Dew Point Depression Calculator
Free Dew point depression Calculator for meteorology & atmospheric science. Enter variables to compute results with formulas and detailed steps.
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Where T is air temperature, Td is dew point temperature, both in Celsius. The Magnus formula converts between dew point and relative humidity. Vapor pressure deficit = saturation vapor pressure minus actual vapor pressure.
Last reviewed: December 2025
Worked Examples
Example 1: Summer Afternoon Assessment
Example 2: Evening Fog Potential
Background & Theory
The Dew Point Depression Calculator applies the following established principles and formulas. Earth science calculators draw on a wide range of measurement scales and physical principles that quantify natural phenomena across geological, atmospheric, and hydrological systems. Earthquake magnitude is most precisely described by the Moment Magnitude Scale (Mw), which replaced the original Richter scale for larger events. Mw is calculated as Mw = (2/3) log10(M0) − 10.7, where M0 is the seismic moment in dyne-centimeters. The Richter scale, while still referenced colloquially, is a local magnitude (ML) measurement derived from peak seismograph amplitude at a standard 100 km distance. Wind intensity is classified using the Beaufort Scale, a 13-point empirical scale (0–12) relating wind speed in knots to observable sea and land effects, with Beaufort 12 corresponding to hurricane-force winds above 64 knots. Tropical cyclone intensity is further categorized by the Saffir-Simpson Hurricane Wind Scale, which assigns Categories 1 through 5 based on sustained wind speed, correlating with expected structural damage. Mineral hardness is quantified on the Mohs scale (1–10), comparing scratch resistance relative to reference minerals from talc (1) to diamond (10). Soil composition analysis measures the proportions of sand, silt, and clay by particle size, alongside organic matter content, bulk density, and porosity, which together determine engineering and agricultural suitability. Seismic wave velocity in rock varies by material: P-waves travel at approximately 5–7 km/s in granite and 1.5 km/s in water, while S-waves travel at roughly 60% of P-wave speeds. Atmospheric pressure decreases with altitude according to the barometric formula: P = P0 × exp(−Mgh / RT), where M is molar mass of air, g is gravitational acceleration, h is altitude, R is the universal gas constant, and T is temperature in Kelvin. Standard sea-level pressure is 101,325 Pa. Tidal calculations use harmonic analysis of gravitational forcing by the Moon and Sun, with the principal lunar semidiurnal tidal constituent (M2) having a period of approximately 12.42 hours.
History
The history behind the Dew Point Depression Calculator traces back through the following developments. The systematic study of Earth's structure and processes spans millennia, but the scientific foundations were laid in the seventeenth century. In 1669, Danish naturalist Nicolas Steno published his principles of stratigraphy, establishing the laws of superposition, original horizontality, and lateral continuity — foundational rules for reading rock layers that remain in use today. Scottish geologist James Hutton introduced the concept of uniformitarianism in 1788, proposing that geological processes observable in the present have operated throughout Earth's history at broadly consistent rates. This idea of deep time challenged prevailing biblical chronologies and set the stage for modern geology. Charles Lyell systematized these ideas in his landmark three-volume work Principles of Geology, published beginning in 1830, which directly influenced Charles Darwin's thinking on biological evolution during the voyage of the Beagle. The nineteenth century saw growing curiosity about continental shapes, but a coherent theory awaited Alfred Wegener, a German meteorologist who proposed continental drift in 1912, arguing that the continents had once formed a supercontinent he called Pangaea. His evidence included matching fossil records and geological formations across the Atlantic, but his mechanism was disputed for decades. The theory gained acceptance in the 1960s when seafloor spreading was confirmed through paleomagnetic studies, and plate tectonics emerged as the unifying framework of modern geoscience. The United States Geological Survey was established by Congress in 1879 to classify public lands and examine the geological structure, mineral resources, and products of the national domain. The twentieth century brought instrumental advances, including the global seismograph network deployed after World War II, initially to monitor nuclear tests, which dramatically improved earthquake detection and characterization. Satellite Earth observation began in earnest with the Landsat program launched in 1972, enabling continuous global monitoring of land use, glacier retreat, and vegetation patterns. Today, GPS networks, LIDAR scanning, and ocean-floor mapping provide centimeter-scale precision for tracking tectonic motion, sea level rise, and volcanic deformation in near real time.
Frequently Asked Questions
Formula
Depression = T - Td; RH = 100 x exp(17.625*Td/(243.04+Td)) / exp(17.625*T/(243.04+T))
Where T is air temperature, Td is dew point temperature, both in Celsius. The Magnus formula converts between dew point and relative humidity. Vapor pressure deficit = saturation vapor pressure minus actual vapor pressure.
