Dewpoint & Relative Humidity Calculator

Enter your dry-bulb and wet-bulb temperatures to calculate dewpoint, relative humidity, estimated cloud base, vapor pressure, and drone fly rating.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Dry-Bulb Temperature (°C)
Input the standard air temperature measured by a dry thermometer, away from direct sunlight.
2
Enter Wet-Bulb Temperature (°C)
Input the temperature read by a thermometer with a wet cloth covering its bulb. This value must be less than or equal to the dry-bulb temperature.
3
Review your results
The calculator will display the relative humidity, dewpoint temperature, dewpoint depression, estimated cloud base height, and vapor pressure.

Example Calculation

An aviation enthusiast wants to determine humidity and cloud base from a dry-bulb temperature of 25°C and a wet-bulb temperature of 18°C.

Dry-Bulb Temperature (°C)

Wet-Bulb Temperature (°C)

Results

47.2

Tips

Interpreting Dewpoint Depression

A small dewpoint depression (e.g., less than 2°C) indicates air near saturation, suggesting a high likelihood of fog or low clouds, especially for aviation.

Cloud Base for Pilots

The estimated cloud base height is crucial for VFR (Visual Flight Rules) pilots and drone operators. A lower cloud base limits visibility and maneuverability.

Wet-Bulb Temperature Significance

The wet-bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water. It's a key indicator of atmospheric moisture and comfort, particularly in hot conditions.

Precision Weather Metrics: Dewpoint & Relative Humidity Calculator

The Dewpoint & Relative Humidity Calculator is an indispensable tool for meteorologists, pilots, and drone operators, providing critical weather parameters from dry-bulb and wet-bulb temperatures. It accurately calculates relative humidity, dewpoint, dewpoint depression, and estimated cloud base height. For a dry-bulb temperature of 25°C and a wet-bulb temperature of 18°C, the calculator reveals a relative humidity of 47.2%, offering vital insights for aviation and drone planning in 2025.

Why Dry-Bulb and Wet-Bulb Measurements are Critical

Dry-bulb and wet-bulb temperatures are critical measurements because, together, they provide a comprehensive picture of atmospheric moisture and its impact on comfort and safety. The dry-bulb temperature is simply the ambient air temperature, while the wet-bulb temperature indicates how much moisture the air can still absorb. Their difference, the wet-bulb depression, is directly related to relative humidity and dewpoint. For aviation, these metrics are crucial for assessing visibility, icing potential, and cloud base height. For human health, a high wet-bulb temperature (e.g., above 32°C / 90°F) signals a dangerous risk of heat stroke, as the body's ability to cool through sweating is severely compromised, a growing concern in increasingly warmer summers.

The Psychrometric Equations Behind Humidity Calculations

The Dewpoint & Relative Humidity Calculator uses psychrometric equations, specifically the Sprung formula, to derive crucial atmospheric properties from dry-bulb and wet-bulb temperatures. This method is highly valued for its accuracy in various scientific and engineering applications.

The key steps and formulas include:

Calculate Saturation Vapor Pressure (es) at Dry-Bulb (T) and Wet-Bulb (Tw): es(t) = 6.112 × exp((17.67 × t) / (t + 243.5))
Calculate Actual Vapor Pressure (e) using Sprung formula: e = es(Tw) - A × P × (T - Tw) (where A is the psychrometric constant and P is atmospheric pressure, typically 1013.25 hPa)
Calculate Relative Humidity (RH): RH = (e / es(T)) × 100
Calculate Dewpoint (Td) by inverting Magnus formula: Td = (243.5 × ln(e / 6.112)) / (17.67 - ln(e / 6.112))

These interconnected formulas allow for the precise derivation of moisture metrics.

💡 Beyond the scientific measurements of humidity, its impact on human sensation is also significant. To understand how moisture affects your perceived comfort, our Humidity Comfort Index Calculator provides a valuable metric.

