Headline: The Dallas Express, 20 June 2023 | ‘Oppressive’ Heat Expected This Week
The opening sentence of the news story: “Oppressive heat” has arrived in the North Texas region and is here to stay.
Around this same time each year, someone will say, “It’s not the temperature; it’s the humidity.”
Neither the temperature nor the humidity alone causes the ‘oppressive heat.’ It’s the enthalpy.
What is Enthlapy?
Encyclopedia Britannica defines enthalpy as the sum of the internal energy and the product of the pressure and volume of a thermodynamic system. Enthalpy is an energy-like property or state function—it has the dimensions of energy (and is thus measured in units of joules or ergs), and its value is determined entirely by the temperature, pressure, and composition of the system and not by its history. In symbols, the enthalpy, H, equals the sum of the internal energy, E, and the product of the pressure, P, and volume, V, of the system: H = E + PV.
Simplified, enthalpy, as it pertains to your physical comfort, is the total heat energy in the air around you. There are three atmospheric variables that affect your personal comfort: Dry-bulb temperature (°F, °C), relative humidity (%RH), and barometric pressure.
Learn your present barometric pressure from the National Weather Service at the nearest major airport; for example the Dallas/Fort Worth International Airport. Smaller regional airports may not have barometric pressure online.
Enthalpy Simplified
Enthalpy is the total heat in the air around you, the product of the dry-bulb temperature (°F), the relative humidity (%RH) and the static pressure of the air at your altitude and location (inches Hg).
A psychrometric chart like the one pictured below can help you quickly identify the present enghalpy at your location.
In the chart above, the atmospheric variables illustrated are 75°F dry-bulb temperature, 40% RH (relative humidity, and barometric pressure of 29.991 inches Hg (Mercury). These conditions indicate an enthalpy of approximately 26.5 BTU/lb. Dry Air.
At the start of the day, June 21, 2023, at 0855 hours, outdoor relative humidty was 83% and outdoor temperature was 84°F. Assuming a barometric pressure of 29.921 inches Hg, the present outdoor enthalpy is 44.5 BTU/lb. Dry Air, very warm and uncomfortable.
If the outdoor air relative humidity were to be 30%, outdoor enthalpy would be about 26.5 BTU/lb. Dry Air, the outdoor on my patio would be very comfortable.
The forecast for this afternoon will be 93°F, relative humidity around 82%, and barometric pressure greater than 29.81 inches Hg. Unable to chart the discomfort, using the Honeywell chart, I estimate an enthalpy around 55 BTU/lb. Dry Air!
You can check out the outdoor heat this afternoon and let me know how it feels. I plan to stay indoors!
John White
Rockwall, Texas
Supplemental Information
For inquiring minds who may ask, “What’s the significance of the barometric pressure on the matter of the enthalpy of air?”
Compressing air drives heat out of the air. My Diesel-powered Mercedes-Benz E300D has no spark plugs like your gasoline-powered car. As with all Diesel engines, the compression ratio is much greater than that of a gasoline engine.
An injector squirts a measured about of Diesel fuel into cylinder where the heat expelled from the compressed air gets very hot and ignites the air-fuel mixture in the cylinder. This is an oversimplified explantion. The point is this: as the barometric pressure rises on a hot day, the air is unable to absorb heat from your skin, thereby causing personal discomfort.
Peole die from excess heat and from excess cold. The climate change activists insist more die from excess heat thant from excess cold.
History
National Weather Service | Heatwave of July 1936
Columbian College of Arts and Sciences | The [1896] Heat Wave that Changed American History
WorldAtlas | Worst Heatwaves In US History
CO2 Coalition | From Princeton Alumni Weekly: And Now, a Different Perspective on Climate Change
The Daily Caller | WRONG AGAIN: Greenland’s Ice Sheet Defies Critics With 4-Year High
John N. White 