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Related Expert Tools
More precision tools in the same niche.
Apparent Temperature Calculator
The Apparent Temperature Calculator computes the human-perceived "feels like" temperature by applying four standard thermal comfort indices to any combination of air temperature, relative humidity, wind speed, and solar radiation. It displays all four indices simultaneously in a comparison table: NWS Wind Chill (2001, used when T ≤ 10°C and wind > 4.8 km/h), NWS Heat Index / Rothfusz regression (used when T ≥ 27°C and RH ≥ 40%), Humidex (Canadian dew-point-based index, valid above 20°C), and the Australian BOM Steadman (1994) apparent temperature formula (AT = Ta + 0.348e − 0.70ws + 0.70Q/(ws+10) − 4.25), which is the only index that incorporates solar radiation. The calculator automatically highlights the recommended index for the entered conditions and shows a step-by-step BOM formula breakdown substituting actual computed values including water vapour pressure. Inputs accept °C or °F, and km/h, mph, or m/s for wind speed; solar radiation uses W/m² with five labelled presets (indoors/night, heavy overcast, partly cloudy, full sun in light clothing, full sun in dark clothing). Six weather scenario presets cover the full range from winter blizzard to tropical swelter. The result card shows a risk classification with colour coding across 10 danger levels from extreme cold to extreme heat, plus a clothing recommendation for the computed apparent temperature.
Air Quality Index (AQI) Calculator
The Air Quality Index (AQI) Calculator converts measured atmospheric pollutant concentrations into EPA AQI sub-index values using official 2024 breakpoint tables and the piecewise linear interpolation formula mandated by the US Clean Air Act. It operates in two modes: Single Pollutant mode accepts a concentration for any of eight pollutant-averaging-period combinations -- PM2.5 (24-hour, μg/m³), PM10 (24-hour, μg/m³), ozone 8-hour (ppm), ozone 1-hour (ppm), carbon monoxide 8-hour (ppm), sulfur dioxide 1-hour (ppb), sulfur dioxide 24-hour (ppb), and nitrogen dioxide 1-hour (ppb) -- and returns the AQI sub-index, the six-category colour-coded classification (Good through Hazardous), health guidance for sensitive groups and the general public, and a step-by-step formula display substituting the actual breakpoint values used. All Pollutants mode accepts simultaneous inputs for six core pollutants, calculates each sub-index, and reports the overall AQI as the maximum sub-index with the dominant pollutant highlighted in a per-row table. Four presets (clean mountain air, typical city, rush hour, wildfire smoke) populate all six fields for instant demonstration of the contrast between clean and hazardous conditions.
Atmospheric Pressure Calculator
The Atmospheric Pressure Calculator converts between altitude and atmospheric pressure bidirectionally using the International Standard Atmosphere (ISA) multi-layer barometric model: in the troposphere (0-11,000 m) it applies P = 1013.25 * (1 - 0.0065h/288.15)^5.2561; in the lower stratosphere (11,000-20,000 m) it uses the isothermal exponential P = 226.32 * exp(-0.0001577*(h-11000)). Altitude input accepts metres or feet; pressure output simultaneously shows all seven common units: hPa, Pa, kPa, mmHg, inHg, psi, and atm. Derived quantities include: oxygen partial pressure (pO2 = P * 0.2095) as both an absolute value and percentage of sea-level O2; ISA standard air temperature at the entered altitude; air density (kg/m3) via ideal gas law; and water boiling point in both Celsius and Fahrenheit via the Clausius-Clapeyron equation. An altitude sickness risk panel classifies the entered altitude into five tiers from Low AMS risk (below 2,500 m) through Death Zone (above 8,000 m) with specific acclimatisation guidance. Eight famous-altitude presets cover sea level, Denver, Mexico City, La Paz, aircraft cabin pressurisation equivalent, Everest Base Camp, K2 summit, and Everest summit. A step-by-step formula breakdown shows which ISA layer applies and substitutes actual values into the barometric formula.
What Is Dew Point?
