Mixing Ratio of Air Calculator
Enter your air temperature, dew point, and station pressure to calculate the actual mixing ratio, saturation mixing ratio, and relative humidity of the atmosphere. The mixing ratio tells you how many grams of water vapor are present for every kilogram of dry air. Switch between Celsius/Fahrenheit and hPa/inHg/mmHg unit options, and the results update as you type.
What is the mixing ratio of air?
The mixing ratio (also called the water vapor mixing ratio) is the mass of water vapor present in a parcel of air divided by the mass of the dry air in that same parcel. It is usually expressed in grams of water vapor per kilogram of dry air (g/kg). Unlike relative humidity, the mixing ratio does not change when an air parcel is lifted or compressed without adding or removing moisture, which makes it a conserved quantity and a preferred measure in meteorology and atmospheric science. The saturation mixing ratio is the maximum amount of water vapor the air can hold at a given temperature and pressure. When the actual mixing ratio equals the saturation mixing ratio, the relative humidity is 100% and condensation or precipitation can begin.
How is the mixing ratio calculated?
The calculation proceeds in two stages. First, the actual vapor pressure (e) is obtained from the dew point temperature using the Magnus approximation: e = 6.112 * exp(17.67 * T_dew / (T_dew + 243.5)) [hPa] Then the mixing ratio is derived from the vapor pressure and the station pressure (p): w = 621.97 * e / (p - e) [g/kg] The saturation mixing ratio (ws) uses the same formula but replaces the dew point with the actual air temperature. Relative humidity is then simply w / ws * 100. Station pressure (the actual pressure at the measurement point) must be used here, not sea-level or altimeter pressure, because the formula depends on the total pressure confining the air parcel.
Dew point depression and condensation risk
The dew point depression is the difference between the air temperature and the dew point. It is one of the most practical numbers in weather forecasting: - A depression of 0 means the air is saturated: fog, dew, or cloud is forming right now. - A depression of 1-5 degrees indicates high humidity. Morning dew or low cloud is common. - A depression greater than 10 degrees indicates dry air; condensation will not occur without significant cooling. Pilots use dew point depression to estimate cloud-base height: every 2.5 degrees Celsius of depression corresponds to roughly 1,000 feet (305 m) of cloud base above the surface, a rule used in general aviation weather briefings worldwide.
Mixing ratio vs. relative humidity: which should you use?
Relative humidity is intuitive and widely reported, but it depends on temperature: the same mass of moisture gives a higher relative humidity at low temperatures and a lower one at high temperatures. This means relative humidity drops during the afternoon (as air warms) even though no moisture has been added or removed. The mixing ratio avoids this limitation. Because it measures the actual mass of water vapor - independent of temperature - it stays constant as air rises and expands adiabatically, as long as no condensation occurs. Meteorologists track the mixing ratio to follow air mass origin and history, and it feeds directly into calculations for lifted condensation level (LCL), convective available potential energy (CAPE), and many other derived stability indices.
Mixing ratio and relative humidity guide
| Relative humidity (%) | Mixing ratio (typical, g/kg) | Atmospheric condition |
|---|---|---|
| 0-25 | 0-5 | Very dry - desert or high-altitude conditions |
| 25-40 | 5-8 | Dry - low condensation risk, good visibility |
| 40-60 | 8-12 | Comfortable - typical mid-latitude range |
| 60-75 | 12-16 | Humid - noticeable moisture, tropical margins |
| 75-90 | 16-22 | Very humid - tropical / maritime conditions |
| 90-100 | 22+ | Near saturated - fog, low cloud, or precipitation |
Typical mixing ratio ranges and their associated relative humidity feel and meteorological significance.
Frequently asked questions
What is a typical mixing ratio for comfortable outdoor air?
A mixing ratio of 8-14 g/kg is generally considered comfortable for most people. Values below 5 g/kg feel very dry and can irritate mucous membranes, while values above 20 g/kg feel oppressively humid. The range varies with climate: a hot, humid tropical day might reach 25 g/kg, while a cold winter day can be well below 2 g/kg even at 80% relative humidity.
Why does my mixing ratio change when I change station pressure?
The mixing ratio formula includes station pressure in the denominator (p - e). At higher altitudes, station pressure is lower, so the same amount of water vapor exerts a larger fraction of the total pressure and the mixing ratio is slightly higher than at sea level. This is why pilots and meteorologists always specify station pressure rather than sea-level (altimeter) pressure when computing mixing ratios.
Can the dew point ever be higher than the air temperature?
No. The dew point is the temperature at which the air becomes saturated with its current moisture content. If the dew point were above the air temperature, the air would already be supersaturated and condensation would be occurring. In practice, relative humidity reaches 100% when the dew point equals the air temperature - enter equal values in the calculator and you will see relative humidity of exactly 100%.
What is the difference between station pressure and altimeter setting?
Station pressure is the actual atmospheric pressure measured at the observation site. Altimeter setting and sea-level pressure are station pressure corrected upward by an estimated column of air to the standard datum (sea level), so that pressure maps can be compared across sites at different elevations. The mixing ratio formula requires the actual (station) pressure that is confining the air parcel, not the corrected sea-level value. Standard sea-level station pressure is 1013.25 hPa / 29.92 inHg / 760 mmHg.
How do I estimate cloud base from the dew point depression?
A simple rule of thumb used in aviation says that cloud base (in feet above ground level) is approximately 400 times the dew point depression in Fahrenheit, or equivalently about 1,000 feet for every 2.5 degrees Celsius of dew point depression. For example, a dew point depression of 8 °C gives an estimated cloud base of around 3,200 feet. This rule applies to convective cloud (cumulus type) forming in a well-mixed boundary layer and can differ for stratiform or orographic cloud.