NATARAJADepressions, fronts & the weather they bring
Learning outcomes & permitted supports
- Follow a depression through its life cycle from wave to occlusion, and say what kills it
- Recite the weather sequence of a frontal passage — pressure, wind, cloud, rain and visibility — from the ship's point of view
- Use the fact that in the northern hemisphere BOTH frontal passages veer the wind
- Contrast the two fronts by slope, cloud, rain, duration and what follows them
- Recognise the warm sector's fog risk, and read an anticyclone, a ridge and a trough
Supports in this lab: No instruments — a barometer and the sky. The frontal sequence is the classical one for a ship passing on the equatorward side of the centre in the northern hemisphere. Golden 1031/1031.
Glossary: frontal depression · warm front · cold front · occlusion · warm sector · barometric tendency
How a depression is born
Along the polar front, cold polar air lies against warm tropical air. A small kink in that boundary lets warm air push north and cold air push south — and Coriolis turns the kink into a circulation. The wave deepens, wraps the warm air into a warm sector between a warm front ahead and a cold front behind, and the whole system runs off to the east along the polar front.
| 1 · Wave | a kink on the polar front; pressure starts to fall |
|---|---|
| 2 · Open | the classic depression: warm front ahead, cold front behind, warm sector between |
| 3 · Occluding | the cold front travels faster and catches the warm one, lifting the warm sector clear of the surface |
| 4 · Occluded and filling | no warm air left at the surface, no energy source, the low fills and dies |
The sequence on watch
This is the part worth knowing by heart, because you can read it from the bridge with no chart at all. Run the clock and watch three things move together: the barometer, the wind direction, and the cloud. In the northern hemisphere, for a ship passing on the equatorward side of the centre, every frontal passage VEERS the wind — that single fact tells you a front has gone through.
Three-hourly tendency right now: -3.3 hPa — falling. That is a warning-level fall: the front is close.
Why the two fronts feel so different
A warm front has a shallow slope — about 1 in 150 — so the warm air climbs over the cold for hundreds of miles ahead of the surface line. That is why you get half a day of warning in the cloud sequence and hours of steady rain. A cold front is steep, about 1 in 50, and undercuts the warm air violently: an hour of towering cumulonimbus, a squall, a sharp veer, and then the sky opens.
| warm front | cold front | |
|---|---|---|
| Slope | 1 in 150 — gentle | 1 in 50 — steep |
| Speed | slower | faster: it catches the warm front and occludes the low |
| Cloud | layer: Ci → Cs → As → Ns, over 12 hours | convective: towering Cu and Cb in an hour |
| Rain | continuous, moderate, long | squally, heavy, brief |
| Pressure | steady fall, then the fall STOPS | sharp rise begins at once |
| Wind | veers moderately | veers sharply, with a squall |
| After it | warm sector: dull, damp, poor visibility, fog risk over a cold sea | bright, gusty showers and excellent visibility |
Highs, ridges and troughs
Between the depressions sit the anticyclones: sinking air, light winds, wide isobar spacing and settled weather — but a summer high traps haze and a winter one traps fog and radiation cooling, so "settled" is not the same as "good visibility". A ridge is a finger of high pressure giving a few hours of respite between two lows; a trough is the opposite, and often carries a squall line even without a full front.
One practical habit ties this lab to MT-01: log the barometer every watch and take the three-hourly tendency. The chart tells you what is there; the tendency tells you what is coming, and it costs nothing.
| Hour 6 — you are | Ahead of the warm front |
|---|---|
| Barometer | 1005.4 hPa, falling steadily |
| Wind | 138° at 24 kn (force 6) — backing SE, freshening |
| Cloud | cirrus → cirrostratus → altostratus → nimbostratus, thickening and lowering |
| Weather · visibility | rain setting in, continuous and heavier as the front nears · good, then moderate in rain |