Actual Vapour Pressure Given Evaporation Loss Per Day Calculator

Formula Used:

\[ v = V - \left( \frac{E}{C' \times (1.465 - (0.00732 \times P_a)) \times (0.44 + (0.0732 \times u))} \right) \]

Maximum Vapour Pressure (V):

cm Hg

Evaporation Loss per Day (E):

Rohwer's Formula Constant (C'):

Atmospheric Pressure (P_a):

cm Hg

Mean Wind Velocity (u):

m/s

Actual Vapour Pressure (v):

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1. What is Actual Vapour Pressure?

Actual Vapour Pressure refers to the pressure exerted by the water vapour present in the air, reflecting the current humidity and temperature. It is typically measured in cm Hg or kPa and is a key parameter in atmospheric science and hydrology.

2. How Does the Calculator Work?

The calculator uses Rohwer's formula:

\[ v = V - \left( \frac{E}{C' \times (1.465 - (0.00732 \times P_a)) \times (0.44 + (0.0732 \times u))} \right) \]

Where:

\( v \) — Actual vapour pressure (cm Hg)
\( V \) — Maximum vapour pressure (cm Hg)
\( E \) — Evaporation loss per day (m)
\( C' \) — Rohwer's formula constant
\( P_a \) — Atmospheric pressure (cm Hg)
\( u \) — Mean wind velocity (m/s)

Explanation: The formula calculates actual vapour pressure by accounting for evaporation loss adjusted by atmospheric conditions and wind velocity.

3. Importance of Vapour Pressure Calculation

Details: Accurate vapour pressure estimation is crucial for weather forecasting, agricultural planning, hydrological studies, and understanding evaporation processes in various environmental conditions.

4. Using the Calculator

Tips: Enter all values in the specified units. Ensure all inputs are positive values. The Rohwer's constant is an empirical coefficient that may vary based on local conditions.

5. Frequently Asked Questions (FAQ)

Q1: What is the difference between actual and maximum vapour pressure?
A: Maximum vapour pressure is the saturation pressure at a given temperature, while actual vapour pressure is the current pressure exerted by water vapour in the air.

Q2: How does wind velocity affect vapour pressure?
A: Higher wind velocities increase evaporation rates, which affects the calculation of actual vapour pressure from maximum vapour pressure.

Q3: What are typical values for Rohwer's constant?
A: Rohwer's constant typically ranges between 0.5-1.0 and is determined empirically for specific geographical locations and conditions.

Q4: Why is atmospheric pressure important in this calculation?
A: Atmospheric pressure affects the evaporation rate and thus influences the relationship between maximum and actual vapour pressure.

Q5: Can this formula be used for all climatic conditions?
A: While Rohwer's formula is widely applicable, it may require calibration for extreme conditions or specific microclimates.