Geography 101: rain, temperature, humidity and weather.

Hi there, I'd like to do a completely different kind of post today - I would like to deal with a geography question: does rain always make the temperature fall? This question has been prompted by the memory of a conversation I had with my father years ago in Singapore. I hate the hot weather in Singapore as the tropical heat would make me start sweating the moment I venture outdoors. However, on the day in question, just as I was about to leave the house, there was a downpour. As I scrambled around to try to find an umbrella, my father made a comment that I should be happy that it is raining. I asked him why he would make that assumption - he said it was because I hated the very hot weather and thus the rain would make the temperature drop, making it cooler and more comfortable for me. I didn't agree with him but as I was frantically searching for the umbrella and running late already, I didn't think that was the right time and place to challenge his assumption but I will do it here today as a blog post. So, is it wrong to assume that rain will make the temperature fall by a few degrees in a tropical country like Singapore and so what happens during a tropical downpour? 

It is not an unreasonable assumption actually - there are several ways that rain can make the temperature fall. Firstly, you need to have ample rain clouds in order to have rain in the first place - those clouds will block out the sun's rays, preventing them from reaching the surface of the ground. Secondly, the moment the rain starts to fall, some of it will start to evaporate - that can happen either whilst it is falling from the sky or after it has hit the ground. The process of evaporation thus takes energy from the air and the ground around it, thus effectively bringing the temperature down. Then there is the factor of the wind, which may be associated with some kinds of rain events like a thunderstorm. Wind naturally has a cooling effect, that is why we talk about the wind chill effect in colder countries and how simply standing in front of a fan can bring much needed relief on those hot, tropical days in Singapore. So if the rain is accompanied by some wind, then it can all come together to make it feel cooler. There is another reason why rains could lower the temperature, but this wouldn't apply in a place like Singapore but it would somewhere like London. If the winds are coming from the south in the summer, it would be dragging up much warmer air from North Africa and continental Europe, leading to hot and dry conditions. But if the wind swings round and comes from the West, then it is dragging in air that is a lot more moist and cool from the Atlantic ocean and that change would bring with it not just rain, but also a significant drop in temperature by up to ten degrees. However in Singapore, being so close to the Equator, it doesn't matter what direction the wind is coming from, the air will still be very hot and being an island surrounded by sea, there isn't a particular direction that would bring drier or wetter conditions. Thus that last reason relating to wind direction doesn't relate to the situation in Singapore, but nonetheless, the other factors are applicable to the context of Singapore. So would it be correct to assume that the temperature in Singapore would drop by a few degrees the moment it starts raining? Not actually, there are a few reasons why this may not be the case. Oh this is turning out to be a physical geography assignment but do indulge me - I'm a physical geography geek and really enjoying talking about the weather. 

Firstly, Singapore gets approximately 12 hours of sunlight a day and this does not vary with the seasons given that  Singapore is so close to the equator. So if the sun has been shining on Singapore for seven or eight hours already before it becomes cloudy, then the ground is already very hot, the temperature would have certainly soared above 30 degrees to around 32 or 33 degrees and that little bit of cloudy weather in the afternoon isn't going to trigger a massive drop of temperature - it would probably stay around the same and any drop would be insignificant. However, if the cloudy conditions occurred in the early morning to coincide with sunrise (which is also the coolest part of the day, usually at around 25  or 26 degrees), then the cloudy conditions can have a much greater impact to prevent the temperatures from soaring into the 30s that day. Theoretically speaking, if there was a rainy day where the cloudy conditions persisted from dawn to dusk, accompanied by constant rain, the temperature that day would not exceed 30 degrees and probably peak at about 28 or 29 degrees. But still, we're moving within quite a small range of a few degrees, ie. 29 instead of 32 degrees, that's a different of 3 degrees. Nonetheless the timing of the cloudy conditions and the associated rain is important, as it can make a big difference whether or not it will have any impact on the temperature. If evaporation is going to play a factor in bringing down the temperature, then you need to consider the relative humidity - this refers to just how much water vapour there is in the air already and how close to saturation (dew point) it is at that temperature. The hotter the air, the more water vapour it can hold. Singapore is a tropical island, so the relative humidity is always very high and it can vary. On average, the relative humidity in Singapore is around 83.9%, it is typically around 90% early in the morning at sunrise but as the temperature soars in the day, it can drop to as low as 60% on dry days but it can easily hit 100% during prolong periods of heavy rain. So if you're counting on evaporation to cause the temperature to fall during a downpour, then that is not going to happen because the relative humidity is already near or at 100%, so very little evaporation can take place under such conditions. That is why your wet laundry will dry a lot faster on sunny days than rainy days, it isn't the temperature per se but the relative humidity. Thus evaporation can never be a significant factor in triggering a significant fall in temperature when it rains in Singapore as the relative humidity is always very high. 