Worked Examples
Example 1: Summer Afternoon Assessment
Problem: T=30 C, Td=20 C, P=1013.25 hPa, wind 15 km/h.
Solution: Depression = 30-20 = 10 C\nRH = 100*exp(17.625*20/263.04)/exp(17.625*30/273.04) = 55.3%\nes=42.43 hPa, ea=23.37 hPa\nVPD = 19.06 hPa\nCloud base = 125*10 = 1250 m
Result: Depression: 10 C | RH: 55.3% | VPD: 19.06 hPa | Cloud base: 1250 m
Example 2: Evening Fog Potential
Problem: T=14 C, Td=12 C, P=1020 hPa, wind 5 km/h.
Solution: Depression = 14-12 = 2 C\nRH = 87.5%\nes=15.98, ea=14.02\nVPD = 1.96 hPa\nCloud base = 250 m\nFog risk: High
Result: Depression: 2 C | RH: 87.5% | High fog risk | Cloud base: 250 m
Frequently Asked Questions
What is dew point depression and what does it indicate?
Dew point depression is the difference between the current air temperature and the dew point temperature, measured in degrees. It indicates how far the air is from reaching saturation. A depression of zero means the air is fully saturated and fog or dew is forming. A small depression of 1 to 3 degrees suggests high humidity and potential for condensation with slight cooling. Larger depressions indicate drier air that is further from saturation. Meteorologists use dew point depression to assess fog potential, estimate cloud base heights, evaluate evaporation rates, and characterize air mass moisture content. It is one of the most important variables in weather observation and forecasting.
How is dew point depression used to forecast fog?
Fog forms when the dew point depression approaches zero, meaning the air has cooled to its dew point temperature and water vapor begins condensing on surfaces and in the air near the ground. Forecasters monitor the evening dew point depression and the rate of nocturnal cooling to predict whether fog will form overnight. If the depression is less than 3 degrees at sunset and conditions favor continued cooling such as clear skies, light winds, and moist soil, fog is probable. The convergence rate of temperature toward the dew point during the night, typically 1 to 3 degrees per hour in favorable conditions, helps estimate the time of fog onset. Dew point depression is the single most useful parameter for fog prediction.
What is the relationship between dew point depression and relative humidity?
Dew point depression and relative humidity are inversely related measures of atmospheric moisture. As the depression decreases, relative humidity increases, reaching 100 percent when the depression is zero. The relationship is not perfectly linear because relative humidity depends on the ratio of actual to saturation vapor pressure, both of which increase exponentially with temperature. At a given depression value, relative humidity is higher at lower temperatures. For example a 5-degree depression at 30 C corresponds to roughly 75 percent relative humidity, while the same depression at 10 C corresponds to roughly 68 percent. The Magnus formula provides the exact conversion between these moisture variables.
What is the mixing ratio and how does it relate to dew point?
The mixing ratio is the mass of water vapor per mass of dry air, typically expressed in grams per kilogram. Unlike relative humidity, mixing ratio does not change as air temperature fluctuates, making it a conserved quantity useful for tracking air masses. The saturation mixing ratio depends on temperature and pressure, increasing approximately exponentially with temperature. The actual mixing ratio is determined by the dew point temperature and ambient pressure. The ratio of actual to saturation mixing ratio closely approximates relative humidity. Mixing ratio differences between air masses drive moisture convergence that fuels precipitation, and forecasters use it to identify moisture boundaries and track moisture transport from tropical source regions.
What is wet bulb temperature and how is it related to dew point depression?
Wet bulb temperature is the lowest temperature that can be achieved by evaporating water into the air at constant pressure. It falls between the air temperature and the dew point, and equals both when the air is saturated. The wet bulb depression (T minus Tw) is always less than or equal to the dew point depression because evaporative cooling cannot bring air below its dew point. Wet bulb temperature is critical for assessing heat stress on humans because the body cools itself through sweat evaporation. When wet bulb temperature exceeds 35 C, the human body can no longer cool itself effectively even in shade with unlimited water. The Stull approximation provides wet bulb from temperature and relative humidity.
How does dew point depression change with altitude?
As air rises in the atmosphere, temperature decreases at the dry adiabatic lapse rate of about 9.8 C per km while the dew point decreases at approximately 1.8 C per km. This means the dew point depression decreases with altitude at a rate of about 8 C per km. At the altitude where the depression reaches zero, the air is saturated and cloud base forms. This convergence rate is the basis for the Espy formula that estimates cloud base height as 125 meters per degree of surface depression. Above the cloud base, both temperature and dew point decrease at the moist adiabatic rate, maintaining saturation. Understanding this vertical variation is essential for predicting cloud formation and atmospheric stability.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy