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- Daytime heating is generally stronger than nighttime cooling? No! The heating during the daytime is about as strong as the cooling at night. Otherwise, each day would always be warmer than the one before it!
- At night the circulation is weaker because the air is more stable? Yes! As a general rule, vertical motion (and circulations that include vertical motions) are stronger when the atmosphere is weakly stratified.
In weak stratification, the temperature decreases rapidly with height. Air that rises and cools does not become much colder than its surroundings, so there's not much resistance to the parcel's continuing to rise. In fact, if the temperature of its surroundings decreases fast enough, the parcel may be less dense than its surroundings and continue accelerating upward unstably. In contrast, with strong stratification the air temperature is constant or increases with height. Air that is rising becomes much colder (and denser) than its surroundings, causing it to want to sink back down. The same sort of thing applies to the descending branch of the circulation.
So what does this have to do with the land and sea breezes? Sea breezes are caused by heating near the ground, which makes the stratification weaker and encourages vertical motion. Land breezes are produced by cooling near the ground, which produces strong stratification and inhibits vertical motion.
3. Nighttime cooling produces a shallower change in temperature, so the flow is not as deep? Yes! In a land breeze, the cooling produces a shallow layer of cold air, so the pressure gradient is felt only by a thin layer of air. In a sea breeze, a deep layer is heated, so the pressure gradient is felt by more air. Thus, the circulation is stronger in a sea breeze.
4. Friction has more of a retarding effect for the land breeze? Yes! Both the shallow flow and stable stratification cause friction to retard land breezes much more than sea breezes.
In summary, with a sea breeze the air is less stably stratified, the affected air is deeper, and friction is not as significant.
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