Resources for the Resourceful; Comparing Climates

In developing the post that will follow this one, I came across a really cool resource called City-Data.com.

In addition to containing representative photos of the area, the site contains a wealth of information for quite a few cities and towns in the USA, including population statistics, maps, home sales statistics, income statistics, tax statistics, cost of living statistics, crime statistics, information on colleges, libraries; you name it, and there probably is some information on the site about it.

I bring this to your attention because the site contains climate data in the form of graphs that compare the local climate in terms of a number of variables to average, maximum and minimum values for the location and the average for the USA.

Here is the temperature graph for Portland Oregon blown up so you can read it.

Here are all of the graphs for Portland.

Here are all of the graphs for Portland, Oregon compared to all of the graphs for Key West, Florida.

Even though the resolution is not good, I put the two sets of graphs side by side because it illustrates how the data on the site can be use to quickly contrast two different locations.  For instance:

  • It’s pretty hot in Key West, compared to Portland and compared to the rest of the United States,
  • Key West is pretty wet in the summer relative to Portland, probably due to the thunderstorms that Florida is famous for, and,
  • Contrary to popular opinion, there are actually more days clear of clouds in Portland (the yellow part of the bottom left graph) than in Key West in the summer; again, probably due to the thunderstorms.

One thing that seemed funny at first was that the graphs said it was more humid in Portland on an average July afternoon than it was in Key West. But then I realized that the graphs are in terms of relative humidity – the amount of water the air has in it relative to the amount it could hold at that temperature.

If you look on a psych chart (you can get a basic electronic psych chart for free from Greenheck that you can upgrade to a very full featured chart for a modest cost), you can see that there is a lot more moisture in the air on Key West’s 85°F/78%RH average July day than there is in Portland’s 69°F/82%RH average day, which is why it will feel more comfortable (my opinion) in Portland in the summer than in Key West.

Specifically, on the average July day, the Portland dew point is about 63°F where as in Key West, its about 77°F (that means if you walk out of a building that’s being controlled at 72°F in Key West, you get condensation on your glasses once you are outside).

In terms of moisture content, on the average July day, there is almost twice as much water per pound of dry air in Key West as there would be in Portland (0.0204 lb water/lb dry air vs. 0.0125 lb water/lb dry air respectively). And that water vapor represents a lot of energy that has to be removed if when you dehumidify the air; 42.8 vs. 30.2 Btu/lb.-°F respectively. I guess that’s why I prefer thinking in terms of specific humidity instead of relative humidity, at least when I have my engineer hat on; it paints a better picture of what the systems I am dealing with are up against.

So, bottom line, while the climate information on City-Data.com is not as detailed and specific as what you might find in the climate nomographs that I discussed in a previous post which are showing up on some of the Western Region NOAA sites, the City-Data information is available for a lot more places. That lets you make a quick comparison of climates you know about with climates you are unfamiliar with, which can be a valuable asset for planning a vacation, a relocation, or your next commissioning or design project.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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