How weather forecasts are made
Modern weather forecasts come from numerical weather prediction models that simulate the atmosphere using current observations (temperature, pressure, humidity, wind, satellite and radar data) as a starting point. Because the atmosphere is chaotic, forecast accuracy decreases the further out the prediction is made — a forecast for tomorrow is far more reliable than one for ten days from now.
What the Air Quality Index (AQI) actually measures
AQI is a standardized scale that converts the concentration of several pollutants — commonly PM2.5, PM10, ozone, NO2, SO2, and CO — into a single number, making it easier to understand overall air quality at a glance than reading raw pollutant concentrations. Different countries use somewhat different AQI scales and breakpoints, so an AQI value isn’t always directly comparable across countries without knowing which scale is used.
Why AQI can vary sharply within a city
Air quality readings can differ meaningfully between monitoring stations even within the same city, depending on nearby traffic, industry, and local weather patterns like wind direction. A single city-wide AQI figure is often an average or a specific station’s reading, not a uniform value across the entire city.
UV Index
The UV Index measures the strength of ultraviolet radiation expected at a given time and place, on a scale used globally to indicate sun protection needs. It typically peaks around midday and is affected by cloud cover, altitude, and season — a UV Index can still be significant on a cloudy day, which surprises many people.
Practical takeaway
Treat short-range weather, AQI, and UV forecasts as reasonably reliable guides for near-term planning, but expect longer-range forecasts to change as the date approaches — checking again closer to the time is more useful than trusting an early forecast rigidly.