Dew Point Depression Calculator
Free Dew point depression Calculator for meteorology & atmospheric science. Enter variables to compute results with formulas and detailed steps.
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.