Weather Forecast Numbers Decoded
Six numbers that most people misread
Every weather app throws out a PoP, a dew point, a UV index, a gust speed, a pressure reading, and a 7-day forecast accuracy. Here is exactly what each one means, where the numbers come from, and what threshold actually matters.
PoP does not mean what most people think
Probability of Precipitation (PoP) is a number produced by a formula: PoP = C x A, where C is the forecaster's confidence that rain falls somewhere in the forecast area, and A is the fraction of that area that would receive measurable rain given it falls at all. For a grid-point or ZIP code forecast, A = 1.0 (the point either gets rain or it does not), so the published number equals C directly. A 40 percent PoP means there is a 40 percent chance your location receives at least 0.01 inches of rain. It says nothing about how hard it will rain or for how long.
"Slight chance" in NWS text = 20 percent PoP
"Chance" = 30 to 50 percent
"Likely" = 60 to 70 percent
No qualifier = 80 to 100 percent (rain is essentially certain)
Dew point beats relative humidity for comfort
Relative humidity tells you how full the air is relative to its capacity at the current temperature. That capacity changes with temperature, so 80 percent humidity on a cool morning feels fine but 80 percent at 32 C is brutal. Dew point is a fixed temperature: the point at which the air would become saturated. It does not change as the temperature changes. Above roughly 21 C (70 F), sweat stops evaporating efficiently and your body struggles to cool itself. That is the threshold to watch.
Dew point comfort scale
| Dew Point (F) | Dew Point (C) | How it feels |
|---|---|---|
| Below 50 F | Below 10 C | Dry, like the high desert |
| 50 to 54 F | 10 to 12 C | Very comfortable |
| 55 to 59 F | 13 to 15 C | Comfortable |
| 60 to 64 F | 16 to 18 C | Slightly sticky, noticeable at upper end |
| 65 to 69 F | 18 to 21 C | Humid, uncomfortable for many |
| 70 to 74 F | 21 to 24 C | Very humid, oppressive |
| 75 F and above | 24 C and above | Extremely oppressive |
UV index: five bands, one rule
The UV index runs from 0 (no sun) to 11 and above (extreme). The scale is joint WHO and EPA. For fair skin (Type I or II), the approximate time to a first burn is listed below. The key threshold is 3: below it you need no protection; at 3 and above, SPF 15 or higher plus a hat is recommended regardless of cloud cover, since UV penetrates light cloud easily.
Approximate fair-skin burn time by UV index (minutes)
Gust vs sustained: two different measurements
The National Weather Service defines sustained wind as the two-minute average wind speed measured at 10 metres above ground. A wind gust is the highest three-second peak within that two-minute window, provided it differs from the minimum by at least 10 knots. In practice, gusts run 30 to 50 percent above sustained in open terrain, and can run higher near fronts or thunderstorm outflow. When a forecast says "20 mph with gusts to 35 mph", that 35 is a 3-second spike, not a sustained condition.
Sustained 25 to 38 mph: NWS threshold for "Windy" advisory
Gusts 46 mph and above: NWS threshold for Wind Advisory
Gusts 58 mph and above: High Wind Warning territory
Barometric pressure trend: the 3-hour rule
Apps show current pressure (around 1013 mb at sea level) but the trend over the past 3 hours is what forecasters watch. A rapid fall signals an approaching low-pressure system and deteriorating weather; a rapid rise signals clearing. The WMO classification below is the standard used in NWS forecaster training.
Pressure tendency over 3 hours (WMO scale)
| Change in 3 hours | Term | What it typically signals |
|---|---|---|
| Less than 0.1 mb | Steady | No significant change expected |
| 0.1 to 1.5 mb | Rising or Falling Slowly | Gradual weather change |
| 1.6 to 3.5 mb | Rising or Falling | Moderate change likely |
| 3.6 to 6.0 mb | Rising or Falling Quickly | Significant change, possible strong winds |
| More than 6.0 mb | Rising or Falling Very Rapidly | Severe weather risk |
A fall of more than 6 mb in 3 hours is a storm-warning threshold. A sustained fall of roughly 24 mb in 24 hours defines a bomb cyclone. Check additional sources and monitor alerts if pressure drops this fast.
How far ahead can you actually trust a forecast?
Modern numerical weather prediction has improved dramatically. NOAA officially states that a 5-day forecast is accurate approximately 90 percent of the time, and a 7-day forecast about 80 percent of the time. Beyond 10 days, accuracy drops to roughly 50 percent, which is little better than a climatological average. Temperature forecasts show mean absolute error growing from roughly 2 to 3 F at day 1 to over 8 F by day 7. For precipitation, skill drops faster.
Forecast accuracy by lead time (NOAA)
| Lead time | NOAA accuracy figure | Practical guidance |
|---|---|---|
| Day 1 to 2 | Very high (better than 90%) | High confidence, act on it |
| Day 3 to 4 | High | Good for plans, review day before |
| Day 5 | Around 90% | Reliable for general planning |
| Day 7 | Around 80% | Treat as probable, not certain |
| Day 10 and beyond | Around 50% | Trend only, not a specific forecast |
Hourly forecasts within 2 to 6 hours draw on radar nowcasting and are substantially more accurate than the same model data for "tonight" 12 to 18 hours out. When precision timing matters, re-check the hourly forecast an hour or two before, not the morning before.