Complex sentence: When the airparcel cools more slowly than the surrounding atmosphere as it is lifted, so it remains relatively less dense, and so continues to rise through the atmosphere until it reaches an area of air denser (cooler) than itself.
Split into simpler sentences: When the airparcel cools more slowly than the surrounding atmosphere as it is lifted, it remains warmer and relatively less dense. The parsel continues to rise through the atmosphere until it reaches an area of air denser (cooler) than itself.