Lake Annecy temperature month by month: when the water passes 20°C, and when it drops again
Water at 9 °C in April, 17 to 19°C in June, 22 to 23°C in July and August, still 18 to 20°C in September. The published ranges month by month, the website that claims 25 to 27°C and why to be wary of it, and the 6 to 8°C waiting below fifteen metres.
The question "how warm is Lake Annecy" has not one answer but twelve, one per month, and the websites that cover it do not agree on the summer months. This page sets out the published ranges, where they come from, and the gap between them.
For the day's reading and how it is measured, there is a dedicated page: the water temperature today. Here, we talk about seasons, which is what matters when you are booking flights months ahead.
What is the water temperature month by month?
The table combines a climate guide and two swimming guides published in 2026. The values are surface averages, measured near the shore: out in the lake and at depth, it is colder.
| Month | Average | Observed extremes |
|---|---|---|
| January | 4.5 °C | 3 to 8°C |
| April | 9 °C | 6 to 13°C |
| June | 17 to 19°C | 15 to 25°C |
| July | 22 to 23°C | 19 to 26°C |
| August | 22 to 23°C | 18 to 26°C |
| September | 18 to 20°C | 16 to 22°C |
| October | 14 °C | 11 to 18°C |
| December | 5.5 °C | 3.5 to 8°C |
For the 2026 season, a swimming guide forecast 17 to 20°C in June, 22 to 25°C in July, 23 to 26°C in August and 18 to 22°C in September. The official reading of 23.4°C on 12 September 2026 was at the top of that range.
What is missing: a long series. No source consulted publishes thirty years of readings with annual minimums and maximums. The ranges above describe recent summers, not the trend.
When can you swim in Lake Annecy?
The guides do not say exactly the same thing. One sets a season from mid-June to mid-September, the others simply say June to September. The tourist office gives an average summer temperature of 23 °C for all the equipped beaches.
For comfortable family swimming, one guide recommends the window from 1 July to 15 August. It is also the period when the beaches are fullest, although no count puts a figure on it.
The lake's surface follows summer with a delay: cool in June, it peaks at the end of July and in August, and stays mild in early September. The late-season days, with water at 20°C and empty beaches, are the best value in the calendar.
Early September is the right window: water still at 20°C, empty beaches, accommodation prices easing. The accommodation search for Annecy in early September shows prices for that window, to compare with the July peak.
Compare accommodation by the lake →Partner link: booking through this link supports the site at no extra cost to you.Why do websites not give the same temperature?
A diving website claims summers at 25 to 27°C and an "ideal temperature between 22 and 27°C". Climate guides give 22 to 23°C in July with a maximum of 26°C, and the Sevrier sailing club 20 to 26°C for the summer.
Four degrees of difference over the same season is the gap between pleasant water and warm water. The institutional readings, around 23°C in early September 2026, bear out the climate guides. The 27°C figure is not confirmed by any measurement consulted.
The reason is method: some websites measure, others model, others copy. The same-day gaps between four sources are detailed in the water temperature today.
Why is the water icy fifteen metres down?
The lake is stratified in summer: the surface layer warms up, while the deep layers stay at 6 to 8°C all year round below about fifteen metres. A diver goes from swimming-pool water to mountain water in a few metres of descent.
This stratification also explains the difference between the two basins early in the season: the small lake, being shallower, warms up faster than the large lake. The depths of the two basins are in the depth of Lake Annecy.
The sources publish no statistics on cold water shock or hypothermia linked to this gap. The common-sense advice remains: get in gradually, especially after time in the sun.
What time of day is the water warmest?
No hourly curve is published. Physics says the surface reaches its maximum in mid-afternoon, around 2pm to 5pm, after a day of solar heating. That is an inference, not a reading.
The guides draw two consistent tips from it: the morning for a calm lake and slightly cooler water, the afternoon for the warmest water and the busiest beaches. Here again, nobody counts the swimmers.
How much does a day of swimming cost?
Access to the lake from a public beach or a municipal pontoon is free. Some equipped beaches charge, with lifeguards, showers and sunloungers, but no source consulted publishes a beach-by-beach price list. Water sports go through private clubs and hire companies, each with its own prices.
The cost that swimming pages forget is parking. By the lake, street parking costs €0.70 per quarter hour, 9am to 6pm, Monday to Sunday, public holidays included, with a €35 flat charge beyond 7 hr 45 min. The details are in parking in Annecy.
For the rest of the stay, Lake Annecy in figures points to each subject, and things to do in Annecy gives the prices of activities and museums.
When the water drops below 20°C, the lake is still there to enjoy: paddleboarding, kayaking and cruises do not depend on swimming temperature. The Annecy activities page gathers the outings with their duration and price.
See activities on Lake Annecy →Partner link: booking through this link supports the site at no extra cost to you.Sources
Office de tourisme du lac d'Annecy (weather page and SRV Annecy sailing club reading, 12 September 2026; beaches page, 2026), greenecho.fr (guide to temperature variations, 2026), annecy-pratique.fr (2026 swimming guide), ryo.co (swimming spots in Annecy, 2026), tourisme-annecy.net (climate, 2021 updated 2026).
annecyvibes.com (swimming in the lake, 2026), cvsevrier.fr (sailing club weather, 2020 updated 2025), profilplongee.fr (temperature, 2026), SILA (PCB and PAH study, 2022). Research consolidated on 12 September 2026. The monthly ranges are surface averages near the shore, not measurements out in the lake.
Frequently asked questions
What is the temperature of Lake Annecy in summer?
When can you swim in Lake Annecy?
What is the temperature of Lake Annecy in winter?
Why is the water of Lake Annecy cold at depth?
Do you have to pay to get into Lake Annecy?
Camille Perrissin
Camille grew up in Sévrier, on the west shore, and swims in the lake from April to November. She has been taking the water temperature herself since 2019 and knows where the bottom suddenly drops away. She writes about the lake without ever turning it into a postcard.
