Are frost and frost the same thing? Here are 7 tips to deal with frost!

Today is Frost Descent, the eighteenth of the twenty-four solar terms and the last solar term of autumn, marking the transition from autumn to winter.

After entering the Frost Descent solar term, cold air activities affecting China have become more frequent, especially in the south of the Yangtze River and southern China, where the temperature fluctuations have become more obvious; while parts of the northwest and northeast have already presented an early winter scene of "cold wind and falling leaves". On average, Frost Descent is the time of the year when the temperature difference between day and night is the largest.

The most prominent feature of the Frost Descent solar term is frost. In life, when we watch the weather forecast, we also hear the word "frost" in addition to frost. Are frost and frost the same thing? Let's find out together.

01What is frost

Frost is white loose ice crystals formed by the water vapor in the air near the ground condensing directly on the ground or objects. It usually appears on clear nights because the temperature of the ground or objects will drop rapidly to below 0℃, allowing the water vapor in the air to condense directly into solid ice crystals. The formation of frost requires two basic conditions: one is that there is enough water vapor in the air, and the other is that the temperature of the ground or objects must be below the freezing point.

02Frost and frost are two different concepts

Frost refers to the phenomenon that the temperature drops rapidly and the surface temperature drops below 0℃, causing damage to crops or even death . Frost is a weather phenomenon , while freezing is an agricultural meteorological disaster . The formation of frost mainly depends on the temperature and water vapor content near the surface; while the formation of freezing is mainly due to the sharp drop in temperature. Frost has little effect on most objects and is mainly a visual change; while freezing has serious destructive power on crops.

Frost forms on plants in Shangri-La, Diqing Tibetan Autonomous Prefecture, Yunnan Province (Photo: Xinhua News Agency)

03What is the frost line

Every autumn, we see red “thorny” lines, lines or coils crawling on the weather map, like traces of earthworms. What are these lines? What do they indicate?

National precipitation forecast map from 14:00 on October 17 to 14:00 on October 18 (Source: China Meteorological Administration)

These lines are called frost lines, which refer to a curve with a surface temperature of 0°C. They are the signs for dividing frost areas . Frost lines only appear on precipitation forecast maps , and are not seen on temperature and wind forecast maps. Frost lines are usually marked in red or other eye-catching colors to distinguish them from other meteorological information. This line divides the forecast area into two parts. The area pointed to by the "thorns" on the red line is prone to frost disasters , and the side without "thorns" is a relatively safe area .

National precipitation forecast map from 08:00 on October 19 to 08:00 on October 20 (Source: China Meteorological Administration)

04The frost line and the 0℃ line are two lines

In meteorology, there is another line called the 0℃ line, which usually refers to the isoline where the temperature is 0℃ at a certain moment or in a certain period of time.

These two lines are easy to distinguish. The frost line is closer to the surface and is greatly affected by the surface temperature; the 0℃ line is located about 1.5 meters above the ground and is affected by the temperature conditions of the entire atmospheric stratification. The frost line refers to the curve with a surface temperature of 0℃, while the 0℃ line refers to the isovalue line of the air temperature of 0℃. Without considering factors such as clear sky radiation, when the daily minimum temperature reaches 2℃~4℃, the surface temperature can reach 0℃ and frost may occur. The frost line is mainly used for the prediction and protection of agricultural meteorological disasters ; while the 0℃ line is more widely used in climate zoning, architectural design and other fields.

The blue line in the picture is the 0℃ line (Source: Central Meteorological Observatory)

05 Characteristics of frost line

The position of the frost line is not fixed, but moves with the changes of seasons, weather systems and topography. In autumn and winter, as the temperature gradually decreases, the frost line will move southward and cover a wider area. In spring, as the temperature rises, the frost line will gradually move northward and reduce its impact range.

The location and movement trend of the frost line are affected by many factors, including atmospheric circulation, topography, ocean temperature, vegetation cover, etc. For example, in mountainous areas, due to the sinking of cold air and the enhancement of radiation cooling, the frost line may appear earlier in valleys and low-lying areas. Near the ocean, due to the regulation of the ocean, the movement of the frost line may be relatively delayed.

