Hour to Hour Forecast: How It Works & How to Read It

An hour to hour forecast shows expected weather conditions for each individual hour over the next 12 to 48 hours, giving you temperature, precipitation chance, wind, and humidity at a glance. You enter your location (or it is detected automatically) and the forecast updates with a new row or column for every hour, so you can plan around that 3 p.m. rain shower or the 6 a.m. temperature drop. This article explains how these forecasts are made, what each number means, and how to use them without getting tripped up by common pitfalls.

Formula

The calculator above converts a precipitation chance and wind speed into simple, comparable numbers. The expected rain amount is estimated as:

Expected rain (mm) = (Precipitation chance / 100) × 2 mm

where 2 mm is a typical hourly rain rate for a moderate shower. Wind speed in m/s is:

Wind (m/s) = Wind (km/h) ÷ 3.6

because 1 m/s equals 3.6 km/h. These are rough conversions—actual forecast models use far more complex physics—but they help you quickly grasp what a “30% chance of rain� might mean in volume or how breezy 15 km/h really is.

Worked Example

Using the default values: 24 hours ahead, 30% precipitation chance, 15 km/h wind.

1. Expected rain = (30 / 100) × 2 = 0.6 mm
2. Wind in m/s = 15 ÷ 3.6 = 4.17 m/s

The calculator displays 0.6 mm and 4.2 m/s. That is a light sprinkle (not enough to soak you) and a gentle breeze (about 15 km/h, or 9 mph).

What Goes Into an Hourly Forecast

Hourly forecasts are generated by numerical weather prediction (NWP) models. These models divide the atmosphere into a three-dimensional grid and solve equations of fluid dynamics and thermodynamics to simulate how air moves, heats, cools, and carries moisture. Supercomputers run these simulations, and the output is then interpolated to specific locations and times.

Key inputs include:

• Observations from weather stations, buoys, ships, and aircraft
• Satellite data (cloud cover, temperature profiles, humidity)
• Radar precipitation estimates
• Previous model runs (to maintain continuity)

For the first few hours, the forecast is heavily influenced by current radar and surface observations—this is called “nowcasting.� Beyond 24 hours, the model physics dominate, and uncertainty grows.

How to Read an Hourly Forecast

Most hourly forecasts show a timeline with columns for each hour. The most useful fields are:

• Temperature – usually in °C or °F, often with a “feels like� value that accounts for wind chill or humidity.
• Precipitation chance – the probability (0–100%) that at least 0.01 inches (0.2 mm) of rain falls at that location during that hour.
• Precipitation amount – expected liquid equivalent in mm or inches. This is a conditional amount: it only applies if precipitation occurs.
• Wind speed and gusts – sustained wind and higher gusts, in km/h, mph, or knots.
• Humidity – relative humidity as a percentage, which affects how hot or cold it feels.
• Cloud cover – percentage of sky covered by clouds, useful for solar planning.

Some forecasts also include UV index, dew point, visibility, and pressure. The “hourly� nature means you can see exactly when a front passes or when the wind picks up, rather than just a daily average.

Common Mistakes and Misconceptions

One widespread mistake is treating precipitation chance as a forecast of how much rain will fall. A 30% chance does not mean 30% of the hour will be rainy—it means that, based on similar past conditions, rain was observed 30% of the time at that location. The amount, if any, is a separate number.

Another pitfall is ignoring the “feels like� temperature. A 20°C day with 80% humidity and no wind feels much hotter than a 20°C day with low humidity and a breeze. Always check the apparent temperature if it is provided.

Rounding errors matter. A forecast of “1 mm� could mean anywhere from 0.5 to 1.5 mm. For critical activities (like outdoor events), look at the exact model output rather than the rounded display.

Finally, hourly forecasts are less accurate for the later hours. The first 12 hours are usually quite reliable; by hour 48, the timing of rain can shift by an hour or two, so check back closer to the time.

How to Choose a Reliable Hourly Forecast Source

Not all hourly forecasts are equal. Here is what to look for:

• Update frequency – forecasts that update every hour or every 3 hours are fresher.
• Model transparency – some sites tell you which model they use (e.g., GFS, ECMWF, ICON).
• Local calibration – forecasts that incorporate local topography and urban heat islands are better for your exact spot.
• Historical accuracy – check a site’s past forecast vs. actual observations; many weather apps publish this.

For most people, the default weather app on a smartphone is good enough for the next 12 hours. For outdoor planning (hiking, sailing, events), use a dedicated weather service that offers a “precipitation probability� and “amount� separately.

Limitations and When to Trust the Forecast

Hourly forecasts have inherent limits due to chaos theory—the atmosphere is a chaotic system, so small errors grow over time. This is why a forecast for 5 days out is less reliable than one for tomorrow. The practical limits:

• 0–6 hours: very reliable for temperature and precipitation, but rain can start/stop 15 minutes earlier or later.
• 6–24 hours: good for general trends; timing of fronts can be off by 1–2 hours.
• 24–48 hours: moderate; precipitation amounts are less certain.
• Beyond 48 hours: use daily forecasts instead of hourly.

Also, mountain valleys and coastal areas are harder to forecast because of local wind patterns and sea breezes. If you live in such a place, expect more variation.

Frequently Asked Questions

What does a 50% chance of rain mean for that hour?

It means that, based on historical model data for similar conditions, rain was observed at that location 50% of the time. It does not mean it will rain for 30 minutes or that half the area will get wet. It is a probability, not a duration or coverage.

Why does the hourly forecast keep changing?

Because new observations come in constantly, and models are re-run every few hours. A small change in wind direction or humidity can shift the expected timing of rain. This is normal; the forecast is meant to be updated, not static.

How accurate is the hourly temperature forecast?

For the next 12 hours, temperature accuracy is usually within 1–2°C (2–4°F). Beyond that, errors grow, especially during weather fronts. The “feels like� temperature is even more sensitive to humidity and wind, so check that as well.

Does an hourly forecast include snow or sleet?

Yes, if the precipitation type is relevant. Most forecasts show “precipitation� as rain, but if the temperature is near freezing, they may show “snow� or “wintry mix.� The amount is still given in liquid equivalent, so 5 mm of snow is roughly 5 cm of snow (depending on the snow-to-liquid ratio, usually 10:1).

Can I use an hourly forecast for planning outdoor activities?

Absolutely, but for events more than 24 hours away, check back the day before. For critical activities like a wedding or a marathon, also have a backup plan. Hourly forecasts are best for deciding when to leave or what to wear, not for making final decisions days ahead.

Related

• Decimal Time to Hours and Minutes
• Minutes to Hours: Divide by 60? Yes, Here’s How
• Minutes to Hours: Formula and Converter

in t | Forecast hours ahead | h | 24 | 1
in p | Precipitation chance | % | 30 | 0
in w | Wind speed | km/h | 15 | 0
out precip | Expected rain amount | mm | (p / 100) * 2 | 1
out wind | Wind speed | m/s | w / 3.6 | 1