As Israel sizzles under prolonged heat, with dry conditions that are once again sparking brushfires, data put together by number cruncher Dr. Gil David underscores the extent to which more and more scorching days in the country are becoming the norm.
David, a data scientist and researcher whose blog Data Science Storytelling provides text, graphs and diagrams on a wealth of subjects including climate change, posted a graph on Monday showing the annual number of heatwave days between 1964 and 2020.
He chose the definition of a heatwave as at least three to four days in which the maximum temperatures exceeded the threshold — the threshold being the 90th percentile of the maximum temperatures in the base period (usually lasting at least 30 years).
The graph shows the inexorable rise in the number of heatwave days over the decades.
Another of David’s graphs, presented at a meeting on Monday of the Environmental Protection Ministry’s Climate Change Preparedness Directorate, showed the number of days in a year in Tel Aviv during which temperatures exceeded 30 degrees Celsius (85°F), between 1964 and 2019.
Between 1964 and 1971, the central city was relatively comfortable, with temperatures above 30°C occurring on anything from 11 to 22 days per year only. The exceptions were 34 days in 1966, 39 in 1968 and 35 in 1970.
Between 2005 and 2010, the number of such days was already creeping up, to 15 to 23 per year, with 2010 setting a record of 53 days.
In recent years, the trend has picked up much further, with 90 such days in 2016, 81 in 2017, 98 in 2018 and 88 in 2019.
Night temperatures have also been rising, according to David’s compilations of raw data.
Minimum temperatures recorded between 1964 and 2020 at Beit Dagan, in central Israel, where the Israel Meteorological Service is located, show that the number of nights in which minimum temperatures exceeded 24°C (75.2°F) has increased exponentially.
Nir Stav, Director General of the Israel Meteorological Service, told The Times of Israel that the lengthy period of very hot weather being experienced across the country and the entire Eastern Mediterranean is due to a phenomenon called a blocking high.
The behavior of upper-level winds — the currents of air that flow around the world from west to east, typically parallel to the latitudes, is gradually changing and scientists believe that this is a consequence of human-induced global warming, he said.
From mainly straight west to east currents, with some waviness from time to time, the upper air streams are becoming more serpentine, like meandering rivers, combining a west to east direction with more north to south or south to north ones.
The north-south bits of the upper air streams transfer air between the latitudes.
Sometimes these wiggling lines create a shape that weather experts call an omega, because of the resemblance of its contours to the Greek letter of that name.
These omegas structures remain static for several days, causing extended periods of whatever abnormal conditions they create.
In the case of Israel’s current heatwave, hot air from tropical Africa moved upwards and northwards, cooling along the way, before descending in a blocking high over the Eastern Mediterranean. During the descent to ground level, the air is subject to increasing atmospheric pressure which compresses and warms it.
The same phenomenon is what has caused scorching weather over much of the western coast of Canada and the northern US, Stav explained.
While conditions within the horseshoe part of the omega are high pressure and cause heat, those immediately adjacent are low pressure and bring colder weather. Thus, abnormal heat along the west coast of North America can coexist with an abnormal cold crunch in the center of the continent, explaining some of the freezing weather reported several months ago in areas such as Texas.
Stav compared the heating of air that descends towards the ground in the blocking high to the action of an air conditioner: Stand next to the unit outside and you will feel the hot air that results from the compression of the gas inside the unit, he said.