Not because of the weather, which is mild by British standards, but because the two on-route taps west of Streatley are both out of service, so the 41 km water gap becomes a full carry across a stretch with no shops. Eight and a half hours of light is the other constraint, and it caps a day at about 22 km on a half of the trail with few places to stop.
7.4-7.7°C high · 1.6-1.7°C low · ~8.4 hrs (sunrise ~08:03, sunset ~16:25 GMT)Watch for: Water, first and last. Low sun straight down the ridge line in the middle of the day on the westward run, which is worth a thought if you are navigating by landmark.The driest of the winter months at both reference stations and an hour and a half more light than January, which is enough to turn a cramped winter day into an ordinary one. The western water problem has not changed.
8.0-8.3°C high · 1.5-1.6°C low · ~9.9 hrs (sunrise ~07:20, sunset ~17:17 GMT)Watch for: Still a full water carry across the downland half.Light and ground both improve fast, but this is the last month before the Sparsholt Firs tap comes back on, so the western half is still a carry. If you can wait three weeks, wait.
10.6-10.9°C high · 2.7-3.0°C low · ~11.8 hrs (sunrise ~06:21, sunset ~18:07, clocks go forward late in the month)The Hill Barn tap at Sparsholt Firs is back in service for the season, which changes the arithmetic of the whole western half, and there is enough light for a 30 km day without starting in the dark.
13.8-13.9°C high · 4.1-4.8°C low · ~13.9 hrs (sunrise ~06:10, sunset ~19:59 BST)Watch for: Ticks become active from about now, and the trail runs through grazed chalk grassland and woodland edge for most of its length.Dry firm chalk, fifteen and a half hours of light so a long day is a choice rather than a race, and warm enough to walk in a shirt without the summer water pressure on the exposed western half.
16.9-17.1°C high · 6.8-7.6°C low · ~15.7 hrs (sunrise ~05:13, sunset ~20:48 BST)Watch for: Peak tick activity is April to June, so check after long grass and woodland edge.The longest days of the year and chalk downland at its best. Sixteen and a half hours of light makes the long western stages, where there is nowhere to stop anyway, an easy proposition.
20.0-20.1°C high · 9.8-10.5°C low · ~16.7 hrs (sunrise ~04:45, sunset ~21:25 BST)Watch for: Heat on the western ridge, which has no shade for hours at a stretch. Ticks are at their peak.The warmest month, and on this trail warmth is a cost rather than a benefit. The western half has no shade, no shop for 69 km and two taps that between them do not add up to a reliable supply, so a hot spell means either a heavy carry or an early start.
22.4-22.5°C high · 11.9-12.6°C low · ~16.3 hrs (sunrise ~05:03, sunset ~21:17 BST)Watch for: Water and heat on the downland half. Start early and drink at the taps rather than past them.Warm and busy, and the wettest of the summer months at both reference stations, which mostly means thundery afternoons rather than days of rain. Nearly two hours less light than June, so the long downland days need a bit more planning.
21.9-22.1°C high · 12.0-12.5°C low · ~14.8 hrs (sunrise ~05:48, sunset ~20:30 BST)Watch for: Same heat and water problem as July on the exposed half.The quiet answer to May. Ground still summer-dry, taps still running, the heat off, the holiday crowds gone, and twelve and a half hours of light, which is comfortably enough for the longest stages on the downs.
19.0°C high · 9.8-10.3°C low · ~12.8 hrs (sunrise ~06:37, sunset ~19:22 BST)Watch for: Tick activity picks up again in early autumn.The wettest month of the year at both stations and the point where the ground turns, but the Chiltern beechwoods are worth the trade and the taps are still on. The clock change late in the month takes an hour off the useful afternoon overnight.
14.7-14.8°C high · 7.3-7.5°C low · ~10.9 hrs (sunrise ~07:26, sunset ~18:13, clocks go back late in the month)The last month the Hill Barn tap at Sparsholt Firs is in service, and the best month for beech colour in the Chilterns. Nine hours of light is the limiting factor, not the weather.
10.3-10.6°C high · 4.1-4.2°C low · ~9.0 hrs (sunrise ~07:22, sunset ~16:17 GMT)Under eight hours of usable light, which is the shortest of the year and puts a hard ceiling on a stage, and the Sparsholt Firs tap has gone off for the winter. As a through-walk this is the hard version of the trail. The eastern half as day sections is a different and much better proposition.
7.7-8.0°C high · 1.8-2.0°C low · ~7.9 hrs (sunrise ~08:04, sunset ~15:55 GMT)Watch for: Finishing in the dark. Sunset before four o'clock means a 25 km day starts at first light.