The Sharav

There are days in the Negev on which the air changes within hours. In the morning the weather is ordinary, and at noon it is hot in a way that is hard to explain, and your lips dry out. Such a day is called sharav, and in Arabic khamsin.

What is going on there? Air whose source is in hot and desert regions flows into the country. It comes dry and it comes hot. On sharav days the temperature may rise within a few hours by 10 degrees Celsius and more. 10 degrees is not a small difference — it is the difference between a pleasant day and a day you have to be careful on.

The second thing that happens is the dryness, and it is the more dangerous part. A sharav day is defined by the humidity: a day on which the average relative humidity is lower than 50 percent in the coastal area, and than 45 percent in the mountain areas. Dry air draws water out of everything — out of the skin, out of the plants, out of the wood and out of the ground.

And so a sharav is not just discomfort. It is the time when fires spread fast, because the vegetation is dry and the wind pushes. In the history of the country there are several sharav waves after which large areas burned.

And there are two kinds of sharav. In one a pressure system drives the hot air from the desert. In the second hot air descends from above downward, warms further on the way, and heats the ground. The first is called a cyclonic sharav and the second a dynamic sharav. To someone standing outside they feel the same, but to someone who forecasts the weather these are two different stories.

And the sharav comes mainly in the transitional seasons — in spring and in autumn — and not necessarily in the middle of summer. That is surprising, and there is a logic to it: in the transitional seasons the pressure systems in the region change, and one of them draws the desert air toward the country.

And it is worth distinguishing between two things that get confused. A hot day is a hot day. A sharav day is a day on which the air was replaced. There can be a sharav at a temperature that is not a record, and it will still be hard on the body — because it is the dryness that makes it hard, and not only the number on the thermometer. And there can be a very hot day in summer with no sharav at all.

And what do you do on a sharav day? You drink before you are thirsty, you stay in the shade during the hot hours, and you do not set out on a long hike. In the desert, a sharav day is the day on which water in your bag is not a recommendation.