Getting Geeky With Grant – Why it gets colder on nights with no clouds present

Weather Wisdom

ELMIRA, N.Y. (WETM) – Have you ever noticed on cloudless nights low temperatures can become cool and sometimes extremely cold in the dead of winter? This all has to do with the diurnal cycle of incoming solar radiation. Solar radiation controls weather on Earth and plays a huge role in our temperatures. As sun rises, incoming solar radiation is absorbed by Earth’s surface. There is a fraction that gets reflected back to space which is called the albedo of Earth and is roughly 30%. This is thanks to clouds and ice cover on Earth’s surface. Albedo is synonymous with “reflectivity”.

This is why during the daytime when it is cloudy, usually temperatures are capped and slightly lower than a day it would be extremely sunny. The surface of Earth absorbs the majority of this incoming radiation and releases some back into the atmosphere. The darker the surface, such as dark soil or asphalt, the more radiation is absorbed. This is why roads and pavement are extremely hot during the summer months. This is also relevant to snowpack. Snowpack is very light, absorbing much less radiation and reflecting much more than the dark surfaces. Meaning it has a higher albedo than the darker surfaces.

When clouds are present in the atmosphere, some of this radiation that is reflected from the surface is once again trapped by the clouds and held closer to the ground. This of this layer of clouds as a form of insulation or a “blanket” for Earth. This moderates temperatures, keeping them warmer at night. When clouds are present the diurnal temperature changes are not as drastic as they would be without clouds.

With the removal of clouds in the atmosphere at a given location, more radiation is able to reach Earth’s surface, allowing for a warmer day to occur. At night, with the incoming radiation shut off as the sun is not present, the daytime absorbed solar radiation now escapes and has nothing to keep it close to Earth’s surface without clouds in the atmosphere. This allows for temperatures to plummet. This creates for cool nights during the summertime and extremely cold nights during the wintertime. Another variable during the wintertime, is snowpack. As mentioned earlier, snowpack has a higher albedo (reflectivity) therefore it doesn’t absorb as much solar radiation throughout the daytime. At night under clear conditions this can create for even colder nights than forecasted as not only do you not have clouds in the sky to keep it close to the surface but you have less radiation in the first place to escape.

Clouds are very common on the East Coast, but in some places in the West, the diurnal temperature changes can be very drastic for this same reason. They have a lack of cloud cover. Therefore in deserts they can have extremely hot daytime temperatures and at night can reach very cold temperatures. Sometimes there can be a forty or fifty degree difference between low and high temperatures in these regions.

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