1.   Moreover, since cold air is heavier than warm air, chill collects in lowlands on calm spring mornings, making valleys more susceptible to frost than hillsides.

2.   Moreover, since cold air is heavier than warm air, chill collects in lowlands on calm mornings making valleys more susceptible to frost than hillsides.

3.   Moreover, since cold air is heavier than warm, chill collects in lowlands on calm mornings making valleys more susceptible to frost than hillsides.

4.   Moreover, since cold air is heavier than warm, chill collects in lowlands on calm spring mornings making valleys more susceptible to frost than hillsides.