The equator region is the hottest part of Earth's surface because of option A: "It warms up and rises."
Here's why:
1. **Solar Angle:** Near the equator, the Sun's rays strike the Earth's surface almost vertically throughout the year. This means that the same amount of solar energy is concentrated over a smaller area, leading to higher temperatures.
2. **Less Variation:** The equatorial region experiences minimal seasonal variation in the angle of sunlight. This consistency in solar input keeps the temperatures consistently high.
3. **Warm Air Rises:** As the Sun heats the surface, the air near the equator also warms up. Warm air is less dense and therefore rises. This rising warm air creates a low-pressure area at the surface, which in turn draws in more warm, moist air from surrounding regions.
4. **Convection and Precipitation:** The rising warm, moist air cools as it ascends and eventually condenses to form clouds and precipitation. This frequent convection and rainfall keep the surface warm and contribute to the lush tropical rainforests found in equatorial regions.
5. **Trade Winds:** The movement of air toward the equator from subtropical high-pressure zones results in the trade winds. These winds help distribute the warm, moist air across the equatorial region.
So, the equatorial region is the hottest part of Earth's surface because of the combination of direct and consistent solar heating, the rising of warm air, and the atmospheric circulation patterns that result from this warming process.