The Observatory has warned that ground frost is likely in the New Territories inland areas on Thursday morning as the cold spell continues. The Cold Weather Warning has been in effect since Sunday as the forecaster said chilly mornings are expected for the next few days. Senior Scientific Officer Gabriel Fan told RTHK people should avoid long periods in the cold, especially in exposed or high-altitude areas. "Ground frost will be formed under clear skies and weak winds, where the radiation cooling effects over the inland areas of the New Territories will be significant. The temperature drop will be fast after sunset," he said. "When the surface temperatures drop to zero degrees or below, the moisture in the air can directly become ice without getting through the face of the liquid." He advised people to put on warm clothes and ensure adequate indoor ventilation.