A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Relative humidity measures saturation as a percentage and varies inversely with temperature, even for constant water vapor content. Dew point (the temperature at which air becomes saturated) directly measures the amount of water vapor present. Together, these indices determine cloud formation potential, evaporation rates, and the nature of atmospheric instability.
Calculate relative humidity and dew point from temperature and mixing ratio; observe how relative humidity changes through diurnal heating cycles; predict dew formation on clear nights.
From your study of saturation and dew point, you understand that air has a temperature-dependent capacity for water vapor, and that the dew point marks the temperature at which the air becomes saturated. Now the question becomes practical: how do meteorologists quantify and use moisture information to predict clouds, precipitation, and atmospheric stability? The answer lies in several complementary moisture indices, each revealing a different aspect of the atmosphere's moisture state.
Relative humidity (RH) is the most familiar index, expressed as the ratio of the air's actual water vapor content to the maximum it could hold at that temperature, multiplied by 100. An RH of 50% means the air contains half of its capacity. The critical insight is that RH changes with temperature even when the actual amount of moisture stays constant. As air cools overnight without gaining or losing water vapor, its capacity shrinks while its content stays fixed — so RH rises. By dawn, RH may reach 100% and dew forms. By afternoon, solar heating raises the capacity and RH drops to 30–40%, even though the air contains the same water vapor. This is why RH alone can be misleading: a desert afternoon at 15% RH and a tropical morning at 95% RH might contain similar absolute amounts of water vapor.
This is where dew point temperature becomes invaluable. Unlike RH, dew point is a direct measure of the actual water vapor content — it does not change with temperature (as long as no moisture is added or removed). A dew point of 20°C means there is a specific, fixed amount of water vapor present, regardless of whether the air temperature is 25°C or 40°C. Forecasters often prefer dew point over RH for this reason: dew points above 20°C signal oppressively humid conditions, while dew points below 10°C feel dry and comfortable. The dew point depression — the gap between temperature and dew point — tells you how far the air is from saturation. A small depression (say 1–2°C) means clouds are likely; a large depression (15–20°C) means the air is far from condensation.
Beyond these basic measures, meteorologists use derived indices to assess atmospheric stability and convective potential. The mixing ratio (grams of water vapor per kilogram of dry air) provides an absolute moisture measure that is conserved as air rises or sinks without condensation. The saturation mixing ratio at a given temperature defines the upper limit — and the ratio between the two gives you RH from a different angle. For forecasting convection, the vertical profile of moisture matters enormously: a moist lower atmosphere beneath a dry mid-level layer creates conditions where evaporative cooling of rain can produce powerful downdrafts, while a uniformly moist column favors widespread stratiform rain. These moisture indices, plotted on a sounding diagram alongside temperature, give forecasters the information they need to predict whether a given day will produce fog, fair-weather cumulus, or violent thunderstorms.