Air temperature tells you what a shielded thermometer measures. Airpult’s Feels Like value estimates how the same weather may affect heat transfer from a standard person. It is useful when choosing clothes or planning time outdoors, but it cannot predict exactly how you will feel and it is not a body or skin temperature.
What Airpult shows
Airpult normally displays the two-metre apparent-temperature field supplied by a numerical forecast model. Our pipeline gives the National Blend of Models value priority where it is available, then uses the Global Forecast System value. NOAA encodes this field as APTMP and defines it broadly as an outdoor temperature perceived through a combination of effects such as air temperature, relative humidity and wind speed.
That broad definition matters because “feels like” is not one universal formula. The National Blend combines calibrated guidance from several models, while the GFS is a single global forecasting system. Providers can also use different rules in mild weather. For example, the US National Digital Forecast Database uses wind chill at 10°C (50°F) or below, heat index above 27°C (80°F), and forecast air temperature between those ranges. The Met Office uses wind chill in colder conditions, heat index in warmer conditions and a combination through the middle. Two services can therefore start with similar weather data and publish different apparent temperatures without either value being a measurement error.
If neither provider field is available for current conditions, Airpult calculates a fallback using an Australian apparent-temperature equation. It uses air temperature, water-vapour pressure derived from relative humidity, and wind speed. Solar radiation enters the version used when that input is available. Hourly and daily values normally continue to use the provider field. Because the provider fields and the fallback do not necessarily handle radiation or transition weather in the same way, a change of data source can change the result. Do not assume that direct sunshine is included unless the underlying method says so.
Wind chill and cold conditions
Your skin warms a thin layer of air beside it. Wind replaces that layer and increases heat loss from exposed skin. The US National Weather Service wind chill index is defined for air temperatures at or below 10°C and wind speeds above 3 mph, about 5 km/h. It models heat loss from exposed skin and assumes no warming from sunshine.
Those limits belong to the index, not to the physics. Above 10°C, wind can still increase convective heat loss and help moisture evaporate, but the standard formula does not describe that situation. Below its wind-speed limit, the equation is not intended to represent calm conditions. Wind chill also leaves the measured air temperature unchanged. A car or water pipe exposed to −5°C air can cool to −5°C faster in wind, but wind alone cannot cool it to a wind-chill value of −12°C.
Exposure changes the result for people too. A forecast wind is usually for an open, standard measurement height; the wind at face level may be weaker behind buildings or stronger on a ridge, bridge or seafront. A wind-resistant outer layer traps more warm air than bare skin. Wet skin and clothing speed evaporative cooling, so rain, sweat or a fall into water can make conditions more dangerous than the displayed number suggests. See Wind Speed Explained for the difference between sustained wind and gusts.
Heat index and hot conditions
Sweat cools the body when it evaporates. High humidity slows evaporation and makes it harder to release heat. The National Weather Service heat index combines air temperature and relative humidity for a person in shade with light wind. It is one specific apparent-temperature method, not a label for every Feels Like calculation.
The assumptions explain some familiar differences. Direct sun adds radiant heat that a shade-based heat index does not represent. Warm walls, paving and sheltered urban spaces can add radiant heat or reduce air movement. Exercise and physical work create metabolic heat, while protective or heavy clothing can slow heat loss. Very hot, dry wind is not always cooling either: when the air is hotter than the skin, convection can add heat even though moving air helps sweat evaporate. For outdoor work or hard exercise in sunshine, an official wet-bulb globe temperature product may be more suitable because it accounts for wind and radiation as well as heat and humidity.
Relative humidity also changes as air temperature changes, even when the amount of water vapour does not. That is one reason a humid morning and a hotter afternoon require more context than a single percentage. Our Humidity Explained resource covers relative humidity and dew point in more detail.
Why the value can change abruptly
Near the range where a provider switches formulas, a small change in forecast temperature, humidity or wind can change which calculation applies. The Feels Like line may then meet the air-temperature line, separate from it or change more sharply than expected. That behaviour can be mathematical rather than a forecast of a sudden physical jolt. Rounding can make the step look larger.
The inputs are forecasts too. Models estimate temperature, humidity, wind and cloud or radiation on a grid, then Airpult interpolates those fields to a location. Hills, coasts, tree cover, street canyons and small showers can produce local conditions that the grid does not resolve. Forecast error in any input carries into the apparent temperature, and uncertainty generally grows further into the forecast. Check nearby hours and the underlying weather fields instead of treating one degree at one time as exact.
Why your experience may differ
Every formula represents a reference person and standard exposure. Age, health, medication, acclimatisation and individual physiology affect heat and cold risk. Clothing, sun or shade, shelter, wetness and activity can matter as much as the published value. Walking may keep you warmer in cool weather, while the same effort adds heat strain on a hot day. A person standing still at a bus stop and a runner on the same street should not expect the same sensation.
Comfort and safety are also different questions. There is no Feels Like threshold below which conditions are safe for everyone. Duration matters, repeated hot days and warm nights matter, and official warning criteria vary by climate and local health impacts. Use the number as a planning aid, not a personal comfort guarantee, medical diagnosis or countdown to frostbite.
Read the forecast safely
Compare air temperature with Feels Like, then check wind, gusts, humidity, cloud cover and precipitation. Think about your actual exposure: full sun or shade, sheltered streets or open ground, dry clothes or rain, resting or strenuous activity. Check the latest warning from the national or local weather service, especially during severe heat or cold, because a warning considers more than one apparent-temperature value.
In cold weather, cover exposed skin, use dry layers and get out of the wind if you become numb, unusually drowsy, confused or uncoordinated. In heat, reduce strenuous activity and move to a cooler place if symptoms develop. Confusion, collapse, seizures or loss of consciousness can indicate a medical emergency; call the local emergency number and follow official first-aid advice.
Use Airpult Explore to find a location, compare the hourly values and see how the conditions change through the day.
