Can the “erratic” heavy rainfall become the “terminator” of the heat wave?

Recently, "high temperature heat wave" has become a real "hot" word. In my country, the Central Plains region has been scorching for days with a temperature of 40 degrees Celsius, which has fried the cement roads and made you doubt your life. People say that Henan is as hot as "Kennan". On the one hand, the North China Plain in my country is experiencing a long-lasting heat wave, while on the other hand, Europe and the United States are also performing a high temperature march at the same time: in the United States, Spain, France and other places, there have been continuous hot weather of more than 40 degrees Celsius, and high temperature red warnings have been issued frequently.

Is high temperature contagious? High temperatures and fever have always been associated with low-latitude areas near the equator. Why are these mid- and high-latitude areas so hot? Will the current heavy rainfall be the "terminator" of the heat wave?

Heat wave sweeps across the Northern Hemisphere

In mid-June, before the summer solstice, a heat wave began to spread across many countries in the Northern Hemisphere. On Sunday, June 12, the beginning of the week in Europe and the United States, in North America, Salt Lake City in Utah, in the northwest of the United States, saw the earliest record of 102 degrees Fahrenheit (about 38.9 degrees Celsius) in U.S. history. The heat wave continued to spread eastward, and on the 13th, the National Weather Service issued a high temperature warning for the Chicago metropolitan area. Subsequent data showed that on the 14th and 15th, Chicago had two consecutive days of maximum temperatures reaching 96 to 98 degrees Fahrenheit (36 to 37 degrees Celsius).

Unexpectedly, the heat wave swept across Europe. On the 16th, the temperature in Bordeaux, a city in southern France, began to soar to over 35 degrees Celsius. On the 17th and 18th, the temperature here soared to over 40 degrees. During this period, France measured a high temperature that broke the national record: 42.3 degrees Celsius. Spain, which is famous for its Mediterranean climate, started the hot and dry mode a week ago. At this time, it was as hot as a "burnt red brick" on the temperature chart, and a high temperature of 44.5 degrees Celsius was measured, which is undoubtedly a new record in the history of European observations. Fortunately, on the 19th, Sunday, with the arrival of Father's Day, this round of abnormal high temperatures came to an end in European and American countries.

However, in my country, the heat wave did not subside so quickly, but intensified and even broke records. In the North China Plain, a long-lasting round of high temperatures began on the 15th, with Henan as the center. The high temperatures in Henan, central and southern Hebei, and western Shandong have lasted for 11 days from the 15th to the 25th.

Zhengzhou's highest temperature curve from June 15 to 25 (unit: ℃)

The period from Grain in Ear to Summer Solstice is the time of the year when solar radiation is the strongest in the Northern Hemisphere. There is not much rain in the north, the sky is transparent, and the ground temperature tends to rise quickly. It is not surprising that high temperatures appear in the North China Plain during this season. However, this is the first time that high temperatures have lasted so long.

The high temperature lasted for a record-breaking period of time, and the degree of heat was also astonishing: the highest temperature in many places in Henan and southern Hebei exceeded 40 degrees, especially on the 24th and 25th, the heat was almost crazy, and the temperature in many places broke records, exceeding 40 degrees or even exceeding 42.3 degrees. This includes Zhengzhou and Jiaozuo in Henan. On the 24th, the highest temperature in Zhengzhou was 42.3 degrees, and Jiaozuo was 43.3 degrees. On the 25th, the heat also "burned" Beijing, Tianjin and Hebei, and the temperature in Beijing reached 39 degrees in the afternoon.

The highest temperature in China on June 24 (Source: China National Meteorological Observatory)

National temperature map at 16:00 on June 25 (Source: China Central Meteorological Observatory)

On the other hand, the hot period is also very long every day. It can be said that it is hot all day. On the 24th, just after 8 o'clock in the morning, the temperature in Zhengzhou had risen to 35 degrees. It was not until 12 o'clock at night that the temperature began to drop below 35 degrees. The time above 35 degrees alone reached 15 hours. Moreover, there were 7 or 8 hours from noon to dusk when the temperature was above 40 degrees, and the surface temperature was above 70 degrees. It felt like you would be melted by the sun if you went out, and frying eggs was not a problem.

Zhengzhou temperature line chart from 08:00 to 24:00 on June 24 (unit: ℃)

The creators of heat waves:

Strong warm high pressure

The dominant force behind this round of extreme high temperature weather in Europe, the United States and my country is the extremely strong continental high pressure and the dry and warm air masses it carries.

On the 18th, when the heat wave in France was at its peak, the corresponding weather situation map showed that the North African high pressure expanded all the way northward, firmly controlling the area from Spain to France. On the same day, in my country, the high temperature was developing in full swing, and at this time a warm continental high pressure ridge was also stationed in the area from the Loess Plateau to the North China Plain.