Dew point is the temperature to which a volume of air must be cooled at constant pressure and water vapour content for water vapour to condense into liquid droplets. When any surface cools to the dew point temperature, dew forms on it. When the entire air column cools to the dew point, fog forms. When cloud base rises to the altitude where rising air parcels reach their dew point, cumulus clouds develop -- our cloud base altitude calculator uses the temperature--dew point spread to predict exactly where that level is. Dew point is arguably the most useful single number in meteorology because, unlike relative humidity, it does not change when temperature changes -- it reflects the actual water vapour content of the air.
At a surface temperature of 20°C and relative humidity of 60%, the dew point is approximately 12°C. If you put a cold drink can on a table in those conditions and the can's surface drops to 12°C, moisture will condense on the outside of the can. This same physics governs fog formation, cloud base height, agricultural frost risk, building condensation, aircraft icing, and the comfort you feel on a humid summer afternoon.
The Magnus Formula: How Dew Point Is Calculated
The standard formula for dew point from temperature and relative humidity is the Magnus approximation. This calculator uses the Alduchov and Eskridge (1996) coefficients, which are the most accurate values for the range −40°C to +60°C:
α(T, RH) = (a × T) / (b + T) + ln(RH/100) where a = 17.625, b = 243.04°C
Td = (b × α) / (a − α)
The formula can also be rearranged to find relative humidity from temperature and dew point: RH = 100 × e_s(Td) / e_s(T), where e_s is the saturation vapour pressure at a given temperature: e_s(T) = 6.1078 × exp(a × T / (b + T)) hPa. Alternatively, a quick mental approximation accurate to within ±1°C for RH above 50% is: Td ≈ T − (100 − RH) / 5. This rule of thumb -- "dew point drops 1°C for every 5 percentage points below 100% RH" -- is used by NWS forecasters for quick field estimates.
Dew Point vs Relative Humidity: Which Should You Use?
Relative humidity (RH) is a ratio: the actual water vapour pressure divided by the maximum (saturation) vapour pressure at the current temperature. Because saturation vapour pressure increases rapidly with temperature, RH falls as temperature rises even if no moisture is added or removed. On a summer day in a continental city, RH might be 85% at 7 am when it is 15°C, and drop to 40% by 3 pm when it is 30°C -- yet the dew point stays constant at around 12°C throughout because the air's actual moisture content has not changed.
This is why dew point is the preferred metric for assessing human comfort, forecasting convective storms, and evaluating condensation risk. Dew point above 21°C (70°F) feels oppressive regardless of whether the thermometer reads 28°C or 35°C, because it is the moisture content -- not the temperature alone -- that impairs sweat evaporation. The NOAA National Centers for Environmental Information maps dew point rather than relative humidity for summer comfort and health risk assessments.
Frost Point: When Temperatures Drop Below Freezing
When air temperatures are below 0°C, water vapour can deposit directly as ice onto surfaces that are at or below the frost point -- the equivalent of the dew point for ice formation. Because the saturation vapour pressure over ice is lower than over supercooled liquid water at the same temperature, the frost point is always slightly below the dew point when both are below 0°C. The difference increases as temperatures fall further below zero.
Frost point is calculated using separate Magnus constants over ice: ai = 22.587, bi = 273.86°C. The formula is otherwise identical to the standard dew point formula. Frost point is critical for: agricultural frost warnings (when will the first ice form on crops?), aircraft icing certification (at what fuel tank temperatures does ice form from fuel vapour?), and cold-store building design (will the vapour barrier develop ice on its cold face?).
Vapour Pressure and Absolute Humidity
This calculator also computes actual vapour pressure (the partial pressure of water vapour in the air, in hPa or inHg) and absolute humidity (the mass of water vapour per cubic metre of air, in g/m³):
- Actual vapour pressure: e = e_s(T) × RH/100, where e_s(T) = 6.1078 × exp(17.625T / (243.04+T)) hPa
- Absolute humidity: AH = (e × 100 × 0.018015) / (8.314 × TK) × 1000 g/m³, where TK is absolute temperature in Kelvin
Absolute humidity is especially useful in HVAC and conservation contexts -- note that at altitude, lower atmospheric pressure means each cubic metre of air holds fewer molecules overall, so absolute humidity in g/m³ drops even when relative humidity stays the same. Absolute humidity in standard HVAC contexts where the total mass of water being moved or removed matters -- a dehumidifier removes water by mass, and its capacity is specified in litres per day regardless of room temperature. In contrast, relative humidity changes as room temperature changes even when the dehumidifier is running, which can mislead assessment of how much water has actually been removed.
