Israeli towns and cities were built to withstand the hot climate. You soon become accustomed to the abundance of air-conditioning, parasols and the easy availability of cold drinks. All this goes to hide the fact that the semi-arid Judean hills can be fatally inhospitable. In the summer, temperatures in Jerusalem frequently rise to the high 90s, which is bad enough, but once you head down toward Jericho and Ein Gedi the temperature can go over 100. In short, any travel outside towns or the main tourist sites requires some basic preparations.
Firstly, water. You should carry substantial quantities of water with you. Even if you are driving, you should carry several pints of drinking water in your car lest you get lost or suffer a breakdown. You should also ensure you have a sun hat and sunglasses unless you want to stumble around confused, and trying to see the path ahead through sore squinting eyes.
Equally important is a good map. Local villagers and Bedouin herdsmen excel at creating numerous paths across the hills, but you cannot always rely on them to be there when you need help deciding which path to follow. It is advisable to choose a route before you set out and to advise someone of your tour and the time you expect to return.
As always, good footwear is also important. The hills around Jerusalem are scraggy and an open invitation for a twisted ankle. Avoid this by wearing solid boots. Snakes can pose a hazard and boots can afford you some degree of protection.
Unfortunately, the area around Jerusalem also suffers from more than its fair share of man-made threats. Most of the areas around the city are in the West Bank, under a mixture of Israeli and Palestinian control. The risk of sporadic fighting here is high at times and you should always seek local advice before setting-off. If there is even the slightest risk of conflict, you should avoid the area, even the very latest in hi-tech trekking gear will not protect you against a tank shell. At times of uneasy quiet many Israelis will travel in these areas only if they are armed. At times like this you should exercise extreme caution and travel only with an organized tour.