Meteorological psychrometrics: atmospheric humidity and its uses

The h-x diagram 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 föhn is a psychrometric phenomenon describable with the h-x diagram. And climate data (EPW 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 9.8 °C/km. 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 (4–7 °C/km) — because condensation releases latent heat. The difference between the two lapse rates is the basis of the föhn effect.

On the h-x diagram, the air starts at point A and cools adiabatically along an isenthalp until it reaches the saturation curve. The temperature at this intersection corresponds to the wet-bulb temperature of the inlet air.

The föhn effect and its psychrometric explanation

The föhn is a warm, dry wind on the leeward side of a mountain range — an asymmetric adiabatic cycle:

1. Windward side: the air rises, first cooling at the dry adiabatic rate (9.8 °C/km), then from the condensation level at the saturated adiabatic rate (≈5 °C/km). Precipitation removes part of the moisture — $x$ falls.
2. Over the ridge: the air reaches the summit with a lower $x$ than at the start.
3. Leeward side: the air descends at the dry adiabatic rate (9.8 °C/km), 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 500 m a.s.l. at 10 °C / 80% RH rises over a 3,000 m ridge. On the leeward side, at the same altitude, it may be 20–22 °C / 30–35% RH — 10 °C warmer and markedly drier.

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 h-x diagram.

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 h-x diagram.

EPW 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 format is EPW (EnergyPlus Weather Format), which for every hour of the year contains 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 and the humidity ratio and relative humidity for every hour.

Climate analysis on the h-x diagram

Plotting a full year of hourly data onto the h-x diagram (a psychrometric climate chart) immediately shows how much of the year the conditions are suitable for free cooling, evaporative cooling or passive ventilation. For Prague, most summer hours lie in the region 15–28 °C / 40–75% RH, a band favorable for passive and hybrid cooling. Extreme hours above 30 °C make up just 1–3% of the year. The winter hours cluster into −5 to 5 °C / 70–90% RH.

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 (4–7 °C/km) is smaller than the dry one (9.8 °C/km).

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

Is an aerological diagram the same as an h-x diagram? 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 h-x diagram shows enthalpy and humidity ratio.