Buoyancy + Stability

Buoyancy

Buoyancy is the ability of an object to float, depending on its density compared to its surrounding environment.

Result: Positively buoyant

The object floats

Result: Negatively Buoyant

The object sinks

Result: Neutrally buoyant

The object neither rises nor sinks

Archimedes' Principle

If an object is placed in a fluid, it displaces a volume of fluid equal to its own volume. Whether it sinks or floats depends on its density relative to the fluid

How does buoyancy apply to the atmosphere?

Air parcels rise or sink based on their density compared to the surrounding air, which is determined using temperature.

What is the formula for atmospheric density using the Ideal Gas Law?

p=P/T

When does Air Parcel Float, based on temperature?

Tparcel>Tenv

When does Air Parcel sink, based on temperature?

Tparcel<Tenv

When does Air Parcel stay at origin, based on temperature?

Tparcel=Tenv

What is the Dry Adiabatic Lapse Rate?

Rate at which unsaturated air cools as it rises

Dry Adiabatic Lapse Rate

-10C/km

Moist Adiabatic Lapse Rate

-6C/km

What is the Moist Adiabatic Lapse Rate?

Rate at which a saturated air parcel cools as it rises.

What is atmospheric stability?

Stability describes how an air parcel behaves when lifted and released

Stable Parcel

Returns to origin

Unstable Parcel

accelerates away from the origin

Nuetrally Stable Parcel

stays in its new position

How is atmospheric stability determined?

By comparing a lifted parcel’s temperature with the environmental temperature at the same altitude.

What are radiosondes?

Weather balloons that measure vertical temperature profiles, recording temperature, pressure, and humidity at different heights.

Atmospheric Stability: Absolutely stable

Lapse rate < Moist Adiabatic Lapse Rate, all air parcels return to original positions

Atmospheric Stability: Absolutely unstable

Lapse rate > Dry Adiabatic Lapse Rate, all air parcels rise.

Atmospheric Stability: Conditionally Unstable

Lapse rate is between MALR and DALR

Surface heating

Warms air near the ground, increasing instability.

Cooling aloft

Lowers temperatures at higher levels, increasing contrast.

Combination: surface heating and cooling aloft

Maximizes instability, leading to cloud formation and storms.