1. Why are there subtropical highs around 30° N and S of the equator?

There are subtropical highs around 30° N and S of the equator as from equatorial area wind rises up and low pressure zone forms. So, beyond 30 degree N/S again winds comes to wards the equator from poles and creates pressure over the belt just above equator.

Explanation:

There are total seven pressure belts in the earth's atmosphere: Equatorial low pressure belt, sub tropical high pressure belt, sub polar low pressure belt and polar high pressure belt. Equatorial low pressure belt is characterized by direct sunlight, rising up warm air and low pressure zone.

And towards these low pressure zone winds come in full force from high pressure lands and creates excessive pressure beyond the pressure belt of the equatorial low pressure zone. That is 30 degree N/S sub tropical high pressure belt.