Factors that cause variations in insolation:

1. Transparency of the atmosphere:
The transparency of the atmosphere plays a crucial role in the variation of insolation. The atmosphere consists of various gases, particles, and clouds, which can absorb, scatter, or reflect solar radiation. The presence of pollutants, such as dust, smoke, and aerosols, can reduce the transparency of the atmosphere, leading to a decrease in insolation. On the other hand, a clean atmosphere allows more solar radiation to reach the Earth's surface, resulting in higher insolation.

2. Configuration of land in terms of its aspect:
The configuration of land, particularly its aspect, affects the distribution of insolation. Aspect refers to the direction a slope or surface faces. The angle at which the surface is inclined determines the amount of solar radiation received. For example, a south-facing slope in the Northern Hemisphere receives more direct sunlight throughout the day, resulting in higher insolation, while a north-facing slope receives less direct sunlight and lower insolation. Similarly, the aspect of coastlines, mountains, and valleys can also influence the distribution of insolation.

3. Length of the day:
The length of the day, or the duration of daylight, is another significant factor that causes variations in insolation. The Earth's axial tilt and its revolution around the sun result in changes in the length of the day throughout the year. During the summer solstice, when a hemisphere is tilted towards the sun, the days are longer, allowing for more solar radiation and higher insolation. Conversely, during the winter solstice, when a hemisphere is tilted away from the sun, the days are shorter, resulting in less solar radiation and lower insolation.

Therefore, all three factors - transparency of the atmosphere, configuration of land in terms of its aspect, and the length of the day - contribute to the variations in insolation. The transparency of the atmosphere determines how much solar radiation reaches the Earth's surface, while the configuration of land affects the angle and intensity of sunlight received. Additionally, the length of the day determines the duration of solar radiation exposure. Understanding these factors is crucial for studying climate, weather patterns, and the distribution of energy on Earth.