Barometric Pressure, Barometers, and Boiling Point Concepts (Transcript Notes)

Barometric pressure is atmospheric pressure measured with a barometer. At sea level, it is approximately $29.92\ \text{inHg}$ , or $15\ \text{psi}$ . This pressure pushes on a pool of mercury, causing it to rise in an evacuated tube until its height (e.g., $29.92\ \text{in}$ ) balances the atmospheric pressure.
The hydrostatic relation is conceptually expressed as $P_{atm} = \rho_{Hg} g h$ , where $\rho_{Hg}$ is mercury density, $g$ is gravitational acceleration, and $h$ is the mercury column height.

Earth's atmosphere acts as a massive, weight-bearing "ocean of air," approximately $7\ \text{miles}$ (or $11\ \text{km}$ ) thick (troposphere). Being at sea level is like being at the bottom of this ocean, experiencing the full weight of the air (approximately $15\ \text{psi}$ ).
This concept is analogous to diving deeper into water, where increased water weight above leads to higher pressure.

There is a direct relationship: lowering atmospheric pressure lowers a liquid's boiling point. For example, water boils at $212^\circ F$ at sea level ( $\approx 29.92\ \text{inHg}$ ).
At an altitude of about $5{,}000\ \text{ft}$ ( $\approx 25\ \text{inHg}$ pressure), water boils around $203^\circ F$ (e.g., Denver).
At very high altitudes, like $30{,}000\ \text{ft}$ ( $\approx 1\ \text{inHg}$ pressure), water would boil at a significantly lower temperature.
This principle is crucial in HVAC and refrigeration, where manipulating pressure controls refrigerant boiling points.

Sea-level atmospheric pressure: $\approx 29.92\ \text{inHg}$ or $\approx 15\ \text{psi}$ .
Water boiling point at sea level: $212^\circ F$ ( $100^\circ C$ ).
Pressure and boiling point are directly coupled: as pressure decreases, boiling point decreases for a given liquid.