Okay but what if we are experiencing a very hot afternoon which triggers a sudden thunderstorm, so the relative humidity is down around 60%, surely those kind of conditions would cause the temperature to fall due to evaporation then? Well, not necessarily or at least, it's not that straightforward. Let's say the air temperature is already 33 degrees, the concrete surfaces, pavements and the roads would be a lot hotter than that as they would have absorbed a lot of the heat over the course of the day, it can lead to some exposed surfaces reaching temperatures of 50 to 60 degrees. A lot of the rain would evaporate very quickly the moment it splashes onto such surfaces in urban areas, but that would instantly raise the relative humidity, creating a mini-sauna effect where the sudden increase in humidity would be akin to the feeling of just stepping into a sauna. It is a stifling, suffocating, uncomfortable sensation, quite the opposite of cooling or pleasant! Furthermore, the heat exchange between the air and the rain is a dynamic process and what this means is that whilst the rain falling can cool the air down as the raindrops are originating from clouds very high in the sky where it is cooler, the rain that you feel on your skin by the time it hits the ground in Singapore isn't cold at all. It is either at or very close to the air temperature, it is not 'cold water'. Thus the air in Singapore is already so hot that it will always effectively heat up all the raindrops, so by the time they reach the ground, they are not really that effective in cooling down the surface as that water is practically at air temperature. So imagine it is one of those hot days, it is 34 degrees and you want to cool down, would you take a cold shower or a hot bath? Obviously, you would choose the cold shower because the hot bath would make you sweat even more. Thus the rain that falls in Singapore really isn't cold enough to have that much of an effect when it comes to bringing down the temperature. We rely on sweating to cool us down and when the relative humidity is lower at around 60% to 70%, that sweat can evaporate much more efficiently than if the relative humidity is much higher at 95% to 100%. When it is that humid, the body reacts by producing even more sweat but that sweat cannot evaporate fast enough and thus a period of rain can actually make it a lot more uncomfortable for people in Singapore as a result of this process. We will feel cooler when our sweat can evaporate and we would feel a lot more uncomfortable when it can't.

