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Welcome to Sigyn’s Guide On Understanding All Of the Temperatures
It’s currently summer in the northern hemisphere and it is, in many parts of the world, an extremely hot one. You’re going to see a lot of different temperatures thrown around, and terminology that can be confusing. The most recent example of this is a post I’ve seen about the extreme heat events currently happening in India - that post specifically noted that some parts of India are experiencing “wet bulb” temperatures of 95F and higher, which can be deadly. If you don’t know what a wet bulb temperature is and what that means, it’s easy to not fully understand this - obviously 95F is very hot outside, hot enough to get heat stroke or exhaustion, but not necessarily what we would consider to be inherently deadly. That’s why the “wet bulb” part of that is crucial to understand.
Temperature is an absolute, objective number. Technically it’s a measure of energy but let’s not worry about that for now. What matters is that temperature is the absolute temperature of the air - the thing you will actually see on the thermometer.
Humidity is, broadly, how much of the air is water. However, there’s a complication. The amount of water air is able to hold as “vapor” increases as the air’s temperature rises. Colder air can hold less water, hotter air can hold more water. For this reason, are actually two different measures of humidity: absolute humidity and relative humidity.
Absolute humidity measures, simply, how much of the air is water by volume.
Relative humidity measures how close the air is to its maximum capacity for holding water at its current temperature.
Absolute humidity is not measured in percentages, but for clarity, I’m going to fake some numbers using percentages, so this is easier to understand.
Let’s imagine the air is 10% water (irl the maximum is around 4% but we want easy math, come with me here). If it’s 40F outside, the maximum amount of water the air can hold is much lower. 10% water in the air might be a relative humidity of 100%. Now let’s imagine it’s 80F. That hot air can hold far more water. Now that same 10% water in the air is only a relative humidity of 50%.
The humidity number in your weather app or your phone is almost always showing you relative humidity.
This is also why higher humidity in hotter weather feels worse - the same humidity percentage on your weather app at 80F is objectively wetter air than at 40F.
The dew point is the temperature at which the current absolute humidity would be 100% relative humidity. Remember, hotter air can hold more water. Therefore the dew point (or frost point) can never be higher than the absolute temperature - it is always equal or lower. As the temperature goes down, so does the air’s capacity for holding vaporized water. The dew point tells you, if the absolute humidity - the amount of water in the air objectively by volume - didn’t change at all, how cold would the air have to get before it can’t hold the water anymore.
When the temperature lowers faster than the humidity, and the air gets colder than the temperature which could hold the amount of absolute water in the air, the water escapes the air, as frost, dew, or fog (or rain but then things get complicated).
It’s often said that higher dew points are correlated with it feeling more unpleasant outside, and this is true, but dew point is really just another way of expressing relative humidity.
The heat index or wind chill is a subjective number, derived from a calculation of heat and humidity. It represents what the current temperature feels like to a human being. In hot weather, the heat index will almost always be higher than the current temperature, as sun and humidity make it feel hotter. In cold weather, the wind chill is almost always a lower temperature, as the wind makes it feel colder.
While heat index is calculated mathematically, it is not an objective scientific measurement - it is an approximation of what the current temperature “feels like.” If you’re planning to be outdoors, it’s really more important than the objective absolute temperature, because it’s a better predictor of how miserable you will or will not be.
Ah, my good friend realfeel.
Realfeel is a term trademarked by the weather forecasting company AccuWeather. It is another subjective, perception based but calculated number. It is, effectively, the heat index. However, AccuWeather has their own algorithm for calculating the so called “realfeel,” which takes into account far more factors than a standard heat index, including the angle of the sun, UV index, atmospheric pressure, cloud cover, etc etc. As someone who spends a lot of time looking at the weather, I find the “RealFeel” is always higher than the calculated heat index and, frankly, often feels more true to the horrors I am experiencing outside in Florida in July.
Wet Bulb temperature is an objective, scientifically measurable temperature.
The wet bulb temperature is a measure of the productiveness of evaporative cooling. Evaporation is how your body cools itself - the process of evaporation creates cooling, for Physics Reasons, and this is why we sweat. Our sweat evaporates, and this in turn lowers our body temperature. The wet bulb temperature is, literally, the temperature that reads on a thermometer wrapped in a wet cloth.
Remember, as we discussed earlier, air has a maximum potential for holding evaporated water. Once it reaches this potential, it can’t hold any more water vapor, and evaporation stops happening. The less evaporation can happen, the less cooling can happen.
This means the wet bulb temperature approaches the absolute temperature as the relative humidity rises, much like the dew point approaches the absolute temperature as the relative humidity rises.
It is important to note that these are NOT the same number - while they both approach the absolute temperature as the humidity rises, the dew point is the temperature at which the air can hold no more water. The wet bulb is the temperature to which, because physics, evaporative cooling can lower a thing (person). The dew point goes up as the relative humidity rises because it measures at what temperature the air would be at 100% relative humidity. If the air IS at 100% relative humidity, they are the same. The wet bulb goes up as the relative humidity rises because it is a measure of evaporative cooling efficiency, and evaporative cooling requires air potential for evaporation.
Wet Bulb temperature is always LOWER than the absolute temperature, as wet bulb temperature is a measure of the effectiveness of cooling the higher the relative humidity = less evaporation = less cooling. Lower relative humidity = more evaporation = more cooling.
When and why wet bulb temperatures become deadly
The wet bulb temperature becomes deadly when it rises above the safe temperature of the human body - actually a couple degrees below. This is because it’s a measure of potential cooling. If the wet bulb temperature is 80F, that’s very unpleasant, but it is not inherently deadly, because it means broadly that your body’s sweating and evaporative cooling can hypothetically cool you to 80F, which is fine. When the wet bulb temperature rises to and above the safe core temperature of the human body - hypothetically 98F - without some sort of artificial cooling, you overheat and die. This is because a wet bulb temperature of for extreme example 100F means that your body’s evaporative cooling system - sweating - can never, on its own, cool you down below 100F. Because this is higher than the safe core temperature of the human body, your body loses the ability to cool itself down - your sweat evaporating cannot cool you below a fever. This causes the body’s temp to continuously rise, which can be deadly over a period of time.
In reality, while the core temp of the human body is “98.6,” wet bulb temps become potentially deadly a few degrees below that - 94 or 95F. Once the “wet bulb” hits that point, your body can no longer cool itself enough to prevent a runaway rise in core temperature.
This is why the wet bulb temperatures being 95+ in India is so deadly. It’s not that the temperature or the heat index is 95F - it’s that with the conditions as they are, a body uncooled in a wet bulb of 95F will experience runaway core temperature rise.
Temperature - Objective measure of heat
Absolute Humidity - the absolute volume of water in the air
Relative humidity - how much water is in the air compared to how much water air of this temperature can hold.
Dew point - the temperature of air that cannot hold any more water than the current absolute humidity.
Heat Index - Subjective measure of apparent heat as experienced by a human being. Calculated based on the temperature and relative humidity, and some other factors. Higher than the current temperature.
RealFeel - Fancier, AccuWeather trademarked heat index.
Wet Bulb Temperature - Objective measure of evaporative cooling potential. Lower than the current temperature.
This has been Sigyn’s fun with weather. Tune in next week for more of me infodumping about SOMETHING.
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