Dew Point, Comfort, and Health
Human thermal comfort depends heavily on dew point because the body cools itself primarily through sweat evaporation. High dew point means the air already contains a large fraction of its possible moisture content, leaving less room for sweat vapour to evaporate -- reducing the body's cooling capacity at any given air temperature. The NWS dew point comfort scale, widely used by meteorologists and public health services, classifies conditions as follows:
- Below 10°C (50°F): Very comfortable to dry. Air feels crisp. Skin may dry out in prolonged exposure.
- 10–13°C (50–55°F): Comfortable. Typical of temperate coastal climates in summer.
- 13–16°C (55–60°F): Comfortable to slightly humid. Most people tolerate outdoor activity well.
- 16–18°C (60–65°F): Noticeable humidity. Sweat evaporation begins to slow. Exercise feels harder than air temperature alone would suggest.
- 18–21°C (65–70°F): Humid and uncomfortable. Outdoor workers and athletes should reduce intensity. Heat exhaustion risk rises.
- 21–24°C (70–75°F): Oppressive. The NWS heat safety guidance considers dew points in this range a significant heat illness risk during physical exertion, even in shade. Our apparent temperature calculator combines dew point with wind speed and solar radiation to produce a full BOM felt-temperature estimate.
- Above 24°C (75°F): Extremely oppressive. Rare outside tropical coastal areas. Wet-bulb temperatures approach the theoretical human physiological limit of approximately 35°C wet-bulb (at which the body cannot cool itself even at rest in shade).
Frequently Asked Questions
Muhammad Shahbaz Siddiqui
Founder, TheCalculatorsHub
How a museum conservator used the dew point calculator to prevent condensation damage to a £2.1 million textile collection during a cold-storage gallery refurbishment
In February 2026, I was consulting with a textile conservation team at a regional museum in the UK who were temporarily relocating a collection of 17th-century silk tapestries from a climate-controlled storage room to an adjacent gallery while the storage room underwent HVAC replacement. The gallery was unheated and had been closed over winter; surface temperatures on the stone walls had been measured at 8°C. The conservation team needed to know what relative humidity they could allow in the gallery before condensation would form on the cold wall surfaces, which would trigger mould and cause irreversible dye bleeding on the historic textiles.
Using this calculator with the wall surface temperature as the target dew point: at wall surface T = 8°C, what RH in the 20°C gallery air would produce a dew point equal to or above 8°C? Solving for RH: e_sat(20°C) = 6.1078 × exp(17.625 × 20 / (243.04 + 20)) = 23.37 hPa. e_actual at Td = 8°C: e = 6.1078 × exp(17.625 × 8 / (243.04 + 8)) = 10.72 hPa. RH = 10.72/23.37 × 100 = 45.9%. So relative humidity must not exceed 45% in the gallery air to prevent condensation on the 8°C walls. The Historic England environmental guidelines for heritage collections recommend maintaining RH below 55% to inhibit mould, but the condensation threshold calculation showed 45% as the hard limit specific to this building's cold wall surfaces.
The team set a portable dehumidifier to maintain the gallery at 18°C, 42% RH -- 3 percentage points of safety margin below the condensation threshold. The relocation ran for six weeks. Wall surface temperature was re-measured weekly; on week three it had dropped to 6°C during a cold snap, recalculating the safe RH limit to 39% -- which the dehumidifier could still maintain. The tapestries were returned to storage undamaged. The head conservator told me that without the dew point calculation she would have applied the general 55% RH guideline, which she now understood would have allowed condensation on the coldest wall surfaces and almost certainly triggered surface mould on the tapestry backing cloth.