Dew Point Calculator

About Dew Point Calculator

The Magnus Formula

The Magnus-Tetens approximation calculates dew point from temperature and relative humidity. It is accurate to within about 0.4°C for temperatures between -45°C and 60°C.

The formula uses two constants: a = 17.27 and b = 237.7°C.

First, calculate alpha:

alpha = (a x T) / (b + T) + ln(RH / 100)

Then the dew point:

Td = (b x alpha) / (a - alpha)

Where T is air temperature in °C and RH is relative humidity as a percentage.

Worked example: Air temperature 25°C, relative humidity 60%:

alpha = (17.27 x 25) / (237.7 + 25) + ln(60/100) = 1.644 + (-0.511) = 1.133

Td = (237.7 x 1.133) / (17.27 - 1.133) = 269.31 / 16.137 = 16.7°C

So at 25°C and 60% humidity, the dew point is about 16.7°C, which feels comfortable.

Dew Point Comfort Scale

Dew point is a better comfort indicator than relative humidity. Here is what the numbers mean in practice:

Dew Point	Comfort Level	Description
Below 50°F / 10°C	Dry	Pleasant, very comfortable. Typical of desert climates and air-conditioned spaces.
50-55°F / 10-13°C	Comfortable	Ideal for most people. Common in spring and autumn in temperate climates.
55-60°F / 13-16°C	Slightly humid	Noticeable moisture but not uncomfortable. Starting to feel sticky during exercise.
60-65°F / 16-18°C	Humid	Uncomfortable for many. Sweat does not evaporate as efficiently. Typical summer day in the UK.
65-70°F / 18-21°C	Muggy	Oppressive. Difficult to cool down through sweating. Common in tropical and subtropical regions.
Above 70°F / 21°C	Miserable	Extremely uncomfortable. Body's cooling system is severely impaired. Heat exhaustion risk rises sharply.

For context: the highest dew point ever recorded on Earth was 95°F (35°C) in Dhahran, Saudi Arabia in 2003. A dew point above 80°F (27°C) is rare and dangerous.

Why Dew Point Beats Relative Humidity

Relative humidity (RH) is the percentage of moisture the air is currently holding compared to the maximum it can hold at that temperature. The problem: warm air can hold far more water than cold air, so the same RH means very different things at different temperatures.

Example: 50% RH at 90°F (32°C) feels horrible. 50% RH at 40°F (4°C) feels dry and pleasant. The dew points are completely different: about 70°F (21°C) versus 23°F (-5°C).

RH also changes throughout the day without any moisture being added or removed. In the early morning when it is cool, RH might be 90%. By afternoon, the same air at a higher temperature might show 40% RH. The dew point stays the same in both cases because the actual amount of moisture has not changed.

This is why weather forecasters increasingly report dew point instead of RH when discussing comfort.

When Condensation Happens

Condensation occurs when any surface drops to or below the dew point temperature. This explains several everyday phenomena:

• Cold drink sweating: The glass surface is below the dew point, so water from the air condenses on it.
• Foggy windows in winter: Indoor air has a higher dew point than the cold window surface. Single-pane windows are much worse than double-glazed for this reason.
• Morning dew on grass: Grass radiates heat overnight and drops below the dew point while the air a few feet up stays warmer.
• Fog: Forms when the air temperature drops to the dew point, usually in the early morning. Radiation fog (inland) and advection fog (coastal) are both fundamentally dew point phenomena.
• Bathroom mirrors: Hot shower steam raises the dew point in the room well above the mirror's temperature.

The calculator warns you when the dew point is within 2-3 degrees of the air temperature, indicating condensation is likely.

Practical Uses for Dew Point

• Home humidity control: If indoor dew point exceeds 60°F (16°C), a dehumidifier will improve comfort. Below 40°F (4°C) is too dry and may cause dry skin and static electricity.
• Painting and coating: Most paint manufacturers recommend the surface temperature be at least 5°F (3°C) above the dew point before applying paint. Painting on a surface near the dew point causes moisture to get trapped under the coating, leading to poor adhesion and bubbling.
• Aviation: Pilots monitor the spread between temperature and dew point to predict fog and cloud formation. When the spread narrows to 2-3°C, fog is likely.
• HVAC systems: Cooling coils must drop air below its dew point to remove moisture (dehumidify). This is why air conditioning both cools and dries the air.
• Agriculture: Dew point affects crop disease risk. Many fungal diseases thrive when leaf surfaces stay wet, which happens when the dew point is close to the air temperature overnight.
• Exercise safety: Dew points above 65°F (18°C) significantly impair the body's ability to cool itself through sweat evaporation. Heat-related illness risk increases sharply.

Dew Point Around the World

Location	Summer Dew Point	How It Feels
Phoenix, Arizona	40-50°F / 4-10°C	Dry heat, very comfortable humidity-wise
London, UK	50-58°F / 10-14°C	Generally comfortable, occasionally sticky
New York City	60-70°F / 16-21°C	Can be quite muggy in July-August
Houston, Texas	68-75°F / 20-24°C	Oppressive, constant stickiness
Singapore	73-77°F / 23-25°C	Relentlessly humid year-round
Persian Gulf coast	75-85°F / 24-29°C	Among the most extreme on Earth

For cold-weather comfort, the wind chill calculator shows how cold the wind makes it feel. For temperature unit conversions, use the temperature converter.

