Highs, lows, troughs, ridges and cols

How do you tell a trough from a ridge on a chart?

The five patterns

One pattern per step, northern hemisphere. Isobars every 2 hPa.

1. Low, depression, cyclone An area ringed by an isobar with the lowest pressure in the middle. Two or more closed isobars (2 hPa apart) make a depression; deeper still, a cyclone. Northern hemisphere: winds blow anticlockwise and converge towards the centre, so air rises: bad weather, but visibility is better than in a high. Winds back.
2. Trough A tongue of low pressure stretching out of a low. Pressure along it is lower than on either side and the isobars are V-shaped. Crossing it the wind changes suddenly and backs. Bad weather. The monsoon trough over the Indo-Gangetic plains is one.
3. High, anticyclone Highest pressure in the middle. Winds blow clockwise, veer and spread outwards. The sinking air gives fair weather, but traps haze, so visibility is poor. A high that sits still and blocks the path of passing lows is a blocking anticyclone. A cold anticyclone forms in the cold air behind a departing low; its sinking air warms it with time.
4. Ridge A wedge of high pressure pushing out from a high. Pressure along it is higher than on either side, the isobars are rounded (not V-shaped), the wind veers and the weather is fair.
5. Col The region between two highs and two lows, set crosswise. The pressure is nearly uniform, the winds light and variable, and the weather a mix of both. Trough: lower pressure than either side, winds back. Ridge: higher, winds veer.

Memory hook: Lows back, highs veer.

Takeaway: Low: anticlockwise, converging, bad weather, better visibility. Trough: V-shaped isobars, wind backs. High: clockwise, diverging, fair but hazy. Ridge: rounded isobars, wind veers. Col: between two highs and two lows, light variable winds.

What do highs, lows, troughs, ridges and cols look like?

Pressure as a landscape

Pressure drawn as height: highs are hills, lows are valleys. White lines are isobars, 2 hPa apart.

• Pressure as height Here the height of the surface is the sea-level pressure. The blue hills (H) are high pressure, the violet valleys (L) low pressure, and each white contour is an isobar.
• A low A low is a valley: lowest pressure in the middle. In the northern hemisphere the wind blows anticlockwise round it (follow the moving arrows) and converges, so air rises and the weather is bad.
• A high A high is a hill: the wind blows clockwise round it (follow the moving arrows) and spreads out, air sinks and the weather is fair.
• Trough and ridge A trough is a tongue of low pressure running out of a low: a valley line. A ridge is a tongue of high pressure: a spur off a hill.
• Col A col is the saddle between two highs and two lows. The pressure is nearly even and the winds light and variable.

Takeaway: A low is a valley, a trough a valley stretched out, a high a hill, a ridge a spur running out from it, and a col the saddle between two highs and two lows.

Which pressure pattern is it?

Highs, lows and cols

Tap a clue, then the pattern.

• Winds converge and air rises
• Bad weather, but better visibility
• Clockwise winds in the NH, diverging
• Fair weather, but poor visibility
• V-shaped isobars, winds back sharply
• Rounded tongue of high pressure, winds veer
• Between two highs and two lows, light variable winds
• The monsoon trough over the Ganga plains

Takeaway: Low: anticlockwise (NH), converging, rising air, bad weather but better visibility. High: clockwise, diverging, sinking air, fair weather but poor visibility. Trough: V-shaped, winds back, bad weather. Ridge: rounded, winds veer, fair. Col: between two highs and two lows, light variable winds.

What does the spacing of isobars tell you?

Gradient and tendency

Two charts and a tendency chart.

1. Pressure gradient The pressure gradient is how fast pressure changes across the isobars, measured at right angles to them and pointing from high to low. Isobars close together: a steep gradient and strong wind.
2. A flat gradient Isobars far apart: the pressure hardly changes, the gradient is flat and the wind is light.
3. Pressure tendency and isallobars The change of pressure over time is the pressure tendency: in India over the past 24 hours, in higher latitudes over the past 3 hours. Isallobars join places with the same change. The biggest fall is the isallobaric low: a surface low tends to move towards it and deepen. An isallobaric high means a system is weakening.

Takeaway: The pressure gradient is the change of pressure across the isobars, from high to low. Close isobars: steep gradient, strong wind. Isallobars join places of equal pressure change; lows move towards the isallobaric low and deepen.