This dew point calculator can be used to explore the relationship between dew point, temperature and relative humidity. 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 the relative humidity in your room is high, you can observe dew forming on the surface of the window. This 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...

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 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, in 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 need to know the definition of absolute humidity, too. It is simply 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 (Sonntag90) 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 is the dew point;
  • T is the temperature;
  • RH is the relative humidity of the air;
  • a and b are coefficients. For Sonntag90 constant set, a=17.62 and b=243.12°C;
  • α(T,RH) = ln(RH/100) +a*T/(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 (which can also serve as 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 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 you are interested in.

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 special case, when the air is maximally saturated with water (the relative humidity is 100%), the dew point is equal to the current temperature.

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

  • Imagine that it's 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 which 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 we don't observe the dew in broad daylight? And how is the 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 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, if the air layer adjacent to the ground is cooled down to the dew point temperature, fog is formed.
  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, on which the dew prefers forming.

  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
  • calm night, no strong wind
  1. Preferred structures on which the dew is forming:
  • thin, exposed objects like leaves, grass blades, petals
  • poor thermal conductivity, good radiators
  • well isolated from the ground

What is comfortable dew point temperature?

High dew point values can be a cause of discomfort. It is because 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
  • 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 to 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...

  • Theoretical maximum possible amount of dew is about 0.8 mm/night, but rarely it exceeds 0.5 mm

  • In some arid regions - such as e.g., Negev Desert in Israel - dew is a really important water source, can you imagine?! 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.
Hanna Pamuła, PhD candidate and Bogna Haponiuk

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