POSTED: Monday, January 20, 2014 - 11:19am
UPDATED: Monday, January 20, 2014 - 1:33pm
Clouds, Moisture and Wind All Play a Part in How Cool or Cold Our Overnight Temperatures Will Be
Monday, January 20th, 2014 — “Why does the temperature drop so fast after the sun sets?” Many of us have wondered why we have seen chilly, below normal overnight low temperatures and followed by warmer than average afternoon high temperatures. A strong ridge of high pressure off the west coast has kept the skies clear, light winds and under a dry flow of air. This works great in the afternoon to warm us up, but those same variables help make for colder overnights. Since we are still in the midst of winter, I thought I would focus on the factors that make for colder overnights and some that keep us not as cold. Most of us know that cloud cover impacts our temperatures, but some may be surprised that good old water vapor or lack there of can determine if we are in for a colder sunrise.
First to understand why we experience nighttime cooling, let us touch on what causes the daytime temperatures to rise. The main source as we all know of all weather is the sun. Solar energy from the sun is sent our way and then everything radiates heat. The ground emits heat continuously to outer space while receiving energy from the sun.
During the daytime, the sun sends us energy that is in excess of the outgoing energy and producing a rising temperature of the ground.
When there is not a colder air mass pushing in, the main reason for cold overnights into the early mornings is all thanks to something we call in meteorology, “radiational cooling”. The incoming solar energy shuts off during the nighttime hours as the ground continues to radiate away heat, the ground temperature falls. This nighttime cooling effect is "radiational cooling". How far the temperature falls at nighttime depends on cloud cover, wind strength and humidity. The maximum amount of cooling occurs under clear skies, light winds and dry conditions.
The effect of clouds on ground temperature is like when we are wrapping the blanket around us to keep our bodies warm. When the skies are cloudy, the clouds will reflect and emit long wave radiation back to the surface. The overnight temperatures are warmer under cloudy skies.
When skies are clear the long wave radiation is better able to escape into space. Temperatures tend to cool during the overnight hours with the low temperature occurring around sunrise.
There are months in which the nights are longer. When night is longer, there is more time for the long wave energy to escape. Therefore those nights are much cooler or colder.
Light winds tend to keep the cool air confined locally, preventing it from getting warm by mixing with surrounding air. When there is wind, the air molecules are constantly moving. Since temperature is a measurement of the speed of air molecules, a steady breeze or better yet a consistent wind will keep the temperature up.
When there is moisture in the air the water vapor keeps the heat on the ground from radiating away. The water vapor in the air will absorb and emit long wave radiation. Humid air can act like cloud cover when it comes to long wave energy trying to escape. It is ideal to have low dew points throughout the atmosphere. Therefore, dry air will cool off much faster than moist air. Fog will keep the temperature up, because the temperature is at or near the dew point. The dew point temperature is the temperature at which air is completely saturated or 100% relative humidity. So the air temperature cannot fall below the dew point. (Read my “Weather Talk”; “Our Friend the Dew Point” http://www.ktsm.com/weather/weather-talk/our-friend-”dew-point”) Even without fog, the evaporating moisture will not let as much long wave energy escape. If it has rained plants and vegetation emit enough moisture to keep the temperature up a bit.
There is not much evaporation into the air when soils are dry. Dry soil reduces the chance of fog developing. There is more cooling when it has not to have rained for the past several days.
Snow is a very efficient emitter and reflector of radiation. At night, especially when it is clear, snow cover will enhance the cooling that takes place. Fresh snow is better at this than old snow or snow that is not covering all the ground. So the temperatures fall faster and more over ground covered in freshly fallen snow.
Rural areas cool off more at night than urban areas. Urban concrete and buildings hold the warmth longer and do not cool down as fast as non-concrete ground with hardly any buildings. The so-called “urban heat island” does keep the cities warmer or hotter during the day and warmer than or not as cold as the surrounding rural areas at night.
So now we know why we have chillier than normal morning with clear skies, dry air and hardly any wind. I would like to see some moisture, rain help our low temperatures and our desert vegetation. We will see plenty of wind in a little over a month when our windy season kicks off. Until our late spring warm up, keep your jacket handy and enjoy the clear sky views of the stars, planets and the moon!