Question:
What is the difference or relationship between hourly probability of precipitation and daily probability of precipitation for example if for a 24 hour period for my town the weather channel is publishing 10% probability of precipitation for each hour of a 24 hour. Period. How is that different from a daily probability of precipitation of 10% if there is a 10% probability of precipitation it will rain between midnight and 1 AM and there’s a 10% chance that it will rain between 1 AM and 2 AM Etc. all day then it seems like the probability of precipitation on that day is nearly 100% I do not mean what is the probability of precipitation like certainty times percentage of area covered my question is different
Answer:
1. Climatology as a Baseline
Climatology in weather forecasting refers to the statistical behavior of the weather over long periods—often 30-year (or longer) datasets that define what is “normal” or average (in mathematical terms) for a given region and time of year. Forecasters use these long-term averages, along with near-term model output, to gauge whether the atmosphere is leaning toward “typical” conditions or deviating significantly.
Example of Incorporating Climatology
- If it is July in a region that historically has about a 20% chance of showers any given afternoon (due to summertime convection), the climatological PoP might be around 20%.
- Now, if short-term weather models indicate no significant systems for your area—e.g., high pressure dominating—forecasters might lower the PoP below the climatological 20%.
- Conversely, if a disturbance is in the forecast, that might push the PoP above the climatological norm.
In other words, climatology provides the “typical” starting point, and real-time weather data or model indications then modify that baseline up or down.
2. Daily vs. Hourly PoP, with Climatology in Mind
When we discuss hourly PoP forecasts (say 10% each hour) versus a daily PoP (10% for the whole day), we must also recognize that these short-term forecasts are informed by both:
Model Physics & Current Observations
- (e.g., radar, satellite data, weather models showing atmospheric instability or moisture levels).
Climatological Norms
Local weather service forecasters might know, for example, that in your area’s rainy season, afternoon storms usually bubble up even if the models are only lukewarm about it. So they may hedge the hourly PoP higher in the afternoon hours. Alternatively, if it’s the dry season—and historically there’s only a 5% chance of precipitation—forecasters might be more confident leaving the hourly PoP low, even if models show a slight possibility of rain.
The short-term model might say, “There’s only a 5-10% chance of a stray shower each hour,” but the forecaster also sees that climatologically, you’d get that kind of scattered shower roughly 20% of the time. If both signals align (low model PoP + low climatological PoP), you’ll end up with an even more confident low daily PoP.
3. Why You Still Can’t Just Sum Hourly PoPs
Even with climatology as a backdrop, the key concept remains:
- Hourly PoPs are not strictly independent events. They often reference the same weather feature (e.g., a passing frontal system or a typical diurnal shower pattern).
- A “10% chance each hour” doesn’t mean a 10% chance of a completely new rain event each hour.Climatologically, if it rains in your area, it might rain for multiple hours in one stretch—or not at all.
Hence, if your daily PoP is 10% (based on the combination of models + climatology), it doesn’t necessarily escalate to near 100% simply because each hour is labeled 10%. Forecasters know from historical patterns how rain typically arrives (e.g., a single wave or a quick convective shower) and incorporate that into the daily number.
4. Putting It All Together
- Climatology sets an expectation for how likely precipitation typically is on that date and in that region.
- Short-term weather models and observations adjust the forecast up or down from that baseline.
- Hourly PoPs are often posted in a simplified format and may show the same small probability across many hours, reflecting a single small system’s uncertain arrival.
- Daily PoP expresses the forecaster’s best estimate of whether measurable rain (≥0.01 inch) will fall at any time during that 24-hour period, taking into account both the synoptic (large-scale) situation and the region’s climatological patterns.
Ultimately, climatology helps forecasters avoid overreacting to short-term model errors by providing a historical context: if the models predict something that’s highly unusual for the season, forecasters will compare it against what typically happens this time of year, then adjust the PoP forecast accordingly.
