Weather and climat exm 2

Adiabatic Process

Temperature change in an air parcel without heat exchange; caused by expansion (cooling) or compression (warming)

Rising air effects

Pressure decreases → volume increases → temperature decreases (adiabatic cooling)

Sinking air effects

Pressure increases → volume decreases → temperature increases (adiabatic warming)

Dry Adiabatic Lapse Rate (DALR)

10°C/km; rate at which unsaturated air cools when rising

Saturated Adiabatic Lapse Rate (SALR)

~5–9°C/km; slower cooling due to latent heat release during condensation

When to use DALR vs SALR

If T > Dew Point → DALR; if T = Dew Point → SALR

Dew Point significance

Temperature at which air becomes saturated and condensation begins

Lifting Condensation Level (LCL)

Height where T = Dew Point; clouds begin forming

Why saturated air cools slower

Condensation releases latent heat, offsetting cooling

Steps for solving adiabatic problems

Check saturation → apply DALR until T = DP → switch to SALR

Environmental Lapse Rate (ELR)

Actual temperature change of the atmosphere; used to determine stability

Absolute stability

ELR < SALR; air resists rising

Absolute instability

ELR > DALR; air rises freely

Conditional instability

SALR < ELR < DALR; air rises only if saturated

Why stability matters

Determines whether air continues rising (storms) or stops (cloud layers)

Why condensation alone cannot produce rain

Droplets are too small and fall too slowly; need growth processes

Collision-Coalescence process

Larger droplets fall faster, collide with smaller ones, and merge into raindrops

Collector drop

Large droplet that falls faster and collects smaller droplets

Factors that encourage CC

High moisture; different droplet sizes; strong updrafts; thick clouds; electrical charge

Why droplet size differences matter

Larger droplets fall faster, increasing collision rates

Bergeron process

Ice crystals grow at expense of supercooled water in cold clouds

Supercooled water

Liquid water below freezing that has not yet frozen

Why ice grows in Bergeron process

Air saturated for water is supersaturated for ice, favoring ice growth

Riming

Supercooled water freezing onto ice crystals (forms graupel)

Aggregation

Ice crystals colliding and sticking to form snowflakes

Precipitation type control

Determined by vertical temperature structure, not just surface temperature

Pressure Gradient Force (PGF)

Force that moves air from high to low pressure; drives wind

Coriolis Force (CF)

Deflects moving air (right in NH, left in SH); increases with speed and latitude

Friction Force (FF)

Opposes wind and reduces speed; important near surface (<1 km)

Geostrophic wind

Balance of PGF and CF; wind flows parallel to isobars (upper atmosphere)

Why surface winds cross isobars

Friction reduces CF, allowing PGF to pull wind toward low pressure

Low pressure system effects

Surface convergence → rising air → cooling → clouds and precipitation

High pressure system effects

Surface divergence → sinking air → warming → clear skies

Hydrostatic balance

Upward pressure force balances gravity, preventing constant vertical acceleration

Why hydrostatic balance matters

Keeps atmosphere stable; vertical motion only occurs when disturbed

Atmospheric circulation cause

Uneven heating of Earth creates global wind patterns

Three-cell model

Hadley (0–30°), Ferrel (30–60°), Polar (60–90°) cells

ITCZ

Equatorial region of rising air, low pressure, and heavy precipitation

Subtropical highs

~30° latitude; sinking air, high pressure, dry conditions (deserts)

Why deserts form at 30°

Sinking air warms and dries, reducing humidity

Westerlies

Mid-latitude winds that move weather systems west to east

Trade winds

Tropical winds that blow toward the equator

Jet streams

Fast upper-level winds caused by strong temperature gradients

Atmospheric motion scales

Micro (tiny), Meso (thunderstorms), Synoptic (cyclones), Planetary (global circulation)

Air mass

Large body of air with uniform temperature and moisture

Air mass types

mT (warm moist), cT (warm dry), mP (cold moist), cP (cold dry), cA (very cold dry)

Front

Boundary between two different air masses

Cold front

Cold air undercuts warm air → steep lifting → thunderstorms

Warm front

Warm air rises gradually over cold air → steady precipitation

Stationary front

Boundary between air masses that does not move