Measuring Relative Humidity and Temperature
Temperature, lighting and humidity are critical environmental factors that profoundly affect every aspect of a tortoise’s life. If we get any of these wrong it is likely to lead to serious consequences for health and survival.
In a recent ‘Myth-Busters’ series we showed how popular conceptions of ‘humid’ microclimates in semi-arid and arid-habitat zones are seriously flawed. From many comments received, however, it is clear that there are a lot of misunderstandings as to what ‘humidity’ actually is. In this short article we will try to provide a simple, but accurate, explanation of what is really meant by ‘humidity’.
At it’s simplest, humidity is ‘the amount of water vapour in the air’.
Nothing is ever quite that simple, however, and in practice humidity is affected by other factors such as the temperature of the air involved, and also by pressure. Of these two factors the temperature of the air involved has by far the most immediate and dramatic effect. In practical terms, any influence by pressure is minimal and unless you are trying to compute ultra-accurate data sets, can be ignored for most common purposes.
There are several ways to measure and describe humidity.
Relative humidity (RH) is usually expressed as a percentage, it measures water vapour relative to the temperature of the air. In other words, it is a measure of the actual amount of water vapour in the air compared to the total amount of vapour that can exist in the air at its current temperature. Warm air can hold more water vapour (moisture) than cold air, so with the same amount of absolute or specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer.
The dew point is the temperature the air needs to be cooled to (at a constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold more water in gaseous form and precipitation, either in the forms of rain, mists or fog will occur. This is why we see ground mists very early in the morning, just before the heat of the rising sun warms the air again, thereby allowing it to retain more water vapour once more.
Typical day-night cycles of temperature and relative humidity measured next to tortoises in their natural habitat in Southern Spain. Note how temperature and RH are INVERSELY related, and as temperatures INCREASE Relative Humidity DECREASES.
Absolute humidity is expressed as either the mass of water vapour per volume of moist air (in grams per cubic meter) or as the mass of water vapour per mass of dry air (usually in grams per kilogram). This is actually a very useful measurement as it provides a good objective baseline that illustrates just how much water vapour is present in a given quantity of air.
Amount of water vapour expressed in g/sq. m. for a constant 40% RH at differing temperatures.
There is one other term you may encounter: Specific humidity, which is the ratio of water vapour mass to the total moist air parcel mass.
Of these terms, the one reptile keepers will encounter by far the most frequently is ‘Relative Humidity’ and unfortunately this is also by far the easiest to misinterpret and misunderstand.
You will often see people ask “what is the correct humidity for a Russian tortoise (insert any other species of your choice here)”. Invariably all of the answers will cite a figure as a percentage based upon relative humidity. The immediate problem is that without a concurrent temperature being given, every single answer is completely meaningless. That is because, as we have seen, relative humidity does have to ‘relate’ to something: temperature! So simply referring to 30%, 60% or 80% ‘humidity’ without reference to the prevailing temperature means absolutely nothing.
The best way to determine what actual humidity ranges a species needs is to take accurate measurements IN THE NATURAL HABITAT. To do this you need both RH and simultaneous temperature readings.
Also, as we have discussed previously, you cannot rely upon general 'climate charts' of an area to fully understand what tortoises - at ground level - experience. The reasons for this are detailed in another article: 'Can you rely on general climate information to know what your tortoise needs?'
The only meaningful answers can be those that specify, say, “35% relative humidity at 32 degrees Celsius” or some other combination. Merely citing relative humidity alone can be incredibly misleading as there is a vast practical difference between say, 50% RH at 10 degrees Celsius and 50% RH at 32 degrees Celsius.
How much difference? The best way to establish that is to refer instead to absolute humidity, which we can easily calculate if we know a) Relative Humidity and b) Temperature.
So, in this case taking 50% RH at 10 Celsius = 3.92 g/kg and 50% relative humidity at 32 Celsius = 15.44 g/kg
Which in practice means that there is almost four times as much moisture vapour in the air at 32C vs 10C for the ‘same’ RH reading of 50%. That’s a very big difference. You would certainly ‘feel’ that difference yourself if in a room with these two different temperatures.
We know, for example, that temperatures outdoors fall sharply in the early hours of the morning, typically reaching their minimum just before dawn. Simultaneously, the relative air humidity INCREASES inversely to this. This is where people get confused. We might, for example, record outdoor air temperatures of 7 Celsius combined with 68% RH. That sounds like the air is quite ‘humid’ possibly? Well, not really, because again we are referring to RELATIVE humidity here. Relative to the temperature.
So, in this case we find that those figures actually mean that there is only 4.42 g/kg of water vapour present. Compare that to 68% RH with an air temperature of 26 Celsius = 14.45 g/kg. Now we have 3.26 times the amount of water vapour in the same quantity of air. The latter will certainly ‘feel’ subjectively humid to a human in terms of sweating and impact upon breathing. The former (cold air) measurement will not. In some rainforest habitats the RH can be 90% or more at 30 degrees Celsius. That means a lot of water vapour in any given volume of air, in this case 24.54 g/kg. That will feel subjectively ‘sauna-like’ and quite unpleasant.
Typical semi-arid Testudo habitat. Relative humidity measured here will normally be very low during the day (often 20-30%), but rises to around 60% just before dawn. However, unless we take temperatures into account we have no idea how much WATER VAPOUR IN THE AIR ('real' humidity) that those percentages actually equate to...
Conversely, in a true desert, we often encounter air temperatures of 38 Celsius and RH readings in single figures. Indeed, the lowest ever recorded was 0.3%. Typically, they are likely to be 5% or less. So taking 5% RH @ 38 Celsius = 2.25 g/kg of water vapour present. Subjectively, that will feel very ‘dry’ indeed.
The whole pivot of this confusion with relative humidity is that people just look at the ‘headline percentage’ while ignoring the one absolutely critical thing that this is relative to: temperature. This results not only in misunderstandings and confusion, but also it can directly lead to animals being subjected to environmental conditions far removed from those that are suitable and appropriate. In some cases, positively harmful conditions can be imposed as a result.
That is why it is essential to make an effort to really understand the terminology being used, and to try to grasp the physical, scientific laws that regulate these processes.