The forecast is for an area, you are at a point
Weather models divide the atmosphere into a three-dimensional grid and calculate one set of values per cell. Global models use cells tens of kilometres across; the higher-resolution regional models that drive consumer forecasts get down to a few kilometres.
A few kilometres sounds precise until you consider what fits inside it: a river valley, a hill, an industrial estate, a park and two housing developments, all averaged into one number.
So a forecast can be entirely correct about the cell and several degrees out for your garden. That is not a failure of prediction. It is a mismatch of scale, and no amount of forecasting skill removes it.
The five things that move your local reading
Elevation
Air cools with height — roughly 6.5 °C per 1,000 metres in freely mixing air, and much more sharply near the ground on a still night. A house 100 metres higher than the reporting station will run measurably cooler by day.
At night the pattern inverts. Cold air is denser and flows downhill, pooling in valleys and hollows. A valley bottom can end up colder than the hillside above it, which is why frost forms there first and why gardeners talk about frost pockets. That mechanism is a temperature inversion.
Surfaces
What is under and around you changes the reading more than most people expect.
Asphalt and dark roofing absorb solar energy through the day and release it slowly after sunset. Grass and trees convert some of that energy into evaporation instead, which cools rather than heats. Water has enormous thermal mass and lags the air, cooling its surroundings in summer and warming them in autumn.
Which is the urban heat island in one sentence: cities are built of materials that store heat, with little vegetation to shed it, so they run warmer than the countryside and hold it longest on clear calm nights.
Shelter and exposure
A courtyard enclosed by walls is a different microclimate from an open field. Walls block wind, radiate stored heat back at night, and stop the ground radiating to the open sky. A south-facing wall in winter sun can be startlingly warm while the air two metres away is near freezing.
The same effect protects plants. A bed against a house wall reliably survives nights that kill the same plant in the open — real, and worth exploiting deliberately.
Distance to the nearest observation
Your reported temperature is interpolated from stations that may be miles away, at a different elevation, over different ground. In a dense network the interpolation is short and reliable. In sparse rural or mountainous areas it is a longer guess.
Airports host a disproportionate share of official stations, and an airport is a large flat expanse of grass and concrete away from buildings — which is good siting for a standard reading and not much like a garden in a terraced street.
Water and coastline
Near a coast or a large lake the wind direction can decide the day. Onshore flow brings the water's temperature inland and holds the air near it; offshore flow lets the land do what it likes. This is why coastal forecasts go wrong so sharply: the boundary between the two regimes can sit a mile from you and move during the afternoon.
Which conditions make the differences big
The gap between your reading and the regional one is not constant. It depends on how well the air is being mixed.
| Conditions | Local spread | Why |
|---|---|---|
| Windy, overcast | Under 1 °C | Air is thoroughly mixed; cloud blocks radiation |
| Breezy, part cloud | 1–2 °C | Some mixing, some local heating |
| Calm, clear day | 2–5 °C | Surfaces heat unevenly with nothing to even them out |
| Calm, clear night | 5–10 °C | Cold air pools; open ground radiates to the sky |
The practical reading of that table: on the nights when the exact temperature matters most — frost, pipes, plants — the local spread is at its widest. The forecast is least specific to you precisely when you need it most.
Learn your own offset. Note how your spot compares with the forecast over a few clear nights and you will find it is consistently a degree or two one way. That offset is worth more than any app, because it is the one piece of information no forecast has.
What this means for the decisions
Every threshold on this site — pipes at about 20 °F, paint at 50 °F, frost on a clear night — is stated against an air temperature. If your spot runs two degrees colder than the forecast on clear nights, then a forecast of 22 °F is a real 20 °F in your garden, and the threshold has already been crossed.
Which is the whole argument for reading a temperature for where you are rather than for your region, and for knowing the difference between the two. The forecast tells you what the area is doing. Your own spot, and your own offset, tell you what to do about it.