Has it ever happened to you that you step out of your house and feel that it's in fact much colder than you thought? Despite the fact that you carefully check outdoor temperature beforehand you may be unpleasantly surprised by the real temperature. Why does it happen and what causes the difference between the measured temperature and the 'true' one? Let's find it out.
The temperature that we usually see when we check it on a website is the temperature of the air and it's not the same as the temperature as we experience it. The 'feels like' temperature takes into account the wind speed and the humidity and therefore is more informative for us. For example, a strong wind in winter can make you feel much colder than the measured temperature indicates. On a humid summer day, the situation is just the opposite - we feel that it's hotter than it actually is. So how to calculate the 'feels like' temperature?
When we're calculating this temperature we take into account several major factors - air temperature, relative humidity and the strength of the wind (measured at around 5 feet - the average height of a person's face).
The heat index or humidex reflects the interrelation of air temperature and relative humidity. When the level of humidity is too high your body's perspiration becomes less efficient and you typically feel the temperature is warmer than it actually is. When the relative humidity is low the situation is just the opposite - perspiration becomes more effective and you feel it's cooler than it is. It's important that the calculation of the heat index is possible only if the relative humidity is 40% or higher and the measured temperature is above 80°F (26.6°C).
When the day is cold even a light wind easily transfers heat from your exposed skin and makes you feel cooler. Until 2001 the calculations of wind-chill were based on the rates at which water froze when the speed of the wind increased but these calculations weren't accurate enough and showed the temperature colder than the real one. Since 2001 the wind-chill calculations are based on the heat transfer of a person's skin exposed to the wind. This index is calculated when the temperature is 50°F (10°C) or lower and when the wind speed is above 3.3 feet (1.33 meters) per second.
Wet-Bulb Globe Temperature
This index is the most complicated one and is therefore mostly used by athletes and the military only. It displays the combination of the effects of air temperature, humidity, wind, and radiation and how all these factors influence a person's perception of temperature. It's rather location-specific and seldom taken into account.
When all these factors are taken into consideration one can get a special formula that allows calculating the temperature as you really feel it when you go outside. Don't forget that your personal temperature perception can be different, too! Keep it in mind and plan your day outdoors wisely!