Vapor Pressure Deficit (VPD): Leaf-to-Air Tetens Formula (kPa)

Use leaf and air temperatures separately

The calculation assumes saturated water vapor inside the leaf and compares it with ambient vapor pressure. It uses the FAO form of the saturation-pressure approximation: 0.6108*EXP(17.27*T/(T+237.3)), with T in degrees Celsius and pressure in kPa.

This interactive example accepts temperatures from 0 to 50°C as an explicit operating range for the exercise, not a universal accuracy claim. It does not model ice saturation. Enter RH as a number from 0 to 100: 65 means 65%, while a spreadsheet value of 0.65 means 0.65% in this formula.

Use measured or deliberately assumed leaf temperature; leaves are not always a fixed number of degrees cooler than air. The result is an estimated pressure difference, not a transpiration rate or a crop-specific target.

Calculate leaf-to-air difference

B2 is leaf temperature, B3 air temperature and B4 RH percentage. Compute leaf saturation pressure minus actual ambient vapor pressure. E2 contains the unrounded result.

The example 26°C leaf, 28°C air and 65% RH gives approximately 0.904 kPa. Crop, stage, radiation, airflow and water availability affect interpretation; this example does not label a universal optimal or harmful interval.

=IF(COUNT(B2:B4)<>3,"Enter numeric inputs",IF(OR(B2<0,B2>50,B3<0,B3>50),"Example temperature range is 0 to 50 C",IF(OR(B4<0,B4>100),"RH must be between 0 and 100",(0.6108*EXP(17.27*B2/(B2+237.3)))-(0.6108*EXP(17.27*B3/(B3+237.3)))*B4/100)))

Inspect leaf saturation pressure

E3 shows the saturation pressure at the leaf temperature.

=IF(COUNT(B2:B4)<>3,"Enter numeric inputs",IF(OR(B2<0,B2>50,B3<0,B3>50),"Example temperature range is 0 to 50 C",IF(OR(B4<0,B4>100),"RH must be between 0 and 100",0.6108*EXP(17.27*B2/(B2+237.3)))))

Inspect actual ambient vapor pressure

E4 multiplies saturation pressure at the air temperature by RH/100.

=IF(COUNT(B2:B4)<>3,"Enter numeric inputs",IF(OR(B2<0,B2>50,B3<0,B3>50),"Example temperature range is 0 to 50 C",IF(OR(B4<0,B4>100),"RH must be between 0 and 100",(0.6108*EXP(17.27*B3/(B3+237.3)))*B4/100)))

Compare with air VPD

E5 uses saturation pressure at the air temperature minus actual ambient pressure. It matches leaf-to-air difference only when leaf and air temperatures are equal.

A cooler leaf can produce a negative leaf-to-air result even with RH at or below 100%. This indicates ambient vapor pressure above saturation at the entered leaf temperature, consistent with condensation potential; preserve the sign rather than silently clamping to zero. Air VPD in the stated RH range remains nonnegative.

=IF(COUNT(B2:B4)<>3,"Enter numeric inputs",IF(OR(B2<0,B2>50,B3<0,B3>50),"Example temperature range is 0 to 50 C",IF(OR(B4<0,B4>100),"RH must be between 0 and 100",(0.6108*EXP(17.27*B3/(B3+237.3)))*(1-B4/100))))

Further reading

• FAO — Meteorological data, saturation and actual vapor pressure
• University of Florida IFAS — Water and Humidity, Vapor Pressure Deficit