Hot, hot, hot! Why does the perceived temperature VS the meteorological temperature feel so different?

I believe many of my friends have had this experience or confusion: the weather forecast says that the highest temperature today will be 30℃, but why does it feel like 40℃ when I’m walking on the street?

Is it that my feelings deceived me, or...the weather station gave a wrong report?

In fact, to figure out this problem, we must first understand: where do the temperature in the weather forecast and "the temperature you feel" come from?

Part 1

The "temperature" of the meteorological department

Let’s first talk about how the temperature in the weather forecast is derived.

The "temperature" mentioned in meteorology refers to the temperature of the air flowing freely in the natural state. Under normal circumstances, it should be as little affected by human interference as possible, and as little affected by other factors in the natural environment except temperature as possible.

So, how do you minimize distractions?

To this end, the World Meteorological Organization has unified the standard, that is, the meteorological department's thermometer should be placed in a louvered box, 1.5 meters above the ground, and the louvered box should be placed on an open lawn.

I believe everyone has seen the white box in the picture below. Yes, this is the legendary "louvered box". The "temperature" used by the weather station refers to the air temperature measured by the thermometer inside it.

So, what makes it different?

Weather shutter box

First of all, the location of the shutter box is special.

The louvered box is generally located in the observation field of the meteorological station. According to my country's "Regulations on the Protection of Meteorological Facilities and Meteorological Detection Environment", the observation field must be open and flat. Generally, it is prohibited to build roads or plant plants over 1 meter in height within 30 meters of the observation field of the meteorological station. It is also prohibited to build buildings and structures within 1000 meters. These can better avoid the influence of human factors on the measurement of temperature and other factors in the observation field.

Secondly, the shape and structure of the shutter box are special.

The first special feature is the color. As can be seen from the picture, the louvered box is all white from head to toe. White is not easy to absorb heat, and most of the sunlight projected onto the louvered box will be reflected, reducing the interference of sunlight on the instruments in the box.

The second special feature is the shutters. The reason why it is called a shutter box is that it has shutters all around. According to my country's "Ground Meteorological Observation Specifications", there are two rows of shutter strips on the box wall, with an angle of about 45° to the horizontal plane, forming a "human" shape. This structure makes the shutter box have good ventilation performance and can also protect the instruments in the box from wind, sun and rain.

The third special feature is the orientation. The door of the louvered box opens to the north because my country is in the northern hemisphere. Opening it to the north can prevent the sunlight from directly shining on the thermometer and other instruments in the box during observation and recording, which will affect the instrument observation results.

It can be seen from this that the temperature of the meteorological department is a purer temperature, which is the air temperature in its natural state after eliminating various interferences .

Part 2

The temperature you feel

Next, let’s talk about “the temperature you feel”. Its scientific name is perceived temperature .

The emphasis of perceived temperature is on "feeling", that is, the human body's feeling of cold and heat, and it cannot be simply understood as the temperature of human skin.

We know that the external environment varies greatly, and it is difficult for people to block out external interference. Studies have shown that under the same temperature conditions, people will have different feelings of cold and warm due to differences in humidity, wind speed, solar radiation (or insolation), clothing color and even mood.

Below is a brief analysis of the impact of factors such as wind speed, sunshine and humidity on the perceived temperature.

The impact of humidity, wind speed, sunshine and other factors on the perceived temperature

If you turn on a fan on a hot day, you will feel cool. This is the effect of wind speed on perceived temperature. A certain wind speed will blow away the heat emitted by the body. Even if the temperature is high, you will feel dry. Therefore, under normal circumstances, the higher the wind speed, the lower the perceived temperature.

The difference between being exposed to the scorching sun and enjoying the coolness under the shade of a tree is like heaven and earth. This reflects the effect of light on the perceived temperature. Direct sunlight on a person will increase the body temperature. Generally speaking, the perceived temperature of a person can differ by 4℃ to 6℃ on a cloudy day and a sunny day, or even more.

When the air humidity is high, the evaporation rate of sweat is very slow, and people feel sticky and uncomfortable. The "sauna days" often encountered in summer are the product of "high temperature + high humidity". Due to the high humidity in southern my country, under the same temperature conditions, people in the south often feel hotter than those in the north.

The figure below better reflects the impact of different humidity on the perceived temperature.

Source: China Weather Network

Studies have shown that when the temperature is 30℃, the relative humidity of the air is around 40%-50%, and the average wind speed is above 3 meters per second, people will not feel very hot; however, under the same temperature conditions, if the relative humidity increases to above 80% and the wind speed is very low, people will feel stuffy and uncomfortable, and the weak may even suffer from heatstroke.

Part 3

The meteorological department and your feelings are not "lying" to you

Finally, let’s make a summary.

The temperature mentioned in the weather forecast refers to the natural air temperature without interference from various human and other natural factors; the temperature we feel is the temperature reported in the weather forecast, combined with humidity, wind speed, sunshine, clothing, and even mood. There are significant differences between the two, especially in the different temperature measurement environments.

Therefore, there is no such thing as the meteorological department reporting a low or incorrect temperature, and your feelings are not deceiving you.

Finally, a friendly reminder, the meteorological department provides a series of services that reflect the perceived temperature from various aspects, such as comfort index, clothing index, UV index and heat stroke index. Therefore, it is recommended that when you pay attention to the daily weather forecast, you may also refer to the relevant life index forecast to arrange your life and travel reasonably.

Produced by: Science Popularization China

Author: Weather Intelligence Team

Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)