Depressions, Fronts and Synoptic Charts

How a temperate depression forms

In short: a temperate depression is born where cold and warm air meet and start to mix. It grows, brings fronts, then fills and dies.

Read first: this is the third topic of the Meteorology area. Read Pressure, Wind and the Coriolis Effect, then Humidity, Dew Point and Fog first. Next: Weather Instruments and Observations.

• In mid-latitudes, cold polar air and warm tropical air flow side by side in opposite directions. Their boundary is the polar front.
• Turbulence between the two opposing airflows of different temperature makes a wave on the boundary.
• Warm air pushes into the cold side and rises over it. Pressure falls at the tip of the wave and a Low forms. The air starts to turn round it (anticlockwise in the NH).
• The wedge of warm air between the two fronts is the warm sector. Its leading edge is the warm front, its back edge the cold front.
• The cold front moves faster than the warm front. It closes the warm sector and lifts the warm air off the surface: the front becomes occluded.
• After occlusion the Low usually stops deepening, slows down and fills (pressure at the centre rises).

Depression or hurricane?

• A temperate depression has fronts, drawn on synoptic charts.
• It is generally larger and moves faster than a hurricane.
• A hurricane (tropical revolving storm) forms over warm tropical sea and has no fronts. It is smaller, but its winds near the centre are usually much stronger. See the TRS topics.
• A deep temperate depression can still give hurricane force winds (Beaufort Force 12).

How depressions move

• In the NH mid-latitudes most depressions travel roughly west to east; most UK weather systems form over the Atlantic (Met Office).
• A young Low tends to move in the direction of the isobars in its warm sector.
• On the bridge: plot the Low’s position from successive charts. Its track and speed tell you which side of you the centre will pass, and when.

Warm, cold and occluded fronts

In short: a front is the boundary between two air masses of different temperature and moisture. Where the warm air is forced up, you get cloud and rain.

• The symbols are drawn on the side towards which the front is moving.
• Cold fronts move faster than warm fronts. That is why occlusion happens.
• Waving front: a wave on a cold front can slow it down and give long spells of rain; models find it hard to forecast (Met Office).
• On the bridge: the warm front gives you hours of warning (high cloud, falling glass). The cold front gives little warning: watch for a dark line of cloud and get ready for a sudden wind shift.

Passage of a Low: what the ship sees

In short: as a frontal Low passes, the barometer, wind, cloud, rain, temperature and visibility change in a fixed order. Learn the order and you can tell where the fronts are from the bridge.

• The sequence below is for the Northern Hemisphere, with the centre passing to the north of the ship (the usual case, through the warm sector).
• In the Southern Hemisphere the pattern is mirrored: where the NH wind backs, the SH wind veers, and the other way round.
• If the centre passes to the south of you (NH), you stay on the cold side: no warm sector, and the wind backs steadily the whole time.

Two details that often go wrong

• Warm sector cloud: you may see nimbostratus listed for the warm sector, with the wind veering and freshening. Standard meteorology places nimbostratus at the warm front; in the warm sector the wind veers and then holds steady, under low stratus or stratocumulus with drizzle and poor visibility. Give the standard answer in the oral.
• Rain ahead of the warm front: it starts as light or intermittent rain and becomes moderate to heavy as the front gets close. It eases or turns to drizzle once the front has passed.

On the bridge

• Log pressure, wind, cloud and visibility every watch. A falling glass with a backing wind and lowering cloud = a warm front is on its way.
• Warm sector: poor visibility — think about restricted-visibility measures and the fog topic.
• Before the cold front: secure the deck, warn the Master and the watch, expect a sudden veer and squalls. Adjust course and speed early.
• Use Buys Ballot’s law to place the centre: NH, back to the wind, the Low is on your left.

Reading a synoptic chart

In short: a synoptic (surface pressure) chart is a snapshot of pressure and fronts over a wide area at one time. Read it in a fixed order.

• 1. Heading: area, date and time in UTC, and whether it is an analysis (observed) or a forecast chart. Met Office surface pressure charts are updated about every 12 hours, around 0730 and 1930 UTC (Met Office).
• 2. Isobars: solid lines of equal pressure, at a fixed interval stated on the chart. Close together = strong wind; far apart = light wind.
• 3. Highs and Lows: marked H and L with their central pressure. Note where they are, how deep, and where they moved since the last chart.
• 4. Fronts: warm (red, semicircles), cold (blue, triangles), occluded (purple, both).
• 5. Other features:
  • Trough — a line of low pressure out from a Low; often unsettled weather.
  • Ridge — a tongue of high pressure out from a High; usually fair weather.
  • Col — the slack area between two Highs and two Lows; light winds, sometimes fog.
• 6. Wind: NH — anticlockwise round a Low, clockwise round a High, roughly along the isobars and turned slightly in towards the Low. Strength from the geostrophic wind scale.
• 7. Your ship: mark your position and route. Work out which way the wind will shift and when the fronts will reach you.

The geostrophic wind scale

In short: the scale printed on the chart turns isobar spacing into a wind speed. It gives the wind above friction, so reduce it for the sea surface.

• The geostrophic wind blows parallel to the isobars, above the layer where surface friction acts (roughly the lowest 500–1000 m).
• How to use the scale:
  1. With dividers, measure the distance between two neighbouring isobars, at right angles to them, at your position.
  2. Lay the dividers on the geostrophic scale at your latitude (the same spacing gives a stronger wind at low latitudes).
  3. Read off the geostrophic wind speed in knots.
• The surface wind is weaker than the geostrophic wind, because of friction. Friction also turns it across the isobars, in towards the Low.
• Curved isobars change the real wind a little (stronger round a High, weaker round a tight Low). Treat the scale as a guide.
• On the bridge: compare the chart wind with your own anemometer and the sea state. If they disagree, trust your observations and check the next chart.