How Accurate Is the Magnus Formula?

The Magnus-Tetens approximation is accurate to within about 0.4°C for temperatures between -45°C and +60°C, which covers nearly every practical weather condition on Earth. The two constants used here (a = 17.27, b = 237.7°C) come from the classic form used by meteorological services for decades.

A refined version published by Alduchov and Eskridge in 1996 (Journal of Applied Meteorology, vol. 35) uses a = 17.625 and b = 243.04°C and trims the error to within ±0.1% across the same range. For professional psychrometric work, engineers often use the Arden Buck equation or the ASHRAE Handbook tables, which account for pressure and ice-phase corrections. For everyday comfort, HVAC sizing, and painting decisions, the standard Magnus form used here is more than accurate enough - the humidity sensor in a typical consumer weather station has an uncertainty of ±3% RH, which is a bigger source of error than the formula itself.

One caveat: the formula assumes water vapour, not ice. Below about -20°C, real-world frost points start to diverge slightly because sublimation over ice follows a different saturation curve. Aviation weather services typically switch to an ice-phase calculation below freezing.

Dew Point vs Wet-Bulb Temperature

Wet-bulb temperature is the lowest temperature air can be cooled to by evaporating water into it, and it is the metric most directly tied to heat-related mortality. Dew point tells you how sticky the air feels; wet bulb tells you whether your body can still cool itself. The two are related but not the same.

Metric	What It Measures	When to Use It
Dew point	Absolute moisture content of the air	Comfort, condensation, painting, HVAC sizing
Wet bulb	Lowest temperature achievable by evaporative cooling	Heat stress risk, cooling tower design, athletic safety
Relative humidity	Percentage of saturation at current temperature	Indoor comfort ranges, material conditioning
Heat index	Perceived temperature combining air temp and RH	Public heat warnings in the US

A wet-bulb temperature of 35°C (95°F) is the theoretical survivability limit for humans because skin can no longer shed heat through sweat evaporation. Research published in Science Advances (Raymond et al. 2020) found that the 35°C wet-bulb threshold has already been briefly exceeded at coastal stations in the Persian Gulf and Indus Valley, decades earlier than most climate models predicted.

Indoor Dew Point and Mould Risk

For healthy indoor air, the World Health Organization and the UK Building Research Establishment recommend keeping indoor relative humidity between 40% and 60%, which corresponds to a dew point of roughly 6-15°C (43-59°F) at typical room temperatures. Below 30% RH, mucous membranes dry out and static electricity builds up. Above 60% RH, dust mites, cockroaches, and mould spores thrive, and allergen load climbs sharply.

The key rule: mould germinates when a surface stays within 3°C of the dew point for extended periods. That is why mould grows behind wardrobes on cold exterior walls, around single-pane windows, and in unheated corners of bathrooms. Raising the surface temperature (insulation, wall heating) or lowering the indoor dew point (ventilation, dehumidifier, extract fans) both work. The most common failure mode is kitchens and bathrooms with no extract ventilation in winter - cooking and showering drive the dew point up, then condensation feeds mould on the coldest surface in the room.

To sanity-check your own home, a £10 hygrometer reading 55% RH at 20°C gives a dew point near 11°C. If any wall surface in the house reads below about 14°C (use an infrared thermometer), you have a mould risk at that spot even though the room feels fine.

Common Mistakes When Reading Dew Point

• Confusing dew point with humidity: A 90% RH reading on a cold morning (say 5°C) corresponds to a dew point of only 3.5°C - genuinely dry air that will feel fine indoors. A 50% RH reading at 30°C corresponds to a dew point of 18.4°C, which feels muggy. Always check the dew point, not the RH percentage.
• Painting too close to the dew point: Most paint and epoxy specifications require the substrate to be at least 3°C (5°F) above the dew point, and humidity below 85%, for at least 4 hours after application. Painting on a cool morning when the substrate is near the dew point traps moisture under the coating and causes blushing, adhesion failure, or delamination.
• Assuming air conditioning always dehumidifies: An oversized AC unit cools fast and shuts off before removing much moisture. Homes in humid climates often feel clammy despite being cool because the unit cycles too quickly. A right-sized or variable-speed unit runs longer at lower capacity and removes more water.
• Ignoring dew point when exercising: The American College of Sports Medicine recommends reducing outdoor training intensity when the dew point exceeds 18°C (65°F) and cancelling strenuous training above 24°C (75°F). Heat cramps and heat exhaustion almost always happen on high dew point days, not necessarily the hottest days by air temperature.
• Mixing up frost point and dew point: Below 0°C the terminology shifts. If a surface temperature is below 0°C and below the frost point, water vapour deposits directly as ice (frost) rather than dew. This matters for aircraft de-icing and refrigeration design.

Sources

• Guinness World Records - Highest Dewpoint Temperature (Dhahran, 2003)
• Tetens Equation - Magnus form and history
• Alduchov & Eskridge (1996) - Improved Magnus Form Approximation, Journal of Applied Meteorology
• WHO Guidelines for Indoor Air Quality - Dampness and Mould
• NOAA JetStream - Heat Index and Humidity
• ACSM - Heat Stress and Exercise Safety
• Raymond et al. (2020) - Emergence of Wet-Bulb Temperature Extremes, Science Advances

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