Explanation of why the hemispheres shine the same from space

23 Feb. () –

Scientists have found a solution to the half-century-old riddle of why Earth’s northern and southern hemispheres they appear just as bright when viewed from space.

This is especially unexpected because the southern hemisphere is mostly covered by dark oceans, while the northern hemisphere has a vast land surface that is much brighter than these oceans. For years, the brightness symmetry between hemispheres has been a mystery.

In a new study, published in Proceedings of the National Academy of Sciences, researchers at the Weizmann Institute of Science and their collaborators reveal a strong correlation between storm intensity, cloud cover, and the rate of reflection of solar energy in each hemisphere. They offer a solution to the mystery, along with an assessment of how climate change could alter the rate of reflection in the future, the institution reports. it’s a statement.

As early as the 1970s, when scientists analyzed data from the first meteorological satellites, they were surprised to find that the two hemispheres reflect the same amount of solar radiation. The reflectivity of solar radiation is known in scientific jargon as “albedo”. To better understand what albedo is, think about driving at night: it’s easy to see the intermittent white lines, which reflect light from the car’s headlights well, but it is difficult to discern the dark asphalt.

The same thing happens when observing the Earth from space: The relationship between the solar energy that falls on the Earth and the energy reflected by each region is determined by several factors. One of them is the relationship between the dark oceans and the bright land, which differ in their reflectivity, just like the asphalt and the intermittent white lines.

The land area of ​​the northern hemisphere is about twice that of the southern, and in fact, when measured near the Earth’s surface, when the sky is clear, there is more than a 10 percent difference in albedo. Even so, both hemispheres appear equally bright from space.

In this study, the team of researchers, led by Professors Yohai Kaspi and Prof. Or Hadas, from Weizmann’s Department of Earth and Planetary Sciences, focused on another factor influencing albedo, one located at a high altitude and that reflects solar radiation: the clouds.

The team analyzed data from the world’s most advanced databases, including cloud data collected via NASA satellites (CERES), as well as data from ERA5, which is a global weather database containing information collected using various airborne and ground-based sources, dating back to 1950. ERA5 data were used to complete the cloud data and to cross-correlate 50 years of these data with information on cyclone intensity and anticyclones.

Next, the scientists classified the storms of the last 50 years into three categories, based on their intensity. They discovered a direct relationship between the intensity of the storm and the number of clouds that form around it.. While the Northern Hemisphere and land areas in general are characterized by weaker storms, over the oceans, in the Southern Hemisphere, moderate and strong storms predominate.