Discuss the four basic types of temperature changes within the year

(1) Diurnal changes in temperature The temperature of the atmospheric boundary layer changes mainly due to the heating and cooling of the ground surface. For example, during the day, the local surface absorbs solar radiation energy and gradually heats up. The heat is transferred to the boundary layer atmosphere through radiation, molecular motion, turbulence, convection motion, and latent heat transport, causing the atmospheric temperature to rise; at night, the surface is heated due to radiation. Cooled by long-wave radiation, the temperature of the boundary layer atmosphere also decreases.

Thus causing diurnal changes in boundary layer atmospheric temperature. The influence of the ground surface on the atmospheric boundary layer temperature is related to the properties of the ground surface (forests, grasslands, deserts, different types of soil, etc.). Warm and cold ocean currents over the vast ocean also affect the atmosphere above the ocean. In addition, both horizontal and vertical movements in the atmosphere will cause changes in local temperature. For example, when warm advection moves in, it will increase the temperature above the local area. When the cold advection moves in, the temperature above the local area will drop. Vertical motion in the atmosphere causes the distribution of heat to become uniform in the vertical direction. When the ground surface is heated, the vertical exchange effect weakens the heating phenomenon on the ground surface. As the surface cools, exchange effects reduce the cooling. The characteristics of diurnal changes in near-surface air temperature are: there is a maximum value within a day, which generally occurs around 14:00 in the afternoon, and a minimum value, which generally occurs around sunrise (Figure 2.30).

The difference between the highest value and the lowest value of temperature in a day is called the diurnal temperature range, and its size reflects the degree of daily temperature change. The solar radiation is strongest at noon of the day, but the highest temperature occurs around two o'clock in the afternoon. This is because the heat in the atmosphere mainly comes from the ground. On the one hand, the ground absorbs short-wave radiation from the sun and gains heat; on the other hand, it transfers heat to the atmosphere and loses heat. If there is a net gain in heat, the temperature will rise. If there is a net loss of heat, the temperature decreases. This means that the level of the ground temperature is not directly determined by the amount of solar radiation absorbed by the ground at that time, but by the amount of heat stored in the ground. As can be seen from Figure 2·30, as the solar radiation increases after sunrise in the morning, the ground gains net heat and the temperature rises. At this time, the heat released by the ground increases as the temperature rises. The atmosphere absorbs the heat released by the ground, and the temperature also rises. Solar radiation reaches its strongest at noon. After noon, although the intensity of solar radiation on the ground begins to weaken, the heat gained is still more than the heat lost. The heat stored on the ground is still increasing, so the ground temperature continues to rise, the long-wave radiation continues to strengthen, and the temperature continues to rise.

By a certain time in the afternoon, the heat gained by the ground is less than the heat lost due to further weakening of solar radiation. At this time, the ground temperature begins to drop. The highest value of the ground temperature occurs when the ground heat changes from storage to loss, and the ground temperature changes from rising to falling. This time is usually around 13:00 in the afternoon. Since it takes a certain amount of time for the heat from the ground to transfer to the air, the highest temperature occurs around 14:00 in the afternoon. Then the temperature gradually drops until the heat stored on the ground is minimized before sunrise in the early morning. So the lowest temperatures occur early in the morning around sunrise, not in the middle of the night. Another characteristic of diurnal temperature variation is that the size of the diurnal range is related to latitude, season and other physical and geographical conditions. The largest diurnal range is in the subtropics and decreases towards the poles. The average diurnal range is about 12°C in the tropics, 8-9°C in the temperate zone, and 3-4°C in the polar circle.

The daily range is greater in summer than in winter, but the maximum value does not occur on the summer solstice. This is because the diurnal temperature range is not only related to the maximum temperature value during the day, but also depends on the minimum temperature value at night. On the summer solstice, although the sun's altitude angle is the highest at noon, the night duration is short and the ground surface does not have time to cool down violently, so the minimum temperature is not low enough. Therefore, the maximum diurnal range in mid-latitudes occurs in early summer and the minimum in winter. The daily range over the ocean is smaller than that over the continent. Due to the slope and little air flow in basins and valleys, daytime heating and nighttime cooling are both large, and the diurnal range is large. In protruding terrain areas such as small mountain peaks, the ground surface has little impact on temperature and the diurnal range is small. The diurnal temperature range is also related to ground characteristics and weather conditions. For example, desert areas have large diurnal variations. The diurnal range is smaller in humid areas.