On the 24th, in my country, when this round of high temperatures reached its peak, the warm high pressure became even stronger on the weather situation map, not only forming a closed center; it was also larger in size, hovering in the sky from the northwest to North China.

Weather conditions on June 18 (Source: China National Meteorological Administration)

Weather conditions on June 24 (Source: China National Meteorological Administration)

First, under the control of high pressure, there are more sunny days and less rain, the sky is transparent, and a large amount of solar radiation can reach the ground; second, the dry and warm air mass carried by high pressure has a warming effect; third, high pressure is equivalent to a "warm cap", which has a heat preservation effect, and heat is constantly accumulated. The superposition of the three effects eventually created a round of extreme high temperature weather that occurred simultaneously across countries in the Northern Hemisphere.

How to solve high temperature? Only cold-low-pressure

Since the powerful warm high pressure in the upper air is the driving force behind the large-scale high temperature weather, perhaps only its opponent - the (upper air) cold low pressure - can "break the deadlock".

As mentioned earlier, starting from the 19th, the northern United States and many parts of Europe were able to get rid of the high temperature, thanks to the emergence of high-altitude cold low pressure, which has a dual cooling effect of "rainfall + cooling". Including France, many parts of Western Europe, due to the continued influence of cold low pressure, the weather is rainy. This week, the maximum temperature once dropped to around 25 degrees, which is really "immediate" cool.

However, the temperature was very high in the early stage, and when the cold air brought by the cold low pressure arrived, it was like pouring cold water on a hot pot, which easily triggered severe convective weather.

Heavy rain is coming, high temperature will ease

In northern my country, there is finally good news. The 25th (this Saturday) is the "last madness" of this round of high temperatures. From the satellite cloud map on the afternoon of the 25th, it can be seen that the rain clouds have gathered in Gansu, Qinghai and Inner Mongolia, and are constantly developing eastward. The strongest rainfall in the north since the beginning of summer this year has begun.

Satellite cloud image on the afternoon of June 25 (Source: Satellite Meteorological Center)

From Northwest China to North China to Northeast China, moderate to heavy rain will generally occur, with local torrential rain to heavy rain. The places with the heaviest rain are basically also the hottest areas at present.

As mentioned earlier, the energy accumulated here in recent days is extremely huge. Such an extremely hot "big pot" and the powerful cold low pressure will pour down cold air, posing a very high risk of "explosion". We must guard against strong convective weather such as thunderstorms, gales, hail, and short-term heavy rainfall.

Precipitation forecast from 26th to 28th (Source: Central Meteorological Observatory)

Severe convective weather forecast on the 26th (Source: Central Meteorological Observatory)

With such heavy rainfall, the high temperature in the North China Plain will be relieved, and the fever here can finally be reduced. It is expected that the 27th will be the coolest day in the recent period in this area. On the maximum temperature forecast map, the red area symbolizing high temperature will disappear completely, and the high temperature here will be eliminated collectively. Until the end of June, there will be no more large-scale high temperatures like before.

Maximum temperature forecast for June 27 (Source: China National Meteorological Administration)

The sweltering heat is “not as hot as hot”

However, it should be noted that in some places in northern Henan, southern Shandong, southern Hebei, including Zhengzhou, the maximum temperature will drop to 32-34 degrees. From a numerical point of view, the temperature is below 35 degrees, which is not considered high temperature. However, the air humidity is much higher than before when it was scorching hot. Most of the time during the day, the relative humidity is 60%-70%, and the lowest temperature is around 25 degrees. It may still be a bit stuffy.

High temperature variations will continue

In my country, the summer solstice marks the beginning of midsummer; in Europe and the United States, it corresponds to the beginning of summer. Regardless of the meaning, it indicates that the high temperature variation has just begun. And the high temperature before the summer solstice in mid-June this year is just a prelude.

my country's climate characteristic of "rain and heat at the same time" means that in midsummer, you can only choose between heavy rain and high temperature (sometimes both). When heavy rain comes, the high temperature in the north will be relieved; while in the south of the Yangtze River, the high temperature will come immediately after the heavy rain. This is because the big boss that dominates my country's summer weather - the subtropical high pressure is expanding its territory and starting to "take over" here.

In the past two days, there have been large areas of high temperature in Shanghai, Zhejiang, Fujian, Jiangxi, Hunan, Guangdong and other places, with the highest temperature of 35~37 degrees. The humidity here is even higher, and the high temperature and high humidity sauna days have already begun.

Generally speaking, in the following July and August, under the control of the subtropical high pressure, the main battlefield of high temperatures begins to shift to the south. This is the hottest period of the year in the south, the so-called "Minor Heat and Greater Heat, steaming up and boiling down."