What Affects Overnight Low Temperatures?

For many of us, temperatures at night will soon, if not already, become a larger factor than normal during these summer months. Simply because it is warm enough to do things outside at night such as going for a swim, a walk along the beach, stargazing, camping, bonfires, concerts and many more. Well, here is a rundown of how low temperatures are forecasted, and some of the factors that contribute to just how low they get.


850 MB Temps

      One of the most important factors to look at is 850 millibar (mb) temperatures. This level in the atmosphere basically represents the top of the planetary boundary layer, the lowest layer of the troposphere where wind is affected by friction, and is the best level to use to determine thermal advection. Thermal advection, simply put, it the transport of temperature from one area to another, and is either categorized as Warm Air Advection (WAA) or Cold Air Advection (CAA). These names are fairly self-explanatory with WAA representing the horizontal transport of warmer air into relatively cooler air, and vice versa for CAA. 850 mb temperatures are also a good indication of the type of air mass over a certain area. When I talk about air mass, it is simply a large body of air with similar temperature and moisture properties, and are determined by their source region (Continental Polar, Maritime Tropical, etc.). Determining the type of air mass over an area gives meteorologists an idea of how warm or cold the air is over the area they are forecasting. After determining air mass, they must look to see if WAA or CAA is occurring, as WAA could force low temperatures to be higher than expected and vice versa for CAA.

Cloud Cover

      The amount of clouds over an area can have a pretty large effect on overnight low temperatures. To understand this effect, we have to understand the importance of clouds in our weather. Simply put, during the day “short-wave” radiation (sunlight) is absorbed by the earth’s, which then reradiates that energy as ‘long-wave” (terrestrial, from the earth) radiation. Long-wave radiation is much more efficiently absorbed by our atmosphere and clouds, and this is the basic premise to the greenhouse effect. At night, the earth’s surface isn’t receiving sunlight, and therefore is only emitting terrestrial radiation which is one reason temperatures usually fall. On a cloudless night, much more terrestrial radiation is allowed to escape the atmosphere, and as a result, overnight low temperatures fall to lower amounts than on an overcast night (ignoring all the other factors mentioned). On an overcast night, assuming the clouds are lower level clouds, terrestrial radiation is absorbed and reradiated back to the earth’s surface which hinders the temperature from falling lower than it normally would. Again, these examples of clouds affecting temperature are only taking into account cloud cover and not other factors mentioned in this article.

Wind

      From the above section on clouds we know that the earth’s surface cools off by radiating terrestrial radiation into the atmosphere. Well, winds can affect the efficiency of this cooling by mixing the air in the previously mentioned planetary boundary layer. This mixing could bring down warmer air from above the surface, which basically counteracts the surface cooling. Although the surface may still cool, it will not cool as much as it would with calm winds allowing the radiation to simply escape into space.

Geographic Location

      The last important factor to consider when forecasting low temperatures is the geographic location of the area you are forecasting for. Basically, is this location near a large body of water, such as an ocean, a large lake or a large river? To understand how water can affect low temperature forecasts, we need to understand a term named specific heat. Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius (~1.8 degrees Fahrenheit). The reason why specific heat is important is because the specific heat of water is very high compared to the earth’s surface and the larger the body of water the longer it takes to heat up (Ocean vs. Pond).  So simply put, it takes a lot more energy to heat water than it does the earth’s surface. Anyone that has a pool has probably experienced the phenomenon when on a hot day the concrete or brick surface around your pool gets very hot while the pool may only warm up by a few degrees all day. This is due to the difference in specific heat between water and the concrete/brick. Specific heat also works the opposite way, where water does not cool as quickly as the earth’s surface does. So, for a location near a body of water, the close proximity to the water doesn’t allow the air to cool as rapidly as if it were inland with no water around. This is due to the inland areas being able to cool much more proficiently due to being surrounded by soil, concrete, etc. and not water which usually ends up being warmer than the land at night. Simply put, inland areas experience large daily changes in temperatures, while shoreline temperatures are hampered by their proximity to large bodies of water. Another generalization that is usually true is that shoreline temperatures are usually cooler during the day and warmer at night.

Forecasting Low Temperatures

      All of these factors mentioned in this article are factors that meteorologists must examine when forecasting overnight low temperatures. Other factors could include precipitation (evaporative cooling), dewpoint, terrain, and more. To forecast low temperatures, many forecasters use numerical forecast models and their knowledge of the area they are forecasting for. A quick resource that forecasters can look at is the Model Output Statistics (MOS) data, which is a statistically derived surface weather condition forecast for a particular location. MOS data for the Global Forecasting System (GFS) and the North American Mesoscale Model (NAM), two of the common forecast models that meteorologists use, are readily available. To make a quick forecast of low temperatures, a forecaster and use the MOS as a reference, and then determine the conditions that they expect to occur such as wind, clouds, 850 temps, precipitation, dewpoint, terrain and more to make their final forecast.

-Aaron DeVivo

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