What is Dew Point? Dew Point Definition
The name can be a bit deceptive — dew point has 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 a window. This happens because the temperature near the window has dropped below the dew point.
Short definition: Dew point is the temperature at which water vapor begins to condense into water.
Complete definition: Dew point is the temperature at which air or gas must be cooled so that the water vapor condenses 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 at the current temperature — in other words, 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 the current temperature)
Or, expressed differently, 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
Absolute humidity is 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–vapor mixture. For saturated air at 30 °C (86 °F), absolute humidity ranges from ~0 to 30 g/m³.
How to Calculate Dew Point and Relative Humidity?
This dew point calculator uses the Magnus-Tetens formula (based on the 2005 article by Mark G. Lawrence), which provides accurate results (uncertainty ≤ 0.35 °C) for temperatures between −40 °C and 60 °C:
Td = (b × α(T, RH)) / (a − α(T, RH))
where α(T, RH) = ln(RH/100) + a·T / (b + T), a = 17.625 and b = 243.04 °C.
To find relative humidity from temperature and dew point, the formula is rearranged:
RH = 100 × exp(a·Td / (b + Td) − a·T / (b + T))
Dew Point vs. Humidity: the Difference
Dew point is an accurate, absolute measure of the moisture content in the air. The higher the dew point, the more moisture there is. It is the better indicator of how humid and uncomfortable conditions really feel.
Relative humidity is a more ambiguous value because it depends on both the amount of moisture and the temperature. The same relative humidity at different temperatures indicates very different amounts of actual moisture.
The key relationship: when relative humidity is 100%, dew point equals the current air temperature. When dew point is close to air temperature, conditions feel sticky and humid.
Morning Dew
Dew usually forms at night because:
- After sunset, surfaces cool through the loss of infrared radiation without solar heating.
- Objects with poor thermal conductivity (leaves, grass, petals) cool faster than deeper layers.
- When the surface temperature drops below the dew point, atmospheric water vapor condenses into droplets.
- Fog forms when the adjacent air layer is also cooled to the dew point temperature.
- After sunrise, the sun heats surfaces and the dew droplets evaporate.
Preferred Conditions for Dew Formation
Weather Factors
- Clear night sky, particularly after a warm day
- Little water vapor in the upper atmosphere
- High humidity in the lowest air layers
- Calm night with no strong wind
Surface Characteristics
- Thin, exposed objects: leaves, grass blades, petals
- Poor thermal conductivity and good radiative cooling
- Well isolated from the warm ground
What is a Comfortable Dew Point Temperature?
High dew point values cause discomfort because the body relies on sweat evaporation for cooling, and this process slows when the air is already saturated with moisture. Here is a general comfort scale:
| Dew Point (°C) | Dew Point (°F) | Comfort Level |
|---|---|---|
| Below 10 | Below 50 | Dry and comfortable |
| 10 – 13 | 50 – 55 | Comfortable |
| 13 – 16 | 55 – 60 | Slightly humid, still pleasant |
| 16 – 18 | 60 – 65 | Noticeably humid |
| 18 – 21 | 65 – 70 | Uncomfortable for most people |
| 21 – 24 | 70 – 75 | Very uncomfortable, oppressive |
| Above 24 | Above 75 | Severely oppressive |
Dew Point Applications
- Meteorology: Dew point is a key indicator of moisture content in the atmosphere and is used in weather forecasts and aviation.
- HVAC and building engineering: Controlling dew point prevents condensation on surfaces, corrosion, and mold growth.
- Aviation: Pilots use dew point spread (difference between temperature and dew point) to assess fog and low-cloud risk.
- Agriculture: Farmers use dew point data to predict frost and dew formation that affects crops.
- Industrial processes: Many manufacturing processes require precise humidity control — dew point sensors are used widely.
- Sports and outdoor activities: Athletes and coaches monitor dew point to assess heat stress risk.
Interesting Facts About Dew
- Spiders and insects rely on morning dew as a water source in dry environments.
- Some desert plants (like the Namib fog basking beetle) collect dew from fog as their primary water source.
- Ancient civilizations collected dew in stone bowls for drinking water.
- Frost is simply dew that forms at temperatures below 0 °C (32 °F), depositing directly as ice crystals.
- The human body can start to feel heat stress at dew points above 16 °C (60 °F), even at moderate temperatures.
Frequently Asked Questions
Can dew point be higher than temperature?
No. By definition, dew point can never exceed the actual air temperature. When dew point equals temperature, relative humidity is 100% and the air is fully saturated.
What is a high dew point?
Dew points above 18 °C (65 °F) are generally considered high and uncomfortable. Above 24 °C (75 °F) is considered oppressive by most standards.
Why does dew point feel worse than relative humidity?
Relative humidity changes throughout the day as temperature changes, even without any change in the actual amount of moisture. Dew point stays relatively constant and directly reflects how much moisture is in the air, making it a better indicator of comfort.
What is the difference between dew and fog?
Dew forms on surfaces when they cool below the dew point. Fog forms when the air itself (not just a surface) cools to the dew point, causing water vapor to condense into tiny droplets suspended in the air.