How OLWeather Calculates

Last updated: 12 September 2026

Provider facts first. Useful context second. OLWeather never invents a number. Every current condition and forecast value shown anywhere on this site comes directly from WeatherAPI.com (or, for official warnings, from NDMA SACHET, NOAA/National Weather Service, or MET Norway — see the Privacy Policy for the full provider list). The screens on this page take those returned values and run them through a public, cited formula to produce a practical yes/no/caution read for one specific task.

This catalog is intentionally the full extent of what's public. OLWeather's mobile app has a considerably deeper decision engine — private weighting, a trained on-device model, satellite/radar fusion, and verified place data — that is deliberately not exposed here. What follows is real, but it is the public education layer, not the app's internal logic.

Every entry below states its equation, its real source citation, what it's useful for, and what it deliberately does not account for. Values update live per location; the formulas themselves are fixed and public. Citations link directly to the official source where it's freely publishable online (government regulations, NASA/NWS technical documents); citations to paywalled industry standards (ISO, AWS, ASABE, ACI) are named but not linked, since the standard body's full text sits behind a purchase, not a stable free URL.

Home

Home outdoor-window check

Useful for: a quick screen before laundry, exterior painting, or outdoor electrical work — whether current or near-term forecast precipitation makes outdoor household tasks a bad idea right now.

Public screen = current precipitation signal OR forecast rain chance ≥ 60%

Source: WeatherAPI.com forecast data; informational household rule
Inputs: Current precipitation (mm), next-period chance of rain (%)
Limitations: This is not a storm warning or electrical-safety certification.

Farm

Livestock temperature-humidity index (THI)

Useful for: deciding when livestock need active heat-stress intervention — shade, ventilation, cooling — versus when ambient conditions are within normal comfort range. Also estimates the practical impact: reduced milk yield and higher water intake as THI climbs.

THI = (1.8T + 32) − (0.55 − 0.0055RH)(1.8T − 26)

Source: NRC livestock THI standard; OLWeather agromet implementation
Inputs: Temperature (°C), humidity (%)
Limitations: Animal species, breed, shade, airflow, and acclimatization are not available to this public endpoint.

Ground-frost and radiative-cooling screen

Useful for: an overnight frost alarm for sensitive crops — estimates how much colder the ground surface will get than the forecast air minimum, since clear, calm, dry nights let heat radiate away fast.

T_ground = T_air,min − (100 − RH)/20 − 1.5 × (1 − cloud fraction)

Source: OLWeather agromet Formula 8: ground frost and nocturnal radiation risk
Inputs: Forecast air minimum (°C), humidity (%), wind (km/h), cloud cover (%)
Limitations: An instantaneous bounded screen; local terrain, canopy, soil moisture, and shelter can dominate actual frost outcomes.

Precision spray Delta-T

Useful for: the classic agricultural spray-timing window — Delta-T too low means wash-off/high-humidity risk, too high means rapid evaporation and spray drift. This is the standard psychrometric check applicators use before spraying.

Delta-T = dry-bulb temperature − Stull wet-bulb temperature

Source: ASABE S572.1 spray droplet guidance; Stull (2011) wet-bulb approximation
Inputs: Temperature (°C), humidity (%)
Limitations: Spray product label, nozzle type, wind direction, buffer zones, and legal requirements are not represented.

Crop wind-drag and lodging screen

Useful for: flagging when gust-driven wind pressure is strong enough to risk lodging (crops bending/breaking) or trellis damage, worth a post-gust field inspection.

q = 0.5 × ρ × V², air density ρ = 1.225 kg/m³

Source: OLWeather agromet Formula 10: dynamic wind drag and lodging risk
Inputs: Gust speed (km/h)
Limitations: Crop height, leaf area, staking, soil anchorage, and wind direction are not available.

Plant vapor-pressure deficit (VPD)

Useful for: gauging plant transpiration stress — high VPD risks foliage scorch and calls for more irrigation/shading; low VPD favors fungal pressure and calls for better ventilation.

VPD = es(T) − ea; es = 0.61078 × exp(17.27T / (T + 237.3)); ea = es × RH/100

Source: Magnus-Tetens vapor-pressure method; OLWeather agromet implementation
Inputs: Temperature (°C), humidity (%)
Limitations: Crop-specific VPD targets and canopy microclimate are not available from a city-level weather feed.

Pasture flystrike and fleece-rot risk

Useful for: flagging warm, humid, calm conditions that favour blowfly strike pressure on sheep and cattle — a cue to inspect fleece and skin for strike and rot.

Risk if 15°C ≤ T ≤ 32°C AND RH ≥ 78% AND wind ≤ 15 km/h

Source: Australian/NZ veterinary blowfly-strike extension guidance; OLWeather agromet implementation
Inputs: Temperature (°C), humidity (%), wind speed (km/h)
Limitations: Breed, fleece length, docking/crutching status, and local fly population are not available to this public endpoint.

