Another phenomenon directly associated with the trough is the abrupt change in the direction of the wind, called a windshift. The wind is often from the south ahead of the trough, and gradually turns to blow from the northwest behind the trough. It is precisely this strong convergence that causes temperature gradients to tighten up and creates fronts in troughs. A high, or anticyclone, is the opposite of a low. In this case, the central area is a zone of maximum high pressure.
Isobars are generally wider apart near the centre of a high than near the centre of a low.
A high forms when its central pressure rises, and is strong when its central value is significantly higher than the surrounding values. In our latitudes, highs are strongest in winter of the cold air. Pressure can sometimes exceed 1040 hPa. Fine weather is generally associated with highs, because the dry cold air of the high is systematically pushed down toward the ground. A high weakens when its central pressure falls.