Today is forecast to be one of the hottest of the year here in the south of the UK. My iPhone will tell me pretty well exactly where I am, where the nearest bookshop is, what time the sun will set, and my Mac will tell me temperatures in every nook and cranny of its innards. Yet neither can tell me how hot it is outside just now. This article considers why not.

How hot is it?

Of course I’ve got apps and sites which will tell me the forecast outdoor temperatures through the day, but they’re only forecasts, and often don’t match what actually happens. Living on the coast, local weather is usually rather different, with cloud instead of promised clear skies, dense fog instead of bright sunshine, and cooling sea breezes when a few miles inland everyone is wilting in the heat.

It’s actually more complicated than that. Some days when it’s forecast to reach 23-25˚C (73-77˚F) it still feels quite pleasant and refreshing here, and other days what’s supposed to be 18˚C (64˚F) feels warm and muggy. This is because what we experience as warmth isn’t just the dry bulb air temperature of weather forecasts, but makes allowance for additional influences like sunshine, relative humidity, and windspeed.

There have been many attempts to roll all those measurements into a single figure, of which the most widely used for warmer temperatures is the WBGT. This is the weighted average of three different sensors: a dry bulb thermometer, one with a wet bulb (for effective evaporation, including both humidity and windspeed), and one inside a blackened globe (for radiant heatload). The latter two aren’t suitable for incorporating into a device as thin and compact as an iPhone, so unlike many of the sensors which can be miniaturised or simulated, they’re unlikely ever to appear in a smartphone.

There’s a great deal of experience in using the WBGT: they’re routinely measured in hot climates, and have been used extensively by US and British forces in Iraq and Afghanistan, for example. Most military training establishments use them during the warmer parts of the year to determine measures they use to prevent heat illness during strenuous activity. But if a TV weather forecaster were to start talking in terms of the forecast WBGT, few viewers would have a clue what they meant.

Forecasting WBGT is considerably more difficult than the regular weather too, because it’s heavily dependent on cloud cover. The sun doesn’t have to come out for long before the temperature of the thermometer inside the blackened globe rises substantially, and the WBGT climbs. This is also very localised: in deep country lanes, one side of the road can be in full sun all day, with a WBGT 10˚C higher than in the shade on the other side, a few feet away. If that doesn’t sound very realistic, try running a few miles on the sunny side, and you’ll quickly appreciate how faithful it is.

How cold is it?

If measuring how hot it is for humans isn’t practical, then why can’t our iPhones at least measure how cold it is? Well, that doesn’t turn out to be so simple either.

Dry bulb temperatures are better representative of cold conditions, but even here there are complications. We’re particularly interested in two different effects of the cold: on exposed flesh like fingers, which could go numb and even freeze, and our whole bodies, which might cool to the point of violent shivering and incipient hypothermia. As we know well, a dominant factor in both is windspeed, something which was investigated first by two American explorer-scientists, Siple and Passel, in the Antarctic just after the Second World War.

This led to the concept of windchill-equivalent temperature, which has been further developed by Steadman and others in Canada. But there’s no international consensus on an index equivalent to the WBGT, and the underlying physics shows that risk of local cold injury (such as frostbite) in fingers is quite separate from that of whole body cooling (and hypothermia).

Another major factor which has been more neglected is the effect of rainfall and other precipitation. If you’re outdoors when it rains and aren’t wearing a waterproof outer layer, displacement of insulating air from within your clothing by water greatly reduces its protection. Evaporation of water from soaked clothing is an even more potent means of cooling – after all, that’s what sweating does to keep us cool in the heat.

Keep it simple

For the heat, we have a fairly reliable and accurate index which can’t be measured using such compact devices as iPhones, and isn’t understood by the general public anyway. Turning to the cold, we’re still not sure what we should even be measuring, nor how best to express it.

What’s most surprising is that this hasn’t changed for several decades, during which finding our location has gone from map and compass to a GPS in every phone. We may not know whether it’s hot or cold, but at least we know where we are.