Meteorological Psychrometrics: Atmospheric Humidity and Its Uses

The psychrometric chart was designed for air conditioning engineering, but the same physical principles — the dependence of saturation vapor pressure on temperature, adiabatic processes, condensation — also apply to the air in the atmosphere. Clouds are nothing other than air whose temperature has dropped below its dew point. The chinook is a psychrometric phenomenon describable with the chart. And climate data (EPW and TMY3 files) is the psychrometric profile of a location across the whole year — the input for building energy simulations.

The condensation level and cloud formation

Air rising over a heated surface cools adiabatically. Until it is saturated, it cools at the dry adiabatic lapse rate — about 5.4°F per 1,000 ft. As soon as the temperature drops to the dew point, condensation begins. This altitude is called the lifted condensation level (LCL) and corresponds to the cloud base. Above it, the air cools more slowly — at the saturated adiabatic lapse rate (2.2–3.8°F per 1,000 ft) — because condensation releases latent heat. The difference between the two lapse rates is the basis of downslope warm winds.

On the chart, the air starts at point A and cools adiabatically along a constant-enthalpy line until it reaches the saturation curve. The temperature at this intersection corresponds to the wet-bulb temperature of the starting air.

The chinook effect and its psychrometric explanation

A chinook (the föhn of the Alps, the Santa Ana of southern California in its own way) is a warm, dry wind on the lee side of a mountain range — an asymmetric adiabatic cycle:

1. Windward side: the air rises, first cooling at the dry adiabatic rate (5.4°F/1,000 ft), then from the condensation level at the saturated adiabatic rate (≈2.7°F/1,000 ft). Precipitation removes part of the moisture — $W$ falls.
2. Over the ridge: the air reaches the summit with a lower $W$ than at the start.
3. Lee side: the air descends at the dry adiabatic rate (5.4°F/1,000 ft), because it is unsaturated — no precipitation. The result is air with the same enthalpy as at the start, but a higher temperature and lower humidity.

Example: air at 1,600 ft elevation at 50°F / 80% RH rises over a 10,000 ft ridge. On the lee side, back at the same elevation, it may be 68–72°F / 30–35% RH — 18°F warmer and markedly drier. Anyone who has watched a Front Range winter afternoon jump twenty degrees has seen this on a chart without knowing it.

Fog is condensation without rising

Fog is a cloud at ground level. It forms when the air cools below its dew point without any vertical motion — by radiative cooling at night (radiation fog), by warm humid air flowing over a cold surface (advection fog), or by the mixing of two air masses (mixing fog). In every case it is a crossing of the saturation curve on the chart.

Measuring humidity in meteorology

Weather stations measure the dry- and wet-bulb temperatures with an aspirated psychrometer or a capacitive sensor. From the two values, all the psychrometric quantities are computed. The vertical profile of the atmosphere is measured by a radiosonde, a balloon-borne instrument with temperature, humidity and pressure sensors. The result is an aerological diagram (skew-T log-P), the meteorological counterpart of the psychrometric chart.

EPW and TMY3 files are climate data for simulations

A building energy simulation (EnergyPlus, IDA ICE, DesignBuilder) needs hourly climate data for the whole year — a so-called TMY (Typical Meteorological Year). The standard formats are TMY3 and EPW (EnergyPlus Weather Format), which for every hour of the year contain the dry-bulb temperature, dew point, relative humidity, pressure, solar radiation, wind and other variables. The key psychrometric variable is the dew point — from it and the dry-bulb temperature, the software computes the enthalpy, the humidity ratio and the relative humidity for every hour.

Climate analysis on the psychrometric chart

Plotting a full year of hourly data onto the chart (a psychrometric climate chart) immediately shows how much of the year the conditions are suitable for free cooling, evaporative cooling or passive ventilation. In a temperate continental climate, most summer hours land in the region 59–82°F / 40–75% RH — favorable for passive and hybrid cooling — while extreme hours above 86°F make up only a few percent of the year, and winter hours cluster into 23–41°F / 70–90% RH. Run the plot against the TMY3 file for your own site before you trust any of those bands: a Gulf Coast year and a high-desert year put the cloud of points in completely different places.

Frequently asked questions

Why does saturated air cool more slowly as it rises than dry air? Condensation of water vapor releases latent heat, which partly offsets the cooling from expansion. That is why the saturated lapse rate (2.2–3.8°F/1,000 ft) is smaller than the dry one (5.4°F/1,000 ft).

Where do I get EPW or TMY3 data for my location? From public portals (climate.onebuilding.org, energyplus.net, NREL for TMY3), which cover thousands of locations. The accuracy of the dew point in the weather file directly affects the results of an air conditioning energy simulation.

Is an aerological diagram the same as a psychrometric chart? They share the physics but differ in their axes. The aerological diagram (skew-T log-P) shows temperature and dew point as a function of pressure (altitude), whereas the psychrometric chart shows dry-bulb temperature against humidity ratio.