How much can a downpour lower the temperature by? Not much at all and the effect doesn't last for long. We're so used to simply taking out our phones and looking at our weather apps to see what the weather forecast is, but imagine if we conducted this experiment over the course of a week in Singapore and we had 100 participants and we would stop them at random times of the day and night and ask them, "what do you think the temperature is right now?" They would have to provide an answer down to one decimal place like, "30.2 degrees Celsius." There will be two obvious trends that will emerge from this experiment, firstly the participants would rely far more on their knowledge and experience of living in Singapore to provide their answers rather than actually rely on how they feel. "It is still early in the morning, so it is probably not that hot yet - something like 27.5?" The second trend would be that you're going to get a range of answers as us humans are not that good at perceiving temperature. The participants will guess the hottest and coolest parts of a day a lot more accurately, but that's mostly down to their knowledge of the climate in Singapore - the hottest part of the day would be around the 33 degree mark whilst the coolest part of the day would be about 26 degrees but it is the part in between the two extremes that the participants would struggle with. Obviously, 25 degrees will feel very different from 35 degrees (both of which are possible in Singapore), but would a thunderstorm trigger such a huge drop in temperature? No, it wouldn't. Looking at the weather data for Singapore over the course of writing this piece, I have looked at what happened to the temperature in Singapore when it does rain and the conclusion is that rain can sometimes bring the temperature down but often, it is never more than just one degree and if the rain stops, then it goes back to what it was before. So for example, the temperature at 12 noon is 33 degrees, we get a short spell of rain and by 1 pm, it falls to 32 degrees, then the rain stops. By 2 pm, it then would go right back to 33 degrees. Thus the effect is at best temporary and very small, it would barely be perceptible by those in the experiment. Let's contrast this to the much bigger contrast experienced by someone in Singapore walking in from the street outside to an air-conditioned room, the temperature outside is 34 degrees whilst the room is 24 degrees. That is a huge fall of ten degrees the moment the person steps through the door into the air-conditioned environment and we can all relate to that instant relief we would experience at that moment: this drop in temperature really needs to be significant enough to provide any respite from the tropical heat. 

Of course, the one other major reason why we enjoy air-conditioning so much is because of the much lower relative humidity it provides: this varies from as low as 30% to around 50% depending on the individual areas and how efficient your air-conditioning is, but remember once the relative humidity is much lower, our sweating mechanism to cool our bodies down can work a lot more efficiently. Hence it is this combination of a lower temperature and a much lower relative humidity that provides that vital relief we have come to expect from air-conditioning. So if we were to compare the two experiences: the first being a thunderstorm in the middle of the day in Singapore and the second being walking from the street outside into an air-conditioned room. The first one would bring with it a drop in temperature of about 1 to 2 degrees (best case scenario) but a rise in relative humidity to nearly 100%. But the second would bring with it a drop of temperature of 10 or even 12 degrees (oh some people in Singapore really like their air-con) and a corresponding drop of  relative humidity to as low as 50,% even 30%. Thus if the subjects are used to the kind of relief and comfort that air-conditioning can bring, then the relatively small drop in temperature during a thunderstorm would barely be perceptible, especially when you then consider the corresponding rise in relative humidity cancelling out any comfort that drop in temperature may bring. If we go back to our experiment, I wouldn't assume that the subjects would experience any kind of relief or comfort when it starts raining heavily - we are merely replacing one kind of discomfort for another. When it is 34 degrees and very sunny, it is uncomfortable but at least the relative humidity would be lower. But when it is 32 degrees and pouring with rain, sure it is two degrees cooler but then it would be a lot more humid - it's just a different kind of tropical weather and quite frankly neither are pleasant, especially when we compare it to the comfort that we can achieve with air-conditioning. So perhaps 40 years ago before Singaporeans became so reliant and addicted to air-conditioning, then the only way they could hope for any kind of relief from the intense, unrelenting tropical heat was if mother nature organized  huge thunderstorm and I can imagine my father as a young man standing in the heavy rain to get soaked, as a desperate way to ty to cool himself down before the days of air-conditioning. I suppose if you did that, of course you would feel much cooler but then how many people would resort to that in the middle of the day in Singapore? 

