Vapor Pressure from Dew Point Calculator
An expert tool to accurately calculate vapor pressure using dew point temperature, based on the August-Roche-Magnus (or Tetens’) formula. Essential for meteorology, HVAC, and scientific applications.
Enter the temperature to which air must be cooled to become saturated with water vapor.
Select the unit for your dew point temperature input.
Vapor Pressure vs. Dew Point Temperature
| Dew Point (°C) | Dew Point (°F) | Vapor Pressure (kPa) | Vapor Pressure (mb) |
|---|---|---|---|
| -10 | 14 | 0.26 | 2.60 |
| 0 | 32 | 0.61 | 6.11 |
| 5 | 41 | 0.87 | 8.72 |
| 10 | 50 | 1.23 | 12.28 |
| 15 | 59 | 1.71 | 17.05 |
| 20 | 68 | 2.34 | 23.39 |
| 25 | 77 | 3.17 | 31.69 |
| 30 | 86 | 4.25 | 42.46 |
What is Vapor Pressure from Dew Point?
Calculating vapor pressure from dew point is a fundamental process in meteorology and thermodynamics. The **dew point** is the temperature at which air becomes saturated with water vapor, causing condensation (dew) to form. The **vapor pressure** itself is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. When you know the dew point, you effectively know the temperature at which the air’s moisture content would reach saturation. Therefore, you can directly calculate the saturation vapor pressure at that specific temperature, which is equal to the actual vapor pressure of the air parcel.
This calculation is crucial for anyone needing to understand atmospheric moisture content, including meteorologists predicting fog or dew, HVAC engineers designing systems to control humidity, and scientists studying environmental processes. A common misunderstanding is confusing actual vapor pressure with saturation vapor pressure at the ambient air temperature. The calculator clarifies this: it determines the *actual* vapor pressure, which is equivalent to the saturation vapor pressure *at the dew point temperature*, not the air temperature.
The Formula to Calculate Vapor Pressure Using Dew Point
The relationship between dew point temperature and saturation vapor pressure is non-linear and is described by empirical formulas derived from the Clausius-Clapeyron relation. One of the most widely used and accurate approximations is the **August-Roche-Magnus formula**, often simplified as Tetens’ formula.
The formula is:
e = 0.6108 * exp( (17.27 * Td) / (Td + 237.3) )
Understanding the variables is key to using this formula correctly.
| Variable | Meaning | Unit (for this formula) | Typical Range |
|---|---|---|---|
| e | Actual Vapor Pressure | kilopascals (kPa) | 0 to ~7.5 kPa |
| Td | Dew Point Temperature | Degrees Celsius (°C) | -40°C to 40°C |
| exp() | The exponential function (e^x) | Unitless | N/A |
| 0.6108, 17.27, 237.3 | Empirical constants | Unitless | Fixed values |
For more advanced analysis, you might consult a psychrometric chart online to visualize these relationships.
Practical Examples
Example 1: A Cool, Damp Morning
Imagine a spring morning where the dew point is measured to be 5°C. What is the actual vapor pressure in the air?
- Input (Dew Point): 5 °C
- Calculation: e = 0.6108 * exp((17.27 * 5) / (5 + 237.3)) = 0.6108 * exp(86.35 / 242.3)
- Result (Vapor Pressure): Approximately 0.87 kPa
Example 2: A Hot and Humid Summer Day
On a sweltering summer day, the dew point is a sticky 25°C. Let’s calculate the vapor pressure.
- Input (Dew Point): 25 °C
- Calculation: e = 0.6108 * exp((17.27 * 25) / (25 + 237.3)) = 0.6108 * exp(431.75 / 262.3)
- Result (Vapor Pressure): Approximately 3.17 kPa
This high vapor pressure signifies a large amount of moisture in the air, which is why it feels “muggy.” This moisture content also affects the air density.
How to Use This Vapor Pressure Calculator
- Enter the Dew Point: Input the known dew point temperature into the “Dew Point Temperature” field.
- Select the Unit: Use the dropdown menu to choose the correct unit for your input data: Celsius (°C), Fahrenheit (°F), or Kelvin (K). The calculator automatically converts the input to Celsius for the formula.
- Click “Calculate”: The calculator will instantly compute the vapor pressure and display it in the results section, along with key intermediate values used in the calculation.
- Interpret the Results: The primary result is the actual vapor pressure in kilopascals (kPa). The chart and table provide additional context for how vapor pressure changes with temperature.
Key Factors That Affect Vapor Pressure
The actual vapor pressure of the air is determined by one primary factor, but its relationship to saturation is affected by others.
- Dew Point Temperature: This is the most direct factor. A higher dew point temperature means there is more moisture in the air, which results in a higher actual vapor pressure.
- Air Temperature: While it doesn’t change the *actual* vapor pressure, air temperature determines the *saturation* vapor pressure—the maximum amount of moisture the air *can* hold. This is why relative humidity drops as air temperature rises, even if the dew point stays the same. To understand this better, a relative humidity calculator can be very helpful.
- Atmospheric Pressure: While the standard formulas assume standard pressure, significant changes in altitude (and thus pressure) can slightly alter vapor pressure behavior. Our atmospheric pressure calculator can provide context here.
- Evaporation Sources: Proximity to large bodies of water, wet ground, or transpiring plants increases the amount of available moisture, potentially raising the dew point and thus the vapor pressure.
- Air Mass Origin: Air masses originating from over oceans (maritime) will have much higher dew points and vapor pressures than those from over large landmasses (continental).
- Wind and Advection: Wind can transport air with different moisture characteristics into a region, rapidly changing the dew point and vapor pressure.
Frequently Asked Questions (FAQ)
- 1. What is the difference between actual vapor pressure and saturation vapor pressure?
- Actual vapor pressure is the pressure currently exerted by water vapor in the air. Saturation vapor pressure is the maximum possible vapor pressure at the current *air temperature*. Air is saturated (100% relative humidity) when these two values are equal.
- 2. Why does the calculator use kilopascals (kPa)?
- Kilopascals are the standard SI unit for pressure and are widely used in scientific and meteorological contexts for consistency.
- 3. How accurate is the Tetens’ formula?
- It is a highly accurate approximation for most environmental conditions, typically within 1% of more complex formulas across a wide range of temperatures.
- 4. Can I calculate dew point from vapor pressure?
- Yes, the formula can be rearranged to solve for dew point if the vapor pressure is known. It’s the inverse of this calculation.
- 5. Does this calculator work for sub-zero temperatures?
- Yes, the formula is valid for both liquid water and supercooled water. For vapor pressure over ice, slightly different constants are sometimes used for maximum precision, but this formula provides a very close estimate. A deeper dive might involve checking a weather station guide for sensor specifics.
- 6. What is a “high” or “low” vapor pressure?
- A vapor pressure below 1.0 kPa generally feels dry, while a value above 2.5 kPa is typically considered humid or “muggy.”
- 7. How is this different from a relative humidity calculator?
- This calculator gives an absolute measure of moisture (pressure). A relative humidity calculator provides a percentage that compares the actual vapor pressure to the saturation vapor pressure at the current air temperature. This tool provides one of the inputs needed for that calculation.
- 8. How does pressure affect the boiling point of water?
- Vapor pressure is directly related to boiling point. A liquid boils when its saturation vapor pressure equals the surrounding atmospheric pressure. This is why understanding pressure is key to using a boiling point altitude calculator.