On a given day, a weather balloon measures the environmental lapse rate as 12°C/km. A dry air parcel is given an initial upward push. What happens next?
AThe parcel returns to its original level — the environment is cooling faster, so the parcel quickly becomes cooler than its surroundings
BThe parcel continues rising on its own — it cools at 9.8°C/km while the environment cools at 12°C/km, keeping the parcel warmer and more buoyant
CThe parcel cools at 12°C/km to match the environment, remaining neutrally buoyant
DThe parcel stops rising when it reaches the altitude where its temperature equals the dry adiabatic lapse rate
Stability is determined by comparing the parcel's temperature to the environment's temperature at each altitude. The parcel cools at 9.8°C/km (DALR). The environment cools faster (12°C/km), so at any altitude above the start, the environment is colder than the parcel. A warmer parcel is less dense and buoyant — it keeps rising without further forcing. This is the unstable condition. Option A has the logic backwards. Option C is wrong: the parcel always cools at the DALR, not the environmental rate — these are independent.
Question 2 Multiple Choice
A Chinook wind event brings warm air to a valley after air crossed a mountain range and descended 3,000 meters dry-adiabatically. Approximately how much warmer is the descending air than it was at the same altitude on the windward side?
AAbout 9.8°C warmer — adiabatic warming only applies over 1 km
BAbout 14.7°C warmer — half the descent distance times the lapse rate
CAbout 29.4°C warmer — 3 km × 9.8°C/km
DNo warmer — temperature changes on ascent and descent cancel exactly
Dry adiabatic descent warms air at 9.8°C per kilometer. Over 3,000 m (3 km): 3 × 9.8 = 29.4°C of warming. This is why Chinook winds can dramatically warm valleys. Note that option D would hold only if ascent and descent were both dry-adiabatic. If the air shed moisture as orographic precipitation on the windward side (ascending at the moist adiabatic rate, which is smaller), the net effect is a temperature gain — the air arrives warmer than it started.
Question 3 True / False
The dry adiabatic lapse rate describes the actual temperature profile of the atmosphere at a given location and time.
TTrue
FFalse
Answer: False
This is the most important misconception to avoid. The DALR (~9.8°C/km) describes how a specific rising air parcel cools due to adiabatic expansion — it is a property of the parcel's thermodynamic process, not of the atmosphere around it. The actual atmospheric temperature profile is the environmental lapse rate (ELR), which varies by location and time based on solar heating, advection, and moisture. The DALR and ELR are independent quantities, and comparing them is how meteorologists assess atmospheric stability.
Question 4 True / False
A dry air parcel starting at 25°C at sea level and one starting at 0°C at a 2,000-meter plateau cool at different rates as they rise, because their initial temperatures differ.
TTrue
FFalse
Answer: False
The dry adiabatic lapse rate is constant at approximately 9.8°C/km regardless of initial temperature, initial pressure, or geographic location. The rate depends only on gravitational acceleration (g) and the specific heat capacity of dry air at constant pressure (Cp) — both effectively constant throughout the lower atmosphere. Initial conditions determine the parcel's temperature at each altitude but not the rate of cooling.
Question 5 Short Answer
Why does an air parcel cool as it rises in the atmosphere, and what source of energy drives this cooling?
Think about your answer, then reveal below.
Model answer: As a parcel rises, surrounding atmospheric pressure decreases, so the parcel expands. This expansion requires the air molecules to do work pushing outward against lower external pressure. If the process is adiabatic (no heat exchanged with surroundings), the energy for this work comes from the parcel's own internal thermal energy — the molecules slow down on average and temperature drops. The cooling is a conversion of internal (thermal) energy into the mechanical work of expansion, with total energy conserved.
The reverse — compression warming during descent — follows the same logic. As air descends, it compresses, and work is done ON the parcel, converting mechanical energy back into thermal energy at the same 9.8°C/km rate. This symmetric process explains both Chinook warming and the constancy of the rate regardless of direction.