High-pressure conditions over the Thar Desert are the primary cause of the "Loo" winds in northern India during the hot weather season. These winds are hot and dry, which can have significant impacts on the region.

Explanation:
1. Thar Desert:
The Thar Desert, also known as the Great Indian Desert, is located in the northwestern part of India. It covers a vast area and experiences extremely high temperatures during the summer season.

2. High-pressure conditions:
During the hot weather season, the Thar Desert experiences the development of a high-pressure system. High-pressure conditions occur when the air over a region becomes denser and sinks towards the surface. This leads to the formation of a high-pressure area.

3. Wind movement:
As a result of the high-pressure conditions over the Thar Desert, the air starts moving outward from the center of the high-pressure system. This movement of air leads to the formation of winds known as the "Loo" winds.

4. Characteristics of Loo winds:
The Loo winds are characterized by their hot and dry nature. They blow in a northwesterly direction from the Thar Desert towards the northern parts of India, including states like Rajasthan, Punjab, Haryana, and Delhi.

5. Temperature and dryness:
The Loo winds bring with them extremely high temperatures, often exceeding 45 degrees Celsius. They also carry dry air, which further exacerbates the heat and can cause dehydration and discomfort.

6. Impact on agriculture and daily life:
The Loo winds have significant implications for agriculture and daily life in northern India. The hot and dry conditions can lead to the drying up of water bodies, reduction in crop yields, and increased evaporation rates. They can also cause heat-related illnesses and discomfort for people residing in the affected regions.

Conclusion:
The primary cause of the "Loo" winds in northern India during the hot weather season is the high-pressure conditions over the Thar Desert. These winds, characterized by their hot and dry nature, have significant impacts on the region's agriculture and daily life.