Season: The answer to the first question depends on your exposure protection, and also to a certain degree on your level of experience. For divers in wetsuits the season (depending on the weather that year) tends to start in May, and starts to drop off towards the beginning of October.

Temperature: During the summer, temperatures at the surface of the water can exceed 24 degrees. This is warmer than the diving in many holiday locations! You see plenty of people swimming in the lake during the summer, so obviously the temperature of the water is warm. As you descend the temperature starts to decrease, and usually you will notice the change in the form of a "thermocline". In summer this thermocline generally sits at around 12 meters of depth and marks the change of temperature from the warmer to the colder water. Sometimes a second thermocline is present at lower depths (perhaps 20 meters) marking the change from cold to very cold water. Once you get into the deeper regions of the lake (30 - 40 meters) the temperature will be around 4-6 degrees.

You can find information on the Internet about water temperatures - for example wiewarm.ch provides current and historical temperatures for Lake Zurich.

Exposure Protection: Don't let these temperatures put you off - remember that when you dive in the tropics you probably used no exposure protection at all. In the lake we use thicker wetsuits (usually 7mm, two piece suits with hood and gloves) or dry suits. But having to wear a thick wetsuit doesn't mean you should miss the opportunity of diving here - would you refuse to leave the house if the weather wasn't good enough to wear a t-shirt?

Divers using wetsuits will probably stick to the shallower sections of the lake - above 18 meters, and are limited to diving during the warmer months (see season details above). They may venture briefly into the deeper sections for limited periods of time, then return to the warmer waters of the shallower sections. All wetsuits will compress with increasing depth, reducing the effectiveness of the exposure protection. Therefore for divers wishing to spend longer in the deeper areas, a dry suit makes a lot of sense.

Dry suit divers have the luxury of making longer dives, to any depth (within their own personal limits) at any time of year. For those who have never dived with a dry suit, additional training with a qualified instructor is strongly recommended. It is not difficult to dive with a dry suit, but it usually takes most people some time to adjust to the transition. A dry suit works on a completely different principle. The diver wears a thick undergarment under the suit, and the suit is filled with air. This offers better overall thermal isolation, allowing the diver to stay submerged for a longer period before noticing the colder temperatures.

Visibility: This can vary greatly depending on depth, location and time of year. The optimal time of year for wet suit divers is the end of April to the beginning of June. At this time of the year the water is starting to warm up, but the visibility has not yet started to degrade. Visibility of up to 8 meters is common between the surface and a depth of 20 meters. As the summer starts to warm up the water, the visibility in the warmer (shallower) sections reduces, and in the worst case could be one to two meters. Large numbers of student divers in a popular training site such as WZT can also help to reduce visibility by stirring up the bottom.

Dropping below the thermocline (see previous section about temperature) will bring you into colder water, but the visibility will vastly improve - it could be as much as 20 to 30 meters depending on the amount of light from the surface. Usually by the time you reach a depth of 40 meters you'll find yourself in very clear water, but it will be dark. A good diving lamp will light the way, and you'll have great visibility limited only by the range of your lamp. The darkness is caused by the poor visibility in the shallow sections reducing the amount of light making it to the depths. In spring it is common to have twilight-like conditions at a depth of 30 meters.