A weather bulletin stating +3 °C does not rule out the possibility of frost-covered grass in the early morning. The official temperature is measured under shelter, 1.50 meters above the ground, while the grass, being much closer to the earth, cools more rapidly. On a clear night, the temperature difference between the air measured under shelter and the grass can be about 3 °C. Therefore, even if the thermometer reads +2 to +3 °C, the surface of the grass can fall below freezing, creating a layer of frost without the weather forecast being incorrect. Both measurements are accurate but refer to different locations. This temperature difference is due to a phenomenon called a radiation frost, which occurs when the ground and vegetation emit infrared radiation into space on clear nights, without receiving heat in return. During the day, the sun compensates for this loss, but at night, especially under clear skies, the ground cools quickly. This cooling effect is more pronounced in areas with light winds and high humidity. Clouds, on the other hand, act as a blanket, trapping some of the heat and making nights milder. The result is a thermal inversion, where the coldest air is at the ground level and the warmer air is above it. This type of frost is common in spring and autumn, when nights are getting longer but the air is still relatively mild. A dry, high-pressure system, known as an anticyclone, creates ideal conditions for radiation frost. Météo-France notes that under these conditions, the temperature difference between the air at ground level and the air measured under shelter can be more than 30 degrees. This difference explains why mornings can look frosty even when the official temperature is above freezing. To assess the risk of frost, it is important to consider not just the temperature but also the sky conditions and wind speed. A clear sky and light wind increase the chance of frost, while clouds and wind reduce it. Local forecasts can be more precise than national ones, as areas like valleys and open meadows tend to be colder than urban or coastal regions. In the garden, the best way to protect sensitive plants is to bring them inside or cover them with a protective layer before nightfall. These precautions can help prevent damage from unexpected frost, especially as the season of the first frosts approaches.