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Air temperature is measured in a shaded enclosure about five feet off the ground, away from surfaces that absorb and radiate heat. It's a precise scientific measurement — but it's not what your skin feels. Your body's experience of temperature depends on two additional factors: wind and humidity.

In cold weather, wind strips away the thin layer of warm air your body generates around itself. The faster the wind, the faster that insulating layer is removed, and the faster your skin loses heat. This is wind chill. A 30°F day with 20 mph winds has a wind chill of about 17°F — that's the rate of heat loss your exposed skin actually experiences, expressed as an equivalent still-air temperature.

In hot weather, the modifier is humidity. Your body cools itself by sweating — as sweat evaporates, it carries heat away from your skin. When the air is already saturated with moisture, evaporation slows dramatically. At 90°F and 90% humidity, the heat index pushes the apparent temperature above 100°F. Your body is working hard to cool down but can't shed heat fast enough.

The practical takeaway: always dress for the feels-like temperature, not the air temperature. On a windy winter day, add a windproof outer layer. On a humid summer day, reduce exertion and stay hydrated even if the thermometer reads lower than you'd expect to feel uncomfortable.

A weather forecast starts with observation. Every hour, thousands of weather stations around the world record temperature, pressure, humidity, wind speed, and wind direction. Weather balloons are launched twice daily from hundreds of locations, measuring conditions at altitudes up to 100,000 feet. Satellites capture cloud cover, sea surface temperatures, and atmospheric moisture from orbit. Aircraft report conditions in real time as they fly. All of this data flows into a global network of meteorological agencies.

That data feeds into numerical weather prediction (NWP) models — mathematical simulations of the atmosphere that divide the Earth into a three-dimensional grid of cells, each roughly 3–10 miles across. The models apply the laws of physics to calculate how temperature, pressure, and moisture in each cell will evolve over the next few hours, then use those results as inputs for the next time step. Running a global forecast model requires some of the most powerful supercomputers on Earth, performing quadrillions of calculations per run.

The major global models — the American GFS (Global Forecast System) and the European ECMWF — run multiple times per day and produce forecasts out to 10–16 days. Meteorologists compare the outputs of multiple models, look for agreement and disagreement, and apply local knowledge to refine the forecast for specific regions. When models agree, confidence is high. When they diverge, uncertainty is higher — which is why forecasters sometimes hedge on specific days beyond the 3–5 day window.

Time & Temp uses Open-Meteo, which aggregates data from multiple global models and applies high-resolution local downscaling to give you the most accurate reading for your specific location. The hourly forecast is updated every hour as new model runs complete, so the data you see is always current.

Heat is the deadliest weather hazard in the United States, responsible for more deaths annually than hurricanes, tornadoes, and floods combined. Heat illness progresses in stages: heat cramps (muscle spasms from electrolyte loss), heat exhaustion (heavy sweating, weakness, cool pale skin), and heat stroke (body temperature above 103°F, hot dry skin, confusion — a medical emergency). The key risk factor isn't just temperature but the combination of heat and humidity that prevents your body from cooling itself.

Cold weather brings its own risks. Hypothermia begins when core body temperature drops below 95°F — it can occur at temperatures well above freezing when wind and wet clothing are factors. Frostbite affects exposed extremities first: fingers, toes, ears, and nose. The wind chill reading on Time & Temp tells you how quickly exposed skin is losing heat — at wind chills below 0°F, frostbite can occur in under 30 minutes.

Barometric pressure changes affect many people physically. A rapid drop in pressure — which typically precedes storms — is associated with increased migraine frequency, joint pain in people with arthritis, and sinus pressure. The mechanism isn't fully understood, but the correlation is well-documented. If you're sensitive to pressure changes, checking the forecast for incoming storm systems can help you prepare.

Air quality is closely tied to weather. Hot, sunny, stagnant days allow ground-level ozone to build up — a respiratory irritant that worsens asthma and lung conditions. Wildfire smoke can travel thousands of miles and dramatically reduce air quality in regions far from the fire. On days with high humidity and low wind, particulate pollution concentrates near the surface. If you have respiratory conditions, checking both the weather and air quality index before outdoor exercise is worthwhile.