Fronts & airmasses

050 MeteorologyChapter 5

Schematic

Cold fronts move faster than warm fronts; warm sector rises over retreating warm air.

Air mass naming

Key fact

Named by temperature (E/T/P/A) + moisture (c/m). Stable at source; modified by track. mTw (Azores): warm, moist → ST/SC, drizzle, poor vis in haze. mPc: cold, moist → CB, showers, good vis. cPc (Siberian): very cold, dry → clear, frost. cTw (Sahara): hot, hazy, clear.

Key fact

Moving south: warms from below → stability decreases, RH decreases → Cu/CB/TS. Moving north: cools from below → stability increases, RH increases → ST/SC, drizzle. Sea track adds moisture; land track dries air.

Front types

Definition — Warm front

Warm air overrides cold; slope ~1:150–100. Clouds ahead: Ci → Cs → As → Ns → St over 200–700 NM. Continuous moderate rain/snow; fog risk ahead. Speed ≈ ⅔ geostrophic wind. Pressure: falls before, lowest at passage, steady/slight rise after.

Definition — Cold front

Cold air undercuts warm; slope ~1:10–80. Cu/Cb, showers, TS possible; weather belt 50–100 NM behind surface. Wind veers with height in NH. Pressure: falls before, rises sharply after passage.

Definition — Occlusion

Cold front catches warm front. Warm occlusion (winter): coldest air ahead; more common winter. Cold occlusion (summer): cold air behind undercuts. Stationary front: wind parallel to front; little perpendicular motion.

Warm sector & jet stream position

Key fact

Warm sector: Tm air, ST/SC, drizzle, mist, moderate visibility; summer fair Cu possible. Jet stream: 300–450 NM ahead of warm front; 50–200 NM behind cold front; crosses occluded fronts. Use Buys Ballot at jet level: cold air on left (NH).

Polar front depression (PFD)

Key fact

Forms on polar front (~50–60°N); ~4 days to deepest low, 10+ days to fill. Warm front travels at ⅔ speed of cold front → occlusion. Surface winds: veer through warm then cold passage (NH). Upper winds back across fronts.

Exam trap

Risk of fog greater ahead of and behind warm front vs cold front. FZRA just in front of warm front surface position → clear ice. Very low cloud ahead of warm front from rain evaporating into cold air and warm air dragged down by precipitation.

Anticyclones

Key fact

Divergence at surface, subsidence at centre → warming adiabatically, inversion, haze/fog (winter), TS over land (summer col). Warm anticyclone (Azores): warm core, pressure bulge upward deep layer. Blocking high: deflects PFDs. Cold anticyclone (Siberian): shallow subsidence, may weaken aloft.

Other depressions & TRS

Key fact

Polar low: mPc over warmer sea, winter, non-frontal, CB/showers. Thermal low (summer land): Cu/CB/TS by day. Secondary depression: forms at cold front or triple point; moves cyclonically around primary.

Definition — Tropical revolving storm

Needs SST ≥ 27°C, 5–25° from equator, low vertical shear. Eye 10–20 NM; whole system ~270 NM. Worst: eyewall. Eye: descending air, under 63 kt, warmer than surroundings. Typhoons NW Pacific most frequent. Decays over land/cold water. Hurricane N Atlantic: ≥ 64 kt, Jun–Nov.

Climate & monsoon (summary)

Key fact

ITCZ moves north NH summer (Jul), south NH winter (Jan); more movement over land. Monsoon: wind direction change ≥ 120° with ITCZ shift. Darwin: NW monsoon Jan (CB/TS), SE monsoon Jul (hot, dry, dusty). Harmattan (Dakar Jan): NE dry dusty; SW monsoon Jul with TS.