11.temperature-pressure-moisture. Temperature, Pressure, and Moisture
Weather is the state of the atmosphere at a given time and place. Three fundamental atmospheric variables — temperature, pressure, and moisture — drive virtually every weather process a pilot encounters, from cloud formation and turbulence to wind, icing, and thunderstorms. Understanding how these three interact is the foundation of aviation weather theory.
Temperature
Temperature is a measure of the kinetic energy of air molecules. The sun heats Earth's surface unevenly because of differing latitudes, surface composition (water vs. land), cloud cover, and terrain. The surface in turn heats the air above it through conduction, convection, and radiation. This uneven heating is the root cause of nearly all atmospheric motion.
Key reference values:
- ISA standard sea-level temperature: 15 °C (59 °F)
- Standard temperature lapse rate: approximately 2 °C (3.5 °F) per 1,000 ft up to the tropopause (~36,000 ft), where temperature stabilizes near −56.5 °C
- Dry adiabatic lapse rate: 3 °C per 1,000 ft (rising unsaturated air)
- Moist (saturated) adiabatic lapse rate: approximately 1.5 °C per 1,000 ft
To convert: °F = (°C × 9/5) + 32, and °C = (°F − 32) × 5/9.
Warm air is less dense than cold air. As temperature rises, air expands, density falls, and pressure aloft over that column increases relative to surrounding columns. This sets up horizontal pressure differences that produce wind.
Pressure
Atmospheric pressure is the weight of the column of air above a point. At sea level under standard conditions, that column exerts:
- 29.92 inches of mercury (in. Hg)
- 1013.2 millibars (hectopascals)
- 14.7 pounds per square inch
Pressure decreases with altitude — roughly 1 in. Hg per 1,000 ft in the lower atmosphere. Pressure also varies horizontally because of temperature differences and large-scale circulation. Areas of relatively low pressure (lows) are associated with rising air, clouds, and precipitation; areas of high pressure (highs) are associated with descending air, clearing, and generally fair weather.
Pressure is measured with a mercury or aneroid barometer and reported as either station pressure or, for aviation, corrected to sea level as the altimeter setting. When set in the Kollsman window, the altimeter setting causes the altimeter to read field elevation when on the ground.
Density altitude is pressure altitude corrected for nonstandard temperature. High DA — caused by high elevation, high temperature, and/or high humidity — degrades aircraft performance dramatically: less thrust, less lift, longer takeoff roll, reduced climb. A useful rule of thumb is that DA increases roughly 120 ft for every 1 °C above ISA.
Moisture (Humidity)
Water vapor is an invisible gas mixed with the dry air. It enters the atmosphere primarily through evaporation from oceans, lakes, and moist soil, and through transpiration from plants. Warm air can hold far more water vapor than cold air — a doubling occurs roughly every 20 °F.
Key terms:
- Relative humidity: the percentage of water vapor present compared to the maximum the air could hold at that temperature. 100% means saturated.
- Dew point: the temperature to which a parcel of air must be cooled, at constant pressure, to reach saturation. The smaller the temperature/dew point spread, the higher the relative humidity.
- Condensation: when air cools to its dew point, water vapor condenses into visible moisture — fog, clouds, dew, or precipitation. Cooling generally occurs by lifting (orographic, convective, frontal, or convergent), by advection over a colder surface, or by radiational cooling at night.
A temperature/dew point spread of 5 °F (about 3 °C) or less is a strong indicator that fog, low stratus, or visible moisture is likely. Pilots use this directly during preflight to anticipate ceilings.
Moisture also affects density. Water vapor (molecular weight 18) is lighter than the dry air it displaces (average molecular weight ~29), so humid air is less dense than dry air at the same temperature and pressure — another reason hot, humid days hurt performance.
How the three interact
The ideal-gas relationship can be written: P = ρRT, where P is pressure, ρ is density, R is the gas constant, and T is absolute temperature. The aviation takeaway:
- Heat the air → it expands, density drops, performance falls.
- Add moisture → density drops further.
- Lower the pressure (or climb to higher elevation) → density drops.
Weather systems arise because the sun heats Earth unevenly, producing pressure gradients; air flows from high to low pressure (modified by Coriolis and friction) to create wind; rising air cools adiabatically until it reaches its dew point, condensing moisture into clouds and precipitation. Every weather phenomenon a pilot studies — stability, fronts, thunderstorms, icing, turbulence — is ultimately a consequence of how temperature, pressure, and moisture distribute themselves in the atmosphere.