Why are there thunderstorms in summer and few or no in winter?

Thunderstorm: Thunderstorm is a local storm caused by strong cumulonimbus clouds, accompanied by lightning, thunder and heavy showers. Lightning and thunder without precipitation are called dry thunderstorms. When a thunderstorm crosses the border, meteorological elements and weather phenomena will undergo drastic changes, such as a sharp rise in air pressure, a sharp change in wind direction, a sharp increase in wind speed, a sudden drop in temperature, and then a downpour. Strong thunderstorms can even bring serious disasters such as hail and tornado. Usually, thunderstorms accompanied by showers are called general thunderstorms, and those accompanied by severe weather phenomena such as heavy rain, strong winds, hail and tornadoes are called strong thunderstorms. Both are formed by strong cumulonimbus clouds called thunderstorm clouds. Thunderstorm process is not just a thunderstorm cloud, but is composed of several or several thunderstorm cell in different development stages. Although these thunderstorm cell are in the same thunderstorm cloud, each cell has an independent circulation in the cloud, but they have all gone through the development stage (updraft running through the cloud), maturity stage (downdraft with precipitation and precipitation drag in the cloud) and dissipation stage (downdraft in the cloud), and they are all in the process of continuous regeneration and disappearance. Thunderstorm activity has certain regional and seasonal characteristics. According to statistics, the frequency of thunderstorms in low latitudes is more than that in middle latitudes, and the middle latitudes are more than that in high latitudes. This is because the low latitude is always hot and rainy, and the air is in a warm and humid unstable state, which is easy to form thunderstorms. In the mid-latitude summer half year, the temperature and humidity of the near-surface atmosphere increase, and the instability of atmospheric stratification increases. At the same time, the weather system is active and there are many thunderstorms. At high latitudes, the temperature is low, the humidity is low, the atmosphere is relatively stable, and thunderstorms rarely occur. At the same latitude, the frequency of thunderstorms is generally more in mountainous areas than in plains, and more in inland areas than in coastal areas. Thunderstorms occur most frequently in summer, followed by spring and autumn, and rarely occur in winter except in warm and humid areas. The movement of thunderstorms is greatly influenced by geographical conditions. In mountainous areas, thunderstorms often move along the mountains. If the mountain is not high, strong thunderstorms can cross the mountain. In coastal areas, rivers and lakes, due to the low water temperature during the day, local downdraft often occurs, which weakens or even disappears the intensity of thunderstorms. However, some weak thunderstorms often cannot cross the water surface and move along the coast, but at night, thunderstorms may intensify. Lightning is a discharge phenomenon in thunderstorm clouds. The formation of thunderstorm clouds should meet certain conditions, that is, there should be enough water vapor in the air, the power to make wet air rise, and the air should be able to produce strong convection. Spring and summer are affected by the warm and humid airflow in the south, with humid air and strong solar radiation. The air near the ground is constantly heated up, and the upper cold air sinks, which is easy to form strong convection, so there are many thunderstorms and even hail. In winter, controlled by the continental cold air mass, the air is cold and dry, the solar radiation is weak, and the air is not easy to form violent convection, so thunderstorms rarely occur. But sometimes the weather is warmer in winter, and the power of warm and humid air is stronger. When the cold air in the north occasionally goes south, the warm and humid air is forced to rise, and the convection intensifies, so-called "thunder into winter" phenomenon will occur. Meteorologists also say that the occurrence of thunderstorms does not depend on the temperature itself, but on the upper and lower distribution of temperature. In other words, although the temperature is not high in winter, if the temperature difference between the upper and lower reaches a certain value, strong convection can also be formed, resulting in thunderstorms. Thunder is rare in China in winter, but it often happens in Toronto in winter.