Dew Point Calculator

Created by Hanna Pamuła, PhD candidate and Bogna Szyk
Reviewed by Steven Wooding and Jack Bowater
Based on research by
Mark G. Lawrence The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications Bulletin of the American Meteorological Society 86(2) (01 Feb 2005)
Last updated: Apr 13, 2022

This dew point calculator can be used to explore the relationship between dew point, temperature, and relative humidity, without the need for a dew point chart. If you have ever wondered "what is dew point?" or how to calculate relative humidity in certain conditions, then this calculator is for you! Also, feel free to check out our wind chill calculator or the heat index calculator if you are interested in the influence of the weather on temperature.

Keep reading to find out more about dew formation, comfortable dew point temperature, and relative humidity.

What is dew point? Dew point definition

The name can be a bit deceptive – dew point has, actually, nothing to do with geometry. It is simply the highest possible temperature at which water vapor can condense to form dew. For example, if your room's relative humidity is high, you can observe dew forming on the surface of the window. This phenomenon happens because the temperature in the vicinity of the window has dropped below the dew point. Once you think about it, you can also find an example of this exact phenomenon in the "Titanic" movie...

Are you simply looking for a short dew point definition? Here you are!

  • Dew point is the temperature at which water vapor begins to condense into water.

or, if you'd like a more complex one:

  • Dew point is the temperature at which air or a gas must be cooled to for the water vapor to condense into dew (or frost, if the temperature is below the freezing point of water).

What is relative humidity?

Relative humidity is expressed as a percentage. It is the ratio of the current absolute humidity to the maximum absolute humidity possible for the current temperature. In other words, it's the amount of moisture in the air compared to what the air can maximally "hold" at that temperature:

  • relative_humidity = 100% × current absolute humidity / max absolute humidity , at current temperature

or, expressed differently, the relative humidity is the ratio of the water vapor pressure Pw to the saturation water vapor pressure Pws at the given temperature:

  • relative_humidity = 100% × Pw / Pws

To understand this definition, you also need to know the meaning of absolute humidity. It is merely the water content in the air, expressed in grams per cubic meter:

absolute humidity = m / V, where m is the mass of water vapor, and V is the volume of the air and water vapor mixture.

For saturated air at 30 °C (86 °F), the absolute humidity in the atmosphere ranges from ~0 to 30 grams per cubic meter.

Did you notice that the formulas don't take the temperature into consideration?

How to calculate dew point? How to calculate relative humidity?

Many equations describing this relationship have been formed. None of them are perfect, though. This dew point calculator uses the Magnus-Tetens formula (Lawrence2005) that allows us to obtain accurate results (with an uncertainty of 0.35 °C) for temperatures ranging from -45 °C to 60 °C.

The dew point is calculated according to the following formula:

Ts = (b × α(T,RH)) / (a - α(T,RH))

where:

  • Ts – Dew point;
  • T – Temperature;
  • RH - Relative humidity of the air;
  • a and b are the Magnus coefficients. As recommended by Alduchov and Eskridge, the value of these are: a = 17.625 and b = 243.04 °C; and
  • α(T,RH) = ln(RH/100) + aT/(b+T).

If you want to calculate relative humidity, you need to know the dew point and temperature to use the equation derived from the above formula. Or, simply type the values into our dew point calculator (you can also use the relative humidity calculator). The result appears in no time!

Dew point vs. humidity: the difference between dew point and humidity

Now that you know the formulas for dew point and humidity, you may wonder, what is the difference between those two terms?

Dew point is an accurate measurement of the moisture content in the air. The higher the dew point is, the more moisture that is in the air. If you want to know whether (or weather, heh) it's comfortable or not to have a morning jog or go on a weekend hiking trip – stick with that term.

Relative humidity is a more confusing value as it depends on temperature and the pressure of the system in question.

Dew point and relative humidity are not the same, but they are closely related: the higher relative humidity is, the closer the dew point is to the current air temperature. In a particular case, when the air is maximally saturated with water (the relative humidity is 100%), the dew point is equal to the current temperature.

To better understand the difference between dew point and humidity, let's look at this example:

  • Imagine that it's a cold autumn morning, 40 °F outside (~4.5 °C). Our forecast shows that the dew point is equal to 40 °F as well – so the relative humidity is 100%.
  • Let's take another example: summer finally has come, we are relaxing near a river, and the temperature is 75 °F (24 °C). The dew point is 60 °F (~15.5 °C), so following the formula, we can find out that the relative humidity is ~60%.
  • And now a paradoxical question: which of those two situations would feel more humid? Definitely the second one! The dew point is the value we should look at if we want to know how dry or humid it is outside, not the relative humidity.