Analyzing a 25°C Dry-Bulb, 18°C Wet-Bulb Scenario

Let's use the default values to calculate the atmospheric conditions. An observer measures:

Dry-Bulb Temperature (°C): 25°C
Wet-Bulb Temperature (°C): 18°C

Step-by-step Calculation (simplified for display):

Calculate Saturation Vapor Pressure at T (25°C): es(25) ≈ 31.69 hPa
Calculate Saturation Vapor Pressure at Tw (18°C): es(18) ≈ 20.62 hPa
Calculate Actual Vapor Pressure (e): Using the Sprung formula with P = 1013.25 hPa and A = 0.000799: e = 20.62 - (0.000799 × 1013.25 × (25 - 18)) ≈ 14.95 hPa
Calculate Relative Humidity (RH): RH = (14.95 / 31.69) × 100 ≈ 47.15%
Calculate Dewpoint Temperature (Td): Using the inverse Magnus formula with e = 14.95 hPa: Td ≈ 12.98°C
Calculate Dewpoint Depression: 25°C - 12.98°C = 12.02°C
Estimate Cloud Base: (12.02 / 2.5) × 1000 ft ≈ 4808 ft AGL

The relative humidity is approximately 47.2%, with a dewpoint of 13.0°C and an estimated cloud base of 4808 feet AGL.

💡 Accurate humidity data is vital for many applications, from aviation to specialized fields like long-range shooting. For those interested in the latter, our Humidity Effect on Ballistics Calculator shows how atmospheric moisture can influence bullet trajectory.

Critical Weather Parameters for Aviation and Drone Operations

Accurate dew point, relative humidity, and cloud base calculations are profoundly important for aviation and drone pilots, directly impacting visibility, icing potential, and flight safety. According to FAA (Federal Aviation Administration) and EASA (European Union Aviation Safety Agency) guidelines, a dew point depression of less than 2°C is a strong indicator of impending fog or low clouds, which can severely limit visibility and ground flights. For drone operations, high humidity (e.g., above 90%) poses a significant risk to electronic components, potentially leading to malfunctions or corrosion, especially for advanced sensors common in 2025 models. Pilots use these metrics to make critical go/no-go decisions, ensuring adherence to visual flight rules (VFR) or instrument flight rules (IFR) minimums.

Psychrometric Equations: Different Approaches to Humidity Calculation

While the Sprung formula is a widely accepted psychrometric equation for deriving actual vapor pressure from wet-bulb and dry-bulb temperatures, other variants exist within scientific and engineering practice. For instance, different coefficients may be applied in the psychrometric constant (A) to account for variations in air pressure, sensor ventilation (e.g., naturally aspirated vs. forced-air psychrometers), or specific temperature ranges. Empirical adjustments might also be incorporated into the formulas for specialized applications that require higher precision, such as in metrology or industrial process control. The fundamental principle, relating the cooling effect of evaporation to the air's moisture content, remains consistent across these models. However, the exact constants and correction factors can vary slightly based on the required precision and the type of measurement equipment, influencing the final relative humidity or dew point result by a fraction of a degree or percentage point.

Frequently Asked Questions

What is wet-bulb temperature and why is it important?

Wet-bulb temperature is the lowest temperature that air can reach by evaporating water into it, measured by a thermometer with a wet wick. It is important because it reflects the air's moisture content and its cooling potential through evaporation. In extremely hot and humid conditions, a high wet-bulb temperature indicates a severe risk of heat stress, as the body's ability to cool itself by sweating is significantly reduced, posing a serious health hazard.

How are dewpoint and relative humidity calculated from dry-bulb and wet-bulb temperatures?

Dewpoint and relative humidity are calculated from dry-bulb and wet-bulb temperatures using psychrometric equations, such as the Sprung formula. These formulas first determine the actual vapor pressure in the air based on the difference between the dry-bulb and wet-bulb readings. From this actual vapor pressure, relative humidity is derived by comparing it to the saturation vapor pressure at the dry-bulb temperature, and the dewpoint is found by inverting the saturation vapor pressure equation.

What is dewpoint depression and its role in aviation?

Dewpoint depression is the difference between the dry-bulb temperature and the dewpoint temperature. It is crucial in aviation because it indicates how close the air is to saturation. A small dewpoint depression (e.g., 2°C or less) means the air is nearly saturated, which significantly increases the risk of fog, mist, or low clouds, severely impacting visibility and flight safety. Pilots use this value to assess weather conditions for takeoff and landing.

How is cloud base height estimated from dewpoint depression?

Cloud base height can be estimated from dewpoint depression using a simple rule of thumb: for every 2.5°C (or 4.4°F) of dewpoint depression, the cloud base rises by approximately 1,000 feet AGL (Above Ground Level). This is because as unsaturated air rises, it cools at the dry adiabatic lapse rate until it reaches its dewpoint, at which point condensation begins to form clouds. This estimation is a vital tool for pilots and meteorologists.