Answer to Question 1
Air pressure is the pressure that results from the weight of the air above the air that is being measured.
Answer to Question 2
Sleet is ice precipitation that forms when a layer of air above freezing temperature overlies a layer of air below freezing. Rain that falls in the upper air layer freezes as it falls through the lower air layer, becoming sleet. Glaze is formed in similar conditions to sleet; however, the thickness of the air layer that is below freezing is not sufficient to completely freeze the rain. Instead, the rain becomes supercooled and freezes upon contact with a solid.
An analogy for how cardiac output, resistance, and blood pressure are related.
Average optimal blood pressure for age
Guidelines for proper blood pressure cuff size
Measuring Blood Pressure
Horizontal Pressure Gradients in the Upper Atmosphere
Physics - Density and Pressure