Can huge drops in temperatures occur during a short period of say 24 hours? Yes, the world record for the biggest drop in temperature in that period was a drop of 56 degrees Celsius from +7 degrees to -49 degrees in Browning, Montana USA in 1916. But of course, something like that can never happen in a country like Singapore. The kind of weather you would experience in Singapore is already determined by Singapore's geography and climate. Singapore does not experience a very wide range of temperatures, given that it mostly stays within the range of 26 to 33 degrees most of the time, that's a tiny range of just about 7 degrees. The hottest temperature recorded is only 37 degrees, which ironically is even lower than the 40 degrees I have experienced in London and the lowest temperature recorded is 19 degrees, which may seem rather low but that is extremely rare as anything in the low 20s in Singapore is already considered cool relative to what the normal temperature is. Thus might it be possible that if locals in Singapore are so used to weather that they can immediately detect any small change in temperatures but of course that might have been the case way back in 1970s before we all got so used to air-conditioning in Singapore but that's certainly not the case today. In any case, I am really the wrong person to pick up any small drop in temperature in Singapore - I have spent more of my life in the UK than in Singapore, sometimes I would go to my garden in just my T-shirt and shorts and spend half an hour there despite it being around 7 or 8 degrees Celsius so anything above 18 degrees feels hot to me and I start feeling discomfort when it is 21 degrees. In fact, I struggle to sleep at night if the temperature doesn't fall below 15 degrees. So whether it is 28, 32 or 34 degrees, it still feels ridiculously hot and uncomfortable to me. Is it possible for a local Singaporean to experience some relief when there is a small drop of temperature of 1 or 2 degrees when it rains? Possibly, but then you have to bear in mind that this relief is nothing compared to what is offered by air-conditioning. So imagine if we were running a marathon in Singapore and there are water stations on the course, some of the water stations offer very big cups of water, whilst others offer tiny cups of water which are barely a mouthful. When a runner drinks a tiny cup of water and we assume that the runners have to keep on running rather than stop there to drink several tiny cups of water, will that one tiny cup of water  satisfy their thirst if they have gotten used to drinking a much bigger cup of water? No, it might provide a little bit of relief, but that runner is still going to be thirsty after that tiny cup of water. So even if you could detect that small drop in temperature when it does start raining, you're still going to turn on the air-con. 

How we perceive temperature varies from person to person. Allow me to use the example of drinking soup - some people prefer to let their soup cool down before drinking as they are not used to putting a very hot spoonful of liquid into their mouth, but I actually get annoyed the moment my soup cools down too much and I'd pop that bowl of soup back into the microwave to get it to the desired hot temperature so I can actually enjoy my soup really hot. Autumn is now approaching in the UK, so on some days, it would be like 17 degrees, I'd still be walking around in shorts and T-shirt as if it was still summer whilst the person next to me would be wearing a winter jacket. We'd both be looking at each other, thinking the other party has dressed inappropriately for the weather when really, both parties are feeling perfectly comfortable the way they are dressed that day. Likewise, different people will set the temperatures of their showers at different levels and there isn't a correct or perfect temperature for the water in your shower, there is only the temperature that you prefer. Another example is when I went swimming this summer in the ponds at Hampstead Heath in London, the water temperature was 23 degrees which was very respectable for that time of the year. I thought the water was perfectly fine but some people couldn't even stay in the water for more than a few minutes, complaining that it was way too cold. Hence on two people can step out of Changi Airport in Singapore and one may say, "oh the weather here is amazing, I can't wait to go to the beach, this is a tropical paradise!" Whilst the other person would say, "it's too hot, I can't bear to leave the air-conditioned airport and step outside, I know I am going to start sweating immediately in this horrible heat." How we experience heat and weather depends on so many factors like our bodies, our health, our age, our experiences, how we are dressed, how prepared we are for the weather and so the only safe thing to assume is that you should never assume that the person next to you would experience the weather or temperature in the same way as you do. Now that I explained it to you like that, you'd say, yes of course and that was why I was irked that my father had made the assumption that the rain in Singapore would have provided sufficient relief for me from the tropical heat. I can believe that he might have experienced some relief when it started raining, but to assume that I would feel the same way was wrong. 

So there you go, that's it from me on this topic, what do you think? What would bring you relief from the heat when you are in a tropical country? Have we become so accustomed to air-conditioning that nothing short of that would bring us relief from the heat? Are some people more able than others to detect even very small changes in temperature? Do you enjoy those really heavy tropical downpours in Singapore? Please leave a comment below and many thanks for reading.