Morning dew

You've probably noticed that dew is usually formed during the night. Our shoes quickly become wet when we set out through some grass at dawn, especially during the summer months. Why is this? Why don't we observe the dew in broad daylight? And how is morning dew formed?

  1. When the sun sets, the surface temperature drops – the sun isn't shining and heating the ground, so the surface cools through the loss of infrared radiation.
  2. Objects with poor thermal conductivity don't keep this energy for too long: the surface is colder than deeper ground layers.
  3. If the surface is cooled to the temperature below the dew point, atmospheric water vapor condenses to form droplets or frost on the surface.
  4. Additionally, fog is formed if the air layer adjacent to the ground is cooled down to the dew point temperature.
  5. When the sun rises high, the dew droplets evaporate into the air.

Preferred conditions to dew formation

We can split the preferred conditions for dew forming into two groups – weather factors and structural characteristics.

  1. Preferred weather conditions:

    • Clear night sky, particularly after a warm day;
    • Little water vapor in the higher atmosphere;
    • High humidity in the lowest layers of air; and
    • Calm night, no strong wind.
  2. Preferred structures on which the dew is forming:

    • Thin, exposed objects like leaves, grass blades, petals;
    • Poor thermal conductivity, good radiators; and
    • Well isolated from the ground.

What is comfortable dew point temperature?

High dew point values can be a cause of discomfort. At high temperatures, our bodies use the evaporation of sweat to achieve a cooling effect. This process is severely slowed down if the air is already saturated with water vapor.

Dew point

Comfort levels

<50 °F (<10 °C)

a bit dry for some

50 - 60 °F (10 - 16 °C)

dry and comfortable

60 - 65 °F (16 - 18 °C)

getting sticky

65 - 70 °F (18 - 21 °C)

unpleasant, lots of moisture in the air

>70 °F (>21 °C)

uncomfortable, oppressive, even dangerous above 75 °F

Dew point applications

You might be surprised, but the dew point calculator may be useful in many different areas. To mention only a few:

  • Meteorology – the most obvious one: the dew point is used to express the amount of moisture in the air and to find the actual mixing ratio that allows tracing the properties of air masses.
  • Aviation – the dew point temperature is calculated to assess the probability of a carburetor icing or fog appearing.
  • Agriculture – to sustain optimal humidity in a greenhouse and avoid water condensation on the plants.
  • Technology – dew point meters are used in the generation and usage of various technical gases (e.g., H2, N2, O2, Ar) and in electronics and optics domains (vapor deposition and thin films).
  • Medicine – e.g., monitoring of the sterilization process.

Interesting facts about dew

Did you know that...

Grass with dew drops
  • Theoretical maximum possible amount of dew is about 0.8 mm/night, but it rarely exceeds 0.5 mm
  • In some arid regions – such as the Negev Desert in Israel – dew is a really important water source. Can you imagine that‽ It is estimated that the desert plants are getting ~50% of their water from the dewfall.
  • People sometimes confuse the dew with another process, called guttation. If plants obtain too much water, droplets at the tip and edges of a leaf are formed. The exuded substance is high in sugars and potassium, so if the drops dry, the white crust remains on the surface. It may look similar to the standard dew, but it's a totally different phenomenon, usually occurring during the day.

FAQ

What is highest dew point ever recorded?

The highest dew point ever recorded was 35 °C, or 95 °F. This occurred on 8th July, 2003, in Dhahran, Saudi Arabia. This happened because the air temperature was 42 °C, or 108 °F, and the relative humidity was 68.5 %. It would have been extremely dangerous to have gone outside.

How do I calculate dew point?

To calculate the dew point:

  1. Measure the temperature and relative humidity of the air.
  2. Multiply 17.625 by the temperature and divide the result by the temperature plus 243.04.
  3. Take the natural logarithm of the relative humidity divided by 100 and add it to the result of the previous step.
  4. Multiply the result of Step 3 by 243.04 and divide it by 17.625 minus the result of Step 3.
  5. Your result is the dew point!

How do I measure dew point?

You cannot measure the dew point directly - it is calculated using the air temperature and the relative humidity. To measure these two values, you can use a sling psychrometer, or find the information online. Once you have it, you can use the dew point formula to calculate it.

What does 70 °F dew point mean?

A dew point of 70 °F, or 21 °C, means that the atmosphere outside is likely to be very uncomfortable, as it will be very humid and you will struggle to properly sweat and regulate your body temperature.

Hanna Pamuła, PhD candidate and Bogna Szyk
Temperature
°F
Relative humidity
%
Dew point
°F
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