Do Stormy-Weather Clouds Cool or Warm The Earth?
People that like to "read" the weather say that the appearance of high, thin clouds in the horizon usually means that a storm is coming. What often follows are gray, ominous clouds that bring with them rain or snow, and then the weather begins to clear again and puffy, white clouds fill the sky. The passage of the storm may last one or two days, and during this time, clouds of all different types parade through the sky.
Does the presence of those clouds in the stormy sky make the atmosphere colder or warmer than it would be without them? At first glance, the answer to this question may appear obvious. After all, we all have experienced the chill in the air when a cloud blocks the sun. We also know, however, that a clear, starry night usually feels colder than a cloudy one. This is because clouds not only block sunlight but also trap the Earth's heat and prevent it from escaping into space.
Some cloud types, like low, thick stratus clouds, are very good in blocking the sun but do little to trap heat, while others, like high thin cirrus clouds, let most of the sunlight in but prevent large amounts of heat from escaping into space. During nightime, when there is no sunlight available to reflect, all clouds are a warming influence on the planet. Whether, then, the clouds that form during the passage of a storm warm or cool the Earth depends on the types of clouds that storms produce and on the amount of sunlight available for them to block.
In a recent study, George Tselioudis, Yuanchong Zhang, and William Rossow from the NASA Goddard Institute for Space Studies (GISS) attempted to quantify how much the passage of a storm affects the radiation, and therefore the temperature, of the Earth. They examined on a day-by-day basis all storms during 1990 that crossed the Northern Hemisphere middle latitude region, the area of the planet between 30 and 60°N and which includes most of the US and Europe and a big part of Asia. They used weather-station measurements of pressure to decide which regions were stormy and which ones were not, and satellite measurements of cloud height and brightness to determine the cloud types in each region. The cloud type measurements were then fed into a mathematical model to calculate what amount of radiation reaches the surface of Earth and what amount escapes into space under both stormy and non-stormy conditions.
The GISS scientists found that, as expected, clouds in areas affected by storm systems (often referred to as low-pressure systems) tend to be more abundant, thicker, and with higher tops that those in all other areas. They also observed, however, that non-stormy areas affected by high-pressure systems often have large concentrations of low, stratus clouds.
When model calculations were made to compare radiation amounts under stormy and non-stormy conditions, the answer turned out to be season dependent. During winter, when the northern midlatitude regions receive small amounts of sunlight for few hours during the day, the passage of a storm has an overall warming effect, as clouds block more heat from escaping into space than sunlight from reaching the surface. An excess heat amount of about 10 Watts for every square meter is added to the Earth's atmosphere under stormy conditions. During all other seasons, however, with more sunlight reaching the midlatitude regions for more hours during the day, the storm clouds' ability to block sunlight dominates and produces a net cooling effect on the planet. An excess sunlight amount of about 30 Watts for every square meter is blocked from reaching the Earth's surface under stormy conditions.
The results of the study have important implications in the work of scientists who use climate models to examine whether our planet's climate will continue to warm in the upcoming century. This is because two leading climate models, one from NASA/GISS and the other from the United Kingdom Meteorological Office, predict that in a future warmer climate, in which the temperature difference between tropics and midlatitudes that fuels storm generation will be smaller, the atmosphere will be producing fewer midlatitude storms. According to this study's results, in winter a reduction in storm events that tend to warm the planet's atmosphere will introduce a cooling effect on climate warming, while in all other seasons a reduction in storm events that tend to cool the planet's surface will introduce an additional warming effect on climate warming. The study made a simple "back of the envelope" calculation and found that those cooling and warming climate effects are potentially significant as they can reach values as large as 2 and 5 Watts per square meter respectively. This points to the need to improve the presently inadequate representation of storm clouds in climate models in order to accurately quantify their potential climate effects.
Tselioudis, G., Y.-C. Zhang, and W. B. Rossow, 1999. Cloud and radiation variations associated with northern midlatitude low and high sea level pressure regimes. J. Climate 13, 312-327.
Please address all inquiries about this research to Dr. George Tselioudis.