When is the coldest time of the day?

The lowest temperature in a day is usually from 5 am to 6 am, which is what northerners often call ghost teeth.

The hot and cold weather generally refers to the temperature. After the sun comes out during the day, the air begins to receive heat, but the air does not receive much heat, but the sun radiates the ground, which receives most of the light and heat, and the ground heats the air through radiation and convection.

In a day, from sunrise to sunset, the sun goes from oblique direct (or near direct) to oblique, and the radiation of the sun is the strongest at noon, that is to say, the radiation intensity of the sun changes from weak to strong and then to weak in a day, and the heat received by the surface is also this law.

Therefore, during the day, from 2 pm to 4 pm, the surface receives the most heat and has the strongest radiation to the air. At this time, the temperature is the highest in a day, and then it gradually weakens. With the decrease of surface temperature, when the air temperature is higher than the surface temperature, the air will form inverse radiation to the surface, and the temperature will gradually decrease, reaching a very low value from 5 am to 6 am the next day.

Extended data

In addition to natural factors, the temperature is also affected by human factors, mainly in the following aspects:

1, the influence of the characteristics of urban underlying surface (the contact surface between atmospheric bottom and surface)

Laying the ground and building walls in the city changes the thermal properties of the underlying surface, reduces the water content of the surface, makes the heat enter the air more easily, and causes the temperature to rise.

2. Urban air pollution

Activities such as nitrogen oxides, carbon dioxide and dust produced by motor vehicles and industrial production absorb a lot of thermal radiation energy, causing the temperature to rise.

3, the influence of artificial heat source

Factories, motor vehicles and residents' lives are all emitting heat.

4. The natural underlying surface in the city is reduced.

The increase of buildings, squares and roads, and the decrease of green space and water bodies have produced the heat island effect, leading to an increase in temperature.

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