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Rain at the time

Recent research has shown that in the future there will be fewer rainy days in our region due to the climate crisis, but there will be more strong rain events. How can this be explained using simple scientific principles?

  • Itamar Karbi, the website of the Davidson Institute, the educational arm of the Weizmann Institute
It's hotter in the summer. Illustration: depositphotos.com
It's hotter in the summer. Illustration: depositphotos.com

In recent decades, the Earth's climate has been undergoing significant temperature-related changes, and this will probably be the case in the foreseeable future as well. Humanity emits greenhouse gases at an unprecedented rate and thus causes warming which is measured in most of the earth.

In the past, the common perception was that the effect of greenhouse gases is limited to warming, but today it is clear that the emission of greenhouse gases affects every aspect of the Earth's climate, including the wind regimes and precipitation patterns in the world. It causes droughts, floods and changes in the seasons. 

How are the rain patterns in Israel expected to change? The answer to this question has far-reaching effects on agriculture, our infrastructure and ecosystems. In this article we will try to understand some of the expected changes using simple scientific principles: the effect of global warming on the amount of water vapor in the atmosphere and on the global wind system.


increase in humidity

How will we understand the connection between the increase in the amount of greenhouse gases and the rains? We will use an equation developed about two hundred years ago by two scientists who gave it its name - Clausius-Calperon equation. The equation shows that warm air can contain more water vapor than cold air. For example, in a pot of water with a lid, the hotter the water and the air, the more water vapor the pot will contain. Two processes happen all the time in the pot: the water evaporates and at the same time the water vapor condenses and turns back into water. As the water is heated, its rate of evaporation increases. However, with the increase in the amount of steam, the rate of condensation also increases. After some time of heating, the increasing rate of condensation will finally equal the rate of evaporation, and the amount of steam in the pot will remain constant. This situation is called equilibrium and the Clausius-Calperon equation calculates the amount of vapor in the air at equilibrium when the air temperature changes. 

If we use the equation not to describe a pot with a lid but the atmosphere and the sea, we find that with a warming of the earth's surface by one degree, the air can contain up to seven percent more water vapor. The equation does not state that the warm air will necessarily contain more moisture, but that it may contain more moisture. That is why there are hot and dry areas in the world, for example deserts. According to the equation, in the future, when it will be warmer, the air will contain more water vapor, and this will be able to condense into clouds and fall as rain. More water above our heads will cause rain events more powerfullyamong others in Israel.


The bucket cell contributes to air rising in the equatorial region, and this air cools, condenses and rains. The air cells on the surface of the earth Designua, Shutterstock

Winds, rain and desert

If so, we saw that in the future there will be more water in the atmosphere and more precipitation, but where will the rain fall? The air contains water vapor and can also hold floating water droplets - the clouds. This is the meaning of equilibrium: water vapor turns into water droplets, and if they are not heavy enough to fall to the ground, they will evaporate and turn back into gas, vapor. Rain, on the other hand, is an example of instability: the floating water droplets grow and when they are heavy enough, they fall as rain. 

This instability can develop in several ways. An important component is the vertical movement of air upwards. The air cools quickly, so the water condenses and turns into drops. The drops grow until they are heavy enough to fall as rain. Israel lies at the meeting point between a temperate Mediterranean climate zone and a desert climate zone, and the instability and vertical movement of the air explain why in Israel rain can fall in winter but not in summer. To understand what causes these differences between the seasons, we must become familiar with the so-called wind system The bucket compartment (Hadley cell).

The bucket chamber is a system of global air circulation created by the temperature difference between the tropical zone around the equator, up to the 23rd parallel, and the subtropical zones, between the 23rd and 35th parallels. The air flow in the chamber causes moist air to rise in the tropical region, around the equator, and dry air to sink in the subtropical regions. This circular and closed flow gave the system the name "cell". In the cell there is a vertical movement of air, which indicates a connection between the cell and the global rainfall regime. Indeed, the bucket chamber contributes to air rising in the equatorial region, and this air cools, condenses and rains. This is how rainforests exist in the tropics, including the Amazon forest. On the other hand, in the subtropical regions, the bucket cell contributes to the deposition of dry air that prevents rain, thus contributing to the formation of the desert belt, which includes the Sahara desert in Africa. 

The cabin is seasonal. In the summer its descending and dry part reaches about 30 latitude, but in the winter the descending part stays closer to the equator and affects a smaller area. Israel lies at the 31st latitude, so in the summer the dry air flow in the cabin prevents precipitation in Israel, and the Israeli summer is almost completely dry. in winter, the bucket cell does not reach our area and thus rain can fall in Israel.


A stormy and short winter

Following the increase in the concentration of greenhouse gases in the atmosphere, The bucket compartment expands And its descending branch, which forms the global desert belt, is expected to migrate north. How will this phenomenon affect the precipitation in Israel? 

In the summer, the expansion will not have a significant effect, because the summer in Israel is almost completely dry anyway. In the winter, the descending branch of the cell will not reach Israel, even when you take into account the future expansion. Therefore, when the cell expands, the most significant drying is expected precisely during the transition seasons. The cell will prevent rains not only in the summer months, June to September, but it moves north and will therefore prevent rains before and after this period as well. The Israeli winter is getting shorter because the bucket cell reaches Israel for a longer time each year.

We came to two insights: from the Clausius-Calperon equation we concluded that in the future the rain events will be stronger because the atmosphere will contain more water, and from the expansion of the bucket chamber we concluded that in the future there will be less precipitation and fewer rainy days during the transitional seasons in Israel. What is actually happening? Are these the changes measured?

A group of researchers from the Hebrew University and Tel Aviv University found that the number of rainy days in Israel has decreased by an average of about two percent per decade since 1975, but the intensity of rain on rainy days has increased by an average of about two percent per decade. The study hypothesizes that the rain events will intensify but the number of rainy days will be shortened by seven days on average every year and we will receive a similar amount of precipitation every year. The meteorological service reached similar conclusions

The researchers also found that during the transition seasons, the rainy days decreased twice on average - by about five percent per decade. This trend corresponds to a forecast based on the expansion of the bucket cell. Thus, the changes in humidity levels and wind regimes allow us to understand how rainfall patterns in Israel are expected to change as a result of greenhouse gas emissions. We have seen that it is possible to understand and predict most of the rain patterns in Israel with the help of basic scientific principles, and not only with the help of complicated computer models.

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For the article on the Davidson Institute website

To the Davidson Institute for Science Education website

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One response

  1. Thanks, Enricher.

    An example regarding the pot - so how come the kettle is boiling? According to how the heating process was described, shouldn't excess steam be produced?

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