there are three NEW designs for license plates. They still are bluebird, cutthroat trout and elk, but they are beautiful.

Idaho Wildlife license plates provide essential funding that benefits the great diversity of native plants and wildlife that are not hunted, fished or trapped—over 10,000 species or 98% of Idaho’s species diversity. Game species that share the same habitats (such as elk, deer, antelope, sage-grouse, salmon, trout) also benefit from these specialty plates.

No state tax dollars are provided for wildlife diversity, conservation education and recreation programs. Neither are any revenues from the sale of hunting or fishing licenses spent on nongame species. Instead, these species depend on direct donations, federal grants, fundraising initiatives—and the Idaho Wildlife license plates.

Both my vehicles have Bluebird Plates. I prefer the bluebird because the nongame program gets 70 percent of the money from bluebird plates, but only 60 percent of the money from elk and trout plates - 10 percent of the money from elk plates supports wildlife disease monitoring and testing programs (to benefit the livestock industry) and 10 percent from cutthroat plates supports non-motorized boat access.

Incidentally, in 2014, the Idaho Legislature denied the Department of Fish and Game the ability to add new plates or even to change the name of the elk and cutthroat plates (very specific) to wildlife and fish plates, a move that would have allowed for changing images occasionally and generating more revenue. It would seem that they believe that we Idahoans don't want a well funded wildlife program.

I think it is time we let the Legislature know that Idahoan support wildlife funding and that we would like to see these generic plates come to fruition.