Estimate dew point from air temperature and relative humidity with a Magnus-style formula.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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The dew point is one of the most practical weather measurements you can know. Unlike relative humidity, which changes as temperature rises and falls throughout the day, dew point stays relatively stable and directly tells you how much moisture is actually in the air. Whether you are a homeowner monitoring indoor air quality, a pilot reading a METAR, a meteorologist forecasting fog, or simply trying to decide whether to step outside in Houston in August, understanding dew point is essential.
Dew point is the temperature to which air must be cooled — at constant pressure and constant water vapour content — for saturation to occur. When air cools to its dew point, water vapour begins to condense into liquid water. This condensation forms dew on surfaces, creates fog at ground level, or produces clouds higher in the atmosphere.
Mathematically, dew point temperature (Td) is calculated using the Magnus formula approximation:
Where T is air temperature in degrees Celsius, RH is relative humidity as a percentage, b is approximately 17.625, and c is approximately 243.04 degrees Celsius. This approximation is accurate to within 0.1 degrees Celsius for temperatures between -40 and 60 degrees Celsius.
Many people confuse dew point with relative humidity, but they measure very different things:
| Measure | What It Tells You | Changes With Temperature? |
|---|---|---|
| Dew Point | Actual moisture content in air | No (stays stable through the day) |
| Relative Humidity | How saturated the air is right now | Yes (drops as temperature rises) |
| Temperature | Heat energy in the air | N/A |
A classic example: on a summer morning in London, the air might be 14 degrees Celsius with 90% relative humidity. By afternoon, temperature rises to 22 degrees Celsius and relative humidity drops to around 56% — yet the dew point remains approximately 12 degrees Celsius throughout, because no moisture was actually added or removed from the air. This is why meteorologists prefer dew point for assessing actual atmospheric moisture.
Dew point is the best single indicator of how uncomfortable humid weather feels. The human body cools itself through evaporation of sweat, and when dew points are high, sweat evaporates more slowly, making you feel hotter and stickier. Here is the internationally recognised comfort scale:
| Dew Point (F) | Dew Point (C) | Comfort Level |
|---|---|---|
| Below 55 F | Below 13 C | Dry and comfortable — typical of UK spring and autumn |
| 55–60 F | 13–16 C | Comfortable, slightly humid |
| 60–65 F | 16–18 C | Noticeable but tolerable |
| 65–70 F | 18–21 C | Sticky and oppressive — common in US Midwest summers |
| Above 70 F | Above 21 C | Oppressive and dangerous — Gulf Coast summer typical |
| 75 F and above | 24 C and above | Extremely oppressive — rare even in tropics |
The UK has a temperate maritime climate, meaning dew points are generally moderate — rarely extremely low or extremely high. The Atlantic Ocean keeps conditions moist year-round but limits the most extreme humidity events common in continental climates.
| Region | Winter Dew Point | Summer Dew Point |
|---|---|---|
| London and South East | 2–6 C | 11–14 C |
| South West and Cornwall | 4–8 C | 12–15 C |
| Wales | 2–6 C | 11–14 C |
| Northern England | 1–4 C | 10–13 C |
| Scotland (lowlands) | 0–3 C | 9–12 C |
| Scotland (Highlands) | -2 to 2 C | 7–10 C |
During heatwaves such as the record-breaking July 2022 event when temperatures exceeded 40 degrees Celsius for the first time in recorded UK history, dew points in the South East briefly reached 17–19 degrees Celsius — unusual for Britain but still well below the oppressive levels experienced in the US Midwest or Gulf Coast. The Met Office includes dew point in its detailed forecasts and SYNOP weather reports for aviation and maritime use.
UK weather forecasting regularly employs dew point to predict morning fog events. When the overnight temperature is forecast to fall close to the dew point, radiation fog is likely — a common hazard on motorways and in low-lying valleys throughout autumn and winter.
The United States experiences far greater dew point extremes than the UK, reflecting its vast continental geography. The National Weather Service (NWS) reports dew point as a standard surface observation on all hourly weather station readings.
| Region | Typical Summer Dew Point | Notes |
|---|---|---|
| Gulf Coast (Houston, New Orleans) | 70–75 F (21–24 C) | Consistently oppressive June through September |
| Midwest (Chicago, St. Louis) | 65–72 F (18–22 C) | July heat waves can push 75 F dew points |
| Southeast (Atlanta, Charlotte) | 65–70 F (18–21 C) | Sticky afternoons common |
| Northeast (New York, Boston) | 55–68 F (13–20 C) | Humid heat waves occur several times per summer |
| Southwest Desert (Phoenix, Las Vegas) | 20–45 F (-7 to 7 C) | Extremely dry; monsoon season raises dew points temporarily |
| Pacific Northwest (Seattle) | 45–55 F (7–13 C) | Comfortable dry summers |
The heat index (or "feels like" temperature) is directly calculated using temperature and relative humidity, but dew point is a better predictor of physiological heat stress than relative humidity alone. When dew point exceeds 65 F (18 C) and air temperature is above 90 F (32 C), heat index values can reach dangerously high levels. The NWS issues heat advisories and warnings based on heat index thresholds, but meteorologists use dew point as the foundational moisture variable in those calculations.
