Dew Point Calculator

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Our Dew Point Calculator is an easy-to-use tool that allows you to determine the dew point based on ambient temperature and relative humidity. The dew point is the value to which air must be cooled in order for the water vapor it contains to become saturated and condense as dew, and is an important indicator in meteorology, HVAC, and many industrial processes.

Features:

This calculator is based on the Magnus-Tetens formula (Mark G. Lawrence, 2005).
It provides an uncertainty of 0.35°C within a range of -40°C to 50°C.

In our daily lives, we are surrounded by meteorological phenomena that we often overlook. One of the most interesting and useful is the dew point. Although it sounds like a term for expert meteorologists, understanding it has surprising practical applications that can help you at home, in the garden, and even when planning your activities.

What Exactly Is the Dew Point?

Imagine a glass of ice-cold water on a hot day. Within minutes, the outside of the glass becomes covered in tiny water droplets. Where do they come from? It's not that the glass is leaking; it's that the air surrounding it has cooled so much upon contact with the cold surface that it can no longer hold all of its moisture. The dew point is exactly that temperature. In simple terms, the dew point is the temperature to which the air needs to cool for the water vapor it contains to condense and turn into liquid water. It's a much more accurate measure of humidity in the environment than "relative humidity," as it tells us at what exact temperature condensation will begin to form.

The Dew Point in Your Daily and Professional Life

Knowing this value is more than just a curiosity. It has a direct impact on our comfort, safety, and in various industries and in your home.

At Home:

Prevent Mold and Dampness: If the dew point outside is high and the temperature of your walls or windows is lower than that, condensation will appear. This can cause mold and damage the structure of your home. Our calculator helps you know when is a good time to ventilate or use a dehumidifier.
Wind Chill: A high dew point (above 20°C) means the air is "sticky" and sweating doesn't cool us effectively. A low dew point (below 10°C) indicates dry, comfortable air.

In Industry:

Prevent Corrosion: When painting or applying coatings to metal surfaces, it is crucial that the surface temperature is at least 3°C above the dew point. Otherwise, an invisible layer of condensation could form and ruin adhesion, causing rust and premature failure.
Storage: In warehouses and storage facilities, ensuring that the temperature does not drop below the dew point is vital to protecting moisture-sensitive products, such as electronics, paper, or food.

In Agriculture:

Protecting Crops from Frost: Farmers monitor the dew point. If the nighttime temperature is expected to drop to the dew point, dew will form on the plants. If that temperature is 0°C or below, frost will form, which can burn and destroy crops.
Pesticide Application: Some treatments are most effective when leaves are wet with dew, while others require them to be dry.

How Does Our Calculator Work?

Our tool uses the Magnus-Tetens formula, a widely accepted and accurate scientific approximation for determining the dew point. You simply need to enter two values, which you can obtain from any local weather service:

Air Temperature (°C)

Relative Humidity (%)

With this data, the calculator applies the formula and provides you with the result instantly, along with an interactive graph that allows you to visualize how the dew point varies as humidity changes, giving you a deeper understanding of the relationship between these variables.

Use our calculator to make more informed decisions, improve the comfort of your home, and better understand the world around you.

Technical Explanation

From a more technical perspective, the dew point (Td) is the temperature to which an air mass, at constant barometric pressure, must cool to reach water vapor saturation. Once this temperature is reached, the rate of condensation of water vapor equals the rate of evaporation, resulting in the formation of liquid water, known as dew.

To fully understand this, we must consider two key concepts:

Partial Pressure of Water Vapor (e): Air is not a single substance, but rather a mixture of gases (nitrogen, oxygen, etc.), including water vapor. The pressure exerted by water vapor alone within that mixture is known as its partial pressure. This pressure is directly related to the number of water molecules in a given volume of air.
Saturation Vapor Pressure (s): For any given temperature, there is a maximum amount of water vapor that the air can hold. The pressure exerted by this maximum water vapor is called the saturation vapor pressure. This saturation pressure is not constant; it increases exponentially with temperature, meaning warm air has a much greater capacity to retain moisture than cold air.

The fundamental relationship is this: The dew point is the temperature at which the saturation vapor pressure (es) becomes equal to the current partial pressure of water vapor (e).

In other words:

Relative Humidity (RH): This is the ratio of the current vapor pressure to the saturation vapor pressure at the current temperature:

Dew Point (Td): This is the temperature at which the current vapor pressure (e) would be the saturation pressure.

The Magnus-Tetens Formula

The calculator uses an empirical approximation known as the Magnus-Tetens formula to solve this relationship. The formula first calculates the saturation vapor pressure at the current temperature (T) and then uses relative humidity (RH) to find the current vapor pressure (e).

The formula is expressed as follows:

Where:

Td is the dew point.
a (17.625) and b (243.04 °C) are the Magnus coefficients, empirical constants derived for water vapor over liquid water.
α(T,RH) is an intermediate term representing the logarithm of the current water vapor pressure, calculated as:

This equation reveals that the dew point depends nonlinearly (exponentially) on temperature and humidity. Unlike relative humidity (which varies with temperature), the dew point is an absolute measure of the water vapor content in the air. Under stable pressure conditions, it will remain constant even if the temperature changes, as long as the amount of water vapor in the air mass remains unchanged.