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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The Fourth Longest Sub-Twenty Cold Spell for Antigua and Barbuda

23 01 2017

Dale C. S. Destin |

Last night’s cold weather makes the current sub-twenty °C (sub 70 °F) cold spell the fourth longest on record at the Airport and most of the rest of Antigua and Barbuda. It tied with February and December of 1973.

With a mean minimum temperature of 18.6 °C (65.5 °F), it is also the second coldest sub-twenty cold spell for the country, on record, lasting more than three consecutive nights. The mean minimum temperature for the past four nights ranged between 11 and 22 °C (71.6 °F) with the vast majority of places experiencing sub-twenty temperatures.

jan2017coldspelltemps

The last time the Airport had a temperature below 18 C was in 2000 – 16 years ago.

We are also looking at the coldest four-night period for January since 1980 – over 35 years ago, and since 2000 for all other months, at least, at the Airport.

Of the eight times we have seen this spell lasting more than three days, it has gone for four days twice and five days thrice, based on data for the Airport.

The record five-day sub-twenty cold spell is shared by March 1997, January 1984 and March 1972. The coldest one is March 1972 with a mean minimum temperature of 18.5 °C (65.3 °F).

So far for January, the mean minimum temperature at the Airport is now 21.9 °C (71.4 °F) – below normal. The mean daily temperature is well below normal with a value of 24.6 °C (76.3 °F).

There is now about a 50/50 chance the record will be tied tonight, as conditions could favour sub-twenty temperatures once again. If it were to happen, this cold spell would likely become the longest as the chances of sub-twenty temperatures are high for Tuesday and Wednesday nights. Usual January temperatures are expected after Wednesday.

The statements above are truest for the Airport and surrounding areas; however, from a qualitative assessment, it is applicable to the rest of the country.

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A Hat-trick of Sub-Twenty Temperatures for most of Antigua and Barbuda

22 01 2017

Dale C. S. Destin |

Last night’s cold weather makes it a hat-trick of sub-twenty temperatures for most of Antigua and Barbuda. This is a fairly rare feat for the country. It has only happened seven other times during January, based on historical data for the Airport. The last time it happened in January was back in 1996 – 20 years ago. The last time it happened for any month was in March 2000.jan2017temperatureWhereas the last hat-trick of sub-twenty temperatures occurred last in January 1996, the coolest such period last took place in 1992. It is also the coolest such period, for all months, last occurred in March 2000. This and the rest of what is said here is an update on the previous blog.

The mean minimum temperature for the past three nights, at the Airport, was 18.5 °C (65.3 °F). This is the eighth coldest for three or more days in a row with sub-twenty temperatures at the Airport. Further, it is the 12 coldest for any three-day sub-twenty spell (overlapping and otherwise).

When we consider such a three-peat of sub-twenty temperatures for all months dating back to 1971, it has only happened 27 previous times.

So far for January, the mean minimum temperature at the Airport of 22.0 °C (72 °F) is below normal. However, up to three days ago, it was bordering on above normal – meaning we were having relatively warm nights for this time of the year. The mean daily temperature is well below normal with a value of 24.7 °C (76.5 °F).

As cold as it has been, it certainly has NOT nearly been cold enough to freeze water. Thus, that picture being circulated suggesting that the cold weather caused a small body of water to freeze in Free Town is a HOAX. For this to happen, we would need to have sub-zero temperatures persisting for days, which will NEVER happen.

There have only being five occasions when sub-twenty degree nights have occurred for more than three consecutive nights. Tonight is likely to be the sixth time this has happened. So far, today has been coldest of the past three days.

After tonight, the weather will warmup to usual temperatures for this time of the year. Then the cold weather will more likely than not return on Wednesday and continue on Thursday. Thereafter, the usual temperatures for this time of the year is expected to prevail for the rest of the month.

Although we are unable to say definitively how cold the country or specific areas have been due scarcity of historical temperature data, it is likely the coldest since 1996. This is based on fact that temperatures across a small homogeneous area like Antigua and Barbuda are highly correlated. And since it is the coldest for the Airport since 1992, it should be likewise for the rest of the islands.

From a quantitative standpoint, the statements above are truest for the Airport and surrounding areas; however, from a qualitative assessment, it is applicable to the rest of the country.

Follow us for all you need to know about this mini-cold spell we are experiencing. We can be followed on twitterfacebookinstagramtumblrflickrgoogle+, and youtube for education and information on all things weather and climate.








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