The relationship is important: the same relative humidity of 60% feels very different at 70 F (21 C) versus 95 F (35 C). However, a dew point of 70 F (21 C) consistently signals dangerous heat stress regardless of the air temperature above it.
Fog forms when the air temperature equals the dew point at ground level, causing air to become saturated and water droplets to suspend in the atmosphere. There are several types of fog tied directly to dew point:
When the dew point temperature is below freezing (0 C / 32 F), it is called the frost point — the temperature at which ice rather than liquid water deposits directly from water vapour. Frost point is critical for agricultural frost forecasting (UK Met Office frost warnings), aviation ice formation on aircraft wings, and road surface temperature forecasting for highway agencies such as UK Highways England and US state DOTs.
In the UK, frost point conditions are common from October through April across most of England and Scotland. The Met Office issues frost warnings based on whether overnight temperatures are forecast to fall to or below the frost point.
Aviation is one of the most technically demanding uses of dew point data. In METAR (Meteorological Aerodrome Report) format — the standard weather observation format used by airports worldwide including all UK and US airports — dew point is reported directly alongside temperature.
Example METAR extract: EGLL 141220Z 24012KT 9999 FEW025 18/12 Q1015 — the "18/12" means temperature 18 C, dew point 12 C. The temperature-dew point spread is critical for pilots: a spread of 4 C or less signals likely fog or low cloud, a spread of 2 C or less indicates imminent fog formation, and a spread of 0 C means saturated air and certain fog. The UK CAA and the US FAA both require pilots to understand dew point for flight planning.
For home and building management, the indoor dew point (derived from indoor temperature and relative humidity) determines condensation risk. The NHS and building standards organisations recommend indoor relative humidity of 40–60% at normal room temperatures of 18–22 C. Translated to dew point, this means an indoor dew point of roughly 6–12 C is ideal.
Problems arise when indoor dew point exceeds 12 C (mould growth risk increases significantly), falls below 3 C (uncomfortable dryness, static electricity, respiratory irritation), or when window surface temperature drops below the indoor dew point (condensation forms on glass). In the UK, condensation damp is estimated to affect one in five homes. Understanding and monitoring indoor dew point is the most accurate way to prevent it. ASHRAE Standard 55, referenced in US and UK building standards, specifies that indoor dew point should not exceed 16.8 C for occupant comfort.
The Met Office uses dew point as a fundamental variable in its Unified Model numerical weather prediction system. Dew point data feeds into cloud base height calculations (Lifting Condensation Level), thunderstorm convective potential (CAPE), fog probability forecasts, frost and ice warnings, and thermal comfort indices during heatwaves.
The US National Weather Service provides dew point data on all standard forecast pages at weather.gov. US forecasters specifically call out dew point in summer heat advisories. During the 2023 summer across the Southern US, dew points of 75–80 F (24–27 C) contributed to heat index values well above 110 F (43 C) across Texas and Florida, leading to multiple Heat Emergency declarations.
Most people find dew points below 60 F (16 C) comfortable. Between 60–65 F (16–18 C) is noticeably humid but tolerable. Above 65 F (18 C) starts to feel sticky and oppressive, and above 70 F (21 C) is considered very uncomfortable and potentially dangerous during exertion. In the UK, summer dew points rarely exceed 16 C, while US Gulf Coast summers routinely exceed 21 C.
Relative humidity tells you how close the air is to being saturated as a percentage — but this percentage changes as temperature changes. Dew point tells you the actual moisture content of the air and stays constant as temperature changes. Dew point is more useful for assessing how muggy or sticky the air actually feels throughout the day.
Pilots use dew point to predict fog formation, cloud base heights, and icing conditions. A small temperature-dew point spread (less than 4 C) means fog or low cloud is likely. All METAR reports include both temperature and dew point, and pilots are required to understand this relationship for safe flight planning at airports in the UK, US, and worldwide.
When the dew point falls below 0 C (32 F), it becomes the frost point. This means water vapour will deposit as ice on surfaces when they cool to that temperature. In the UK, frost point conditions are common October through April. A frost point of -3 C or below with clear skies and calm winds usually produces a ground frost.
Mould grows when surface temperatures fall below the indoor air's dew point, causing condensation. To prevent mould, keep indoor dew point below 12 C — achievable by keeping humidity below 60% at 21 C — and ensure good ventilation. In the UK, condensation damp caused by high indoor dew points is one of the most common property issues affecting around one in five homes.
Within the contiguous United States, dew points of 80 F (27 C) have been recorded in Gulf Coast states during peak summer. These extreme levels are found in Houston, New Orleans, and coastal Florida during July and August and are associated with dangerous heat index values and elevated heat illness risk.
Use the Magnus formula approximation: calculate gamma = (17.625 x T) / (243.04 + T) + ln(RH/100), then calculate dew point Td = 243.04 x gamma / (17.625 - gamma), where T is temperature in degrees Celsius and RH is relative humidity as a percentage. Our dew point calculator above handles this automatically for both Celsius and Fahrenheit.
A dew point of 70 F (21 C) is considered oppressive. At this level, sweat evaporates slowly, the body struggles to cool itself, and prolonged outdoor activity carries a real risk of heat illness. This dew point level is common in Houston, New Orleans, and other Gulf Coast cities during summer, and is exceptional or virtually unknown in the United Kingdom.