Consider the following statements:

1. The proportion of water vapour generally decreases with increasing altitude and from the equator towards the poles.
2. Water vapour may constitute about four per cent of the air in warm, wet tropical regions but less than one per cent in cold or dry regions.
3. Dust-particle concentration is generally higher in equatorial and polar regions than in subtropical and temperate regions.
4. Dust and salt particles can serve as nuclei around which water vapour condenses to form clouds.

Which of the statements given above are correct?