Work

Tower-crane wind limit

Useful for: a fast pre-lift wind screen — flags when effective wind (the higher of sustained or gust) reaches the standard lift-suspension threshold, and separately the full-shutdown threshold.

q = 0.6 × V² (V in m/s); effective wind = max(sustained, gust)

Source: Industry convention (EN 13000 / IS 13367) — not an OSHA numeric limit. OSHA 29 CFR 1926.1431 governs crane-suspended personnel platforms specifically, not ordinary load lifts; the operative OSHA rule for standard lifts (1926.1417(b)) requires following the crane manufacturer's own rated wind limit rather than a fixed figure.
Inputs: Wind speed, gust speed (km/h)
Limitations: The crane model, load area, boom configuration, and site engineer's instructions are not visible to this screen. If the manufacturer's rated limit is lower, the manufacturer's limit governs.

Concrete-pour evaporation risk

Useful for: deciding whether to halt an unshaded concrete pour, add misting/curing controls, or proceed normally — based on estimated surface-moisture evaporation rate, the standard plastic-shrinkage-cracking risk indicator.

E = 5.275 × ([Tc+17.778]²·⁵ − RH[Ta+17.778]²·⁵) × (V+4.02336) × 10⁻⁶

Source: Uno-Menzel evaporation formula / ACI 305R / IS 456 / IS 7861
Inputs: Temperature (°C), humidity (%), wind (km/h)
Limitations: No calibrated solar-radiation field is available from the public feed; a fixed low-solar proxy is used. Confirm site surface and local concrete spec before pouring.

MIG/TIG and stick-welding wind check

Useful for: knowing when gas-shielded welding needs wind screens, when stick welding also becomes unsafe, and when active precipitation rules out arc welding entirely.

MIG/TIG screen threshold: wind > 8 km/h; stick threshold: wind > 20 km/h; any rain = stop

Source: AWS D1.1 §5.12.1 (8 km/h gas-shielded wind-screen requirement, verified) / ISO 15614-1. The 20 km/h stick-welding figure is an OLWeather conservative screen, not an AWS D1.1 requirement — AWS publishes no wind limit for SMAW.
Inputs: Wind (km/h), precipitation (mm)
Limitations: A weather screen only — not a welding procedure qualification or site permit.

Protective-coating dew-point margin

Useful for: the standard pre-coating check — confirming the substrate is far enough above the dew point (and humidity low enough) to avoid condensation-driven coating failure.

Dew point via Magnus-Tetens; coating window = substrate − dew point ≥ 3°C and RH ≤ 85%

Source: ISO 8502-4 / SSPC-PA 2 / NACE SP0188
Inputs: Temperature (°C), humidity (%)
Limitations: Ambient air temperature is only a substrate proxy; a real surface-temperature reading is still required on-site.

NASA Horne hydroplaning speed estimate

Useful for: an educational baseline for the speed at which a standard tyre can hydroplane on a wet road at a given pressure — a reminder to slow down and increase following distance when precipitation is active.

Vp = 6.36 × √p(kPa)

Source: NASA Technical Note D-2056 (Horne & Dreher, 1963) / AASHTO Green Book / IRC:73
Inputs: Precipitation (mm); assumes 32 psi tyre pressure
Limitations: WeatherAPI's current precipitation is not a calibrated rain-rate measurement; this does not replace road-safety judgment.

Wind-chill cold-stress screen

Useful for: flagging when outdoor crews need windproof layers and scheduled warm-up breaks, up to the frostbite-risk threshold where crews should rotate into heated shelter.

WCT = 13.12 + 0.6215T − 11.37V⁰·¹⁶ + 0.3965TV⁰·¹⁶

Source: NWS wind-chill method; ACGIH/OSHA cold-stress guidance
Inputs: Temperature (°C), wind (km/h)
Limitations: A screening calculation only; clothing, exposure time, shelter, and worker health controls remain decisive.

Adventure

Commercial UAV weather screen

Useful for: a fast pre-flight check against a real regulatory wind limit plus visibility/precipitation, before the licensed operator's own full pre-flight process.

No-go if effective wind (sustained or gust) ≥ 18.5 km/h (10 kt) OR visibility < 5 km OR precipitation > 0

Source: DGCA Drone Rules 2021 (10 kt surface wind limit — the one real source with an actual wind number). FAA 14 CFR 107.51 and EASA UAS.OPEN govern groundspeed/altitude/visibility, not wind, and are not cited for the wind threshold.
Inputs: Wind, gust (km/h), visibility (km), precipitation (mm)
Limitations: Not aviation authorization — cannot assess airspace, battery, pilot, aircraft, or local restrictions. Always use the manufacturer's own rated wind limit if it's more restrictive.

Pedestrian wind safety and comfort

Useful for: two real Lawson (1978) tiers — a comfort threshold (strolling becomes uncomfortable) below the safety/danger thresholds (real fall/loose-object risk).

Comfort: effective wind ≤ 28.8 km/h. Safety: unsafe above mean wind 54 km/h, dangerous above effective gust 72 km/h.

Source: Lawson (1978) pedestrian-wind comfort and safety criteria
Inputs: Wind, gust (km/h)
Limitations: Buildings, canyon effects, terrain, and wind direction are not represented. Lawson's criteria are defined statistically over a long-run wind climate, not a single instantaneous reading — applied here as an approximation.

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