Why Was It So Cold Night Before Last?

19 01 2019

Night before last was the coldest for Antigua and Barbuda for the year and the coldest for some areas in years. Some areas had temperatures as low as 15 °C (59 °F). Why was it so cold, relatively speaking, and is this unusual?

Min Temp for Antigua and Barbuda
Temperatures recorded January 17/18, 2019

The short answer to the question is radiational/radiative cooling. The long answer involves explaining what such cooling is and how it works. Radiational cooling is the process by which the ground and the adjacent air cool by emitting heat (infrared – IR energy).

The relationship between dew point (atmospheric moisture) and min temperature.
On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate (heat) infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.) Graphic courtesy Meteorology Today.

As we all know, as the sun goes down, the heat from it decreases. Consequently, at some point late in any given day, the ground and the air near it lose more heat that it receives.

The ground being denser than air cools more quickly than the air above it. Hence, after the sun goes down, the ground is cooler than the air directly above it.

For any two objects in contact with each other, heat will flow from the warmer to the cooler. Similarly, the warmer air above gives up heat to the ground, which the ground quickly emits away.

As the night progresses, the ground and the air near it continue to cool more rapidly. Air is a poor conductor (transferrer) of heat. As a result, it takes a while for the air to reach it coolest. However, this is normally reached just before dawn, in the Antigua and Caribbean context.

Now, radiational cooling happens 365 nights a year, so what was different last night? The main difference was the fact that the winds were calm, and the skies were clear – the main ingredients.

Additionally, we are in the Northern Hemisphere’s winter – shorter days, as the sun is away over the Southern Hemisphere; consequently, the coldest time of the year. So, we had almost the perfect recipe for radiational cooling to be at its optimum; thus, our colder than normal weather.

If you live at the bottom of a valley, you may have felt colder than most. This is because cold dense air, which originates from the cold hill tops, slowly flows down the hill slopes and settled in the valley – making for colder weather than non-valley areas.

If you live on a hill top you would have been coldest, as the higher you go, the cold it gets generally – 1 °C (1.8 °F) for every 100 metres (330 ft) you go up. Radiational cooling has a greater impact on hill tops that elsewhere.

There is nothing unusual with us having such cold night. No records were broken, which is indicative of the fact that we have had colder temperatures. Actually, as our climate warms like almost every other place on earth, these “extremes” temperature are occurring less frequently.

When I was a child, I recall that it was very common for my siblings and I to see our breaths in the mornings, at this time of the year – due the cold temperatures. We used to make the mirrors frosty with our breaths and then write stuff on them. That has become a rarity, at least for me – anecdotally indicating that our climate is warming and the reduced frequency of such low temperatures.

Radiational cooling operates at it maximum under clear skies, dry air, calm winds and long nights, which are synonymous with the winter months – December to February. So, we are in the period when radiational cooling operates at it best. The winds have the effect of disrupting the cooling; consequently, with the winds not likely to returning to calm over the next several days, a repeat of night before last is unlikely – fortunately or unfortunately.

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