The frost line has obvious regional and seasonal characteristics. Due to differences in climate conditions and topography in different regions, the location and movement trend of the frost line will also vary.

06The impact of frost line

In areas covered by the frost line, when the surface temperature drops below 0°C, crops are vulnerable to frost damage. Such low temperature conditions can damage the cell structure of crops, causing damage to leaf and stem tissues, and in severe cases may even cause plant death. After frost occurs, the growth cycle of crops may be delayed, resulting in delayed harvest time, affecting the rhythm and plan of agricultural production.

Frost not only reduces the survival rate of crops, but also may affect the formation and development of fruits, resulting in a significant decrease in yield. At the same time, the quality of crops after frost often decreases, such as poor taste and reduced nutritional value, which affects market competitiveness.

07Frost line forecast

The prediction of the frost line is mainly based on the actual surface temperature in the previous period, and is combined with factors such as the cold air movement speed, surface temperature and minimum temperature predicted by the reference model.

For advection frost, the key to prediction is to understand the direction and speed of cold air. The arrival of cold air is often accompanied by a sharp drop in temperature, so accurately predicting the movement path and speed of cold air is crucial to determining the location of the frost line.

Without considering factors such as clear sky radiation, when the daily minimum temperature reaches 2°C to 4°C, the surface temperature may drop to 0°C. Based on this, forecasters can combine the model forecast results to preliminarily determine the location of the frost line.

The position and movement of the frost line are also affected by the terrain and environment. For example, low-lying areas may enter the frost zone later due to the sinking and warming of cold air, while areas close to lakes or rivers may experience frost later due to the regulating effect of water on the climate.

08 Here are 7 tips to deal with frost →

Frost can cause certain harm to agricultural production. Therefore, the focus should be on taking countermeasures in the agricultural field. The main methods are:

Fumigation method : about 1 hour before the arrival of frost, burn materials that can produce a lot of smoke, such as straw, weeds, etc. These smoke can reduce the loss of heat from the ground and increase the air temperature near the ground by 1 to 2 degrees Celsius, thereby protecting crops from frost damage.

Irrigation method : Irrigate the field in the evening before frost is expected. The evaporation and condensation of water will release latent heat, which helps to conserve ground heat, increase soil temperature by about 2°C, and delay the temperature drop near the surface. However, it should be noted that this method is not suitable for potatoes during the tuber stage.

Covering method : Cover crops with straw, weeds, plastic film, etc. This method can directly increase the temperature of the soil and the temperature around the crops, generally increasing the soil temperature by 1-3℃, or even up to 6-7℃, effectively preventing frost.

Fertilization method : 3-4 days before the arrival of frost, apply manure, compost and wood ash to the crop fields. These fertilizers will release heat during the decomposition process, which can not only increase the soil temperature, but also increase the soil aggregate structure and improve the soil fertility, thereby enhancing the cold resistance of crops.

Wind barrier method : Before the frost comes, set up a wind barrier to the northwest of the field. The wind barrier can effectively block the cold wind, reduce the direct impact of cold air on crops, and protect crops from the harm of low temperature and frost.

Spraying growth regulators : Spray nutrient solution or growth regulators, such as potassium dihydrogen phosphate, one day before frost arrives or after frost damage occurs to enhance crop resistance and prevent or reduce frost damage.

Strengthen field management : timely carry out inter-row cultivation, weeding, soil cultivation, and irrigation and drainage to keep the soil moist. Apply more phosphorus and potassium fertilizers, foliar fertilizers, or thermal farmyard manure such as plant ash to promote plant growth and enhance the ability to resist cold.

In addition, according to the location and changing trend of the frost line, the crop planting structure should be reasonably adjusted, and crop varieties with strong cold resistance and appropriate planting time should be selected. Agricultural insurance services should be provided to farmers to reduce the impact of natural disasters such as frost on agricultural production.

Author: Wang Yuanhong, Senior Weather and Climate Engineer

Produced by: Science Popularization China