Sleep tight – what controls night-time temperatures? By Aidan McGivern
Many different factors influence the temperature at night. Inland temperatures tend to drop faster than coastal locations due to the moderating influence of the sea. Rural air tends to cool quicker than urban. Even the type of soil has an impact. Met Office Meteorologist Aidan Mcgivern describes the factors that determine how much sleep he, and his young daughter, might get.
The first weather prediction I made when I joined the Met Office more than ten years ago was for the minimum temperature in Exeter that night. During my training, I learnt how to construct a temperature curve that could predict the temperature at any point during the night. Taking into account the long November night, the clear skies and the gentle breeze, my temperature curve resembled a beautiful upside-down-S shape and I had full confidence in my prediction. The next morning, I discovered I was three degrees out. The temperature had fallen quicker than expected, my curve wasn’t steep enough.
Fast forward to April 2017 and I am just as concerned with temperature at night. But this time it’s because of my five-month old who, for everyone’s benefit, really could do with having a long and comfortable night’s sleep. The temperature in her room at bedtime is perfect, but the skies outside are clear and the winds are light. My thoughts return to my very first forecast and the temperature curve.
On a still, cloudless night, temperatures do not tend to fall at a constant rate. The air outside starts cooling slowly following the warmest time of day, typically mid-afternoon. Cooling then accelerates after sunset before slowing again as dawn approaches. The lowest temperature often occurs half an hour after daybreak, due to a time lag. This is the temperature curve, the upside-down-S shape.
Behind the curve
Many different factors will influence the shape of this temperature curve.
Of course, the weather overnight is not always clear and still. Any clouds in the sky will act as a blanket, stopping warmer air from escaping into the heavens. A breeze will have a similar effect, helping to mix cold air close to the ground with milder air elsewhere, preventing a steep temperature curve.
The weather, especially in the UK, is notoriously changeable. Overnight temperatures vary not just from season to season and from day to day but can also fluctuate dramatically within a single night.
These sudden changes can be brought by weather fronts. You may have seen these abstract colourful lines on weather maps. The blue lines with triangles – cold fronts – can clear the air and cause a steep drop in outside temperature. The red lines with semi-circles – warm fronts – can increase the humidity to uncomfortable levels, especially during summer.
Maintaining a comfortable temperature
With the temperature outside varying depending on location, cloud, wind, time of year and weather fronts, it is not surprising that it can be so difficult to maintain a constant temperature in your child’s bedroom.
Thanks to modern insulation, heating and air conditioning bedroom temperatures are unlikely to change through the night as much as outdoor temperatures. But it’s still useful to understand how abruptly the temperature outside can rise and fall and how this might impact your baby’s sleeping patterns.
By April 2017, I no longer needed my less than reliable temperature curve. Like many sleep-deprived parents, I was now an expert in the precise temperature variation in my little girl’s bedroom throughout the night – whatever the weather.