The official National Weather Service forecast for Alton — 43.4523°N, 71.2176°W, 17.7 miles from I-93.
| Belknap County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 30 | 34 | 42 | 56 | 68 | 76 | 81 | 79 | 72 | 59 | 46 | 35 | 56 |
| Avg low °F | 13 | 14 | 22 | 32 | 43 | 53 | 58 | 56 | 49 | 38 | 29 | 19 | 36 |
| Precip in | 3.3 | 3.1 | 3.7 | 4.0 | 4.0 | 4.5 | 4.6 | 3.9 | 3.9 | 4.9 | 3.7 | 4.5 | 48.1 |
1996–2025 normals for Belknap County — the climate division Alton sits in. All-time county record high 97.5°F (2025-06-25); record low -29.1°F (1957-01-14). Source: NOAA / National Weather Service.