As far as weather conditions are concerned, if there are clouds, the ground will receive less solar radiation during the day and the maximum temperature will be lower than on a sunny day. At night, cloud cover makes it difficult for ground heat to dissipate, and the minimum temperature is higher than on sunny days. Therefore, the diurnal temperature range on cloudy days is smaller than that on sunny days (Figure 2·31). It can be seen that at any location, the diurnal changes in temperature every day have certain regularity, and are not a simple repetition of the diurnal changes in temperature of the previous day. Instead, the comprehensive influence of the above factors must be considered. The occurrence time of the extreme value of the daily temperature variation increases with the height above the ground, and the amplitude decreases with the height above the ground. In winter, the diurnal vibration is no longer obvious at an altitude of about 0.5km, but in summer, the diurnal vibration can extend to an altitude of 1.5km to 2km.

(2) Annual changes in temperature The annual changes and diurnal changes in temperature have different characteristics in some aspects. For example, in most areas on the earth, the average monthly temperature in a year has a a maximum value and a minimum value. Due to the heat stored on the ground, the highest and lowest temperatures occur not on the days when solar radiation is strongest and weakest (summer solstice and winter solstice in the Northern Hemisphere), nor in the month when solar radiation is strongest and weakest ( June and December in the Northern Hemisphere), but 1 to 2 months behind this period.

Generally speaking, there is more backwardness on the ocean and less on land. The coastal areas are more backward and the inland areas are less backward. As far as the northern hemisphere is concerned, the temperature in mid- and high-latitude inland areas is highest in July and lowest in January. The temperature on the ocean is highest in August and lowest in February. The difference between the highest and lowest monthly average temperatures in a year is called the annual temperature range.

The size of the annual temperature range is related to factors such as latitude, sea and land distribution. Near the equator, the length of day and night is almost equal, the heat budget of the hottest month and the coldest month is almost the same, and the annual temperature range is very small. As we get to high latitudes, the distinction between winter and summer is obvious, and the annual temperature range is very large. For example, the annual temperature range in my country’s Paracel Islands (16°50’N) is only 6°C, Shanghai (31°N) is 25°C, and Hailar (49°13’N) reaches 46.7°C. Figure 2·32 shows the annual changes in temperature at different latitudes. The annual temperature range in low latitudes is very small, while in high latitudes the annual temperature range can reach 40-50°C. For example, comparing the sea and land at the same latitude, the difference in heat budget between winter and summer in the continental area is larger than that in the ocean, so the annual temperature difference on land is much larger than that in the ocean. Under normal circumstances, the annual temperature range in temperate oceans is 11°C, and the annual temperature range in continents can reach 20-60°C.

According to the size of the annual temperature difference and the time when the highest and lowest values ??occur, the annual changes in temperature can be divided into four types by latitude. 1. The equatorial type is characterized by two highest values ??in a year, which occur after the spring and autumn equinoxes respectively, because the sun is at the zenith at noon during the spring and autumn equinoxes in the equatorial region. The two lowest values ??occur after the winter solstice and summer solstice, when the noon solar altitude angle is the minimum value of the year. The annual range here is very small, only about 1 ℃ on the ocean, and only about 5-10 ℃ on the continent. This is because the amount of solar radiation income in the region changes very little within a year. 2. The tropical type is characterized by a maximum (after the summer solstice) and a minimum (after the winter solstice) in a year. The annual range is not large (but larger than the equatorial type). It is generally 5 ℃ in the ocean and about 5℃ on land. is about 20 ℃. 3. The temperate type also has a maximum value in a year, which occurs in July after the summer solstice.

A minimum value occurs in January after the winter solstice. Its annual range is large and increases with increasing latitude. The annual temperature range in the ocean is 10-15°C, while inland the temperature generally reaches 40-50°C, with a maximum temperature of 60°C. In addition, the occurrence of extreme values ??in the ocean is later than that on the continent, with the highest value appearing in August and the lowest value appearing in February. 4. The polar type also has a maximum value and a minimum value in a year. It is characterized by long and cold winters, short and warm summers, and large annual ranges. What is particularly important to point out here is that as the latitude increases, the daily temperature range decreases while the annual temperature range increases. This is mainly due to the fact that in high latitudes, the diurnal variation in solar radiation intensity is smaller than in low latitudes. That is, in areas with high latitudes, the change in solar altitude angle within a day is smaller than in areas with lower latitudes, while the annual variation in solar radiation is at high latitudes. The reason is that the area is larger than the low latitude areas.