An undercurrent is a type of current which runs below the surface of air or water currents. The direction of an undercurrent is typically opposite that of surface currents, and the strength of the undercurrent varies, depending on the situation and the circumstances. Meteorologists often consider undercurrents when making predictions, and the study of undercurrents is an important part of the field of oceanography as well, since undercurrents play a major role in the water cycle which mixes the world's oceans.
In terms of meteorology, undercurrents can have a dramatic effect on the weather, pulling clouds and storm systems in unexpected directions. Many undercurrents are charted, so meteorologists can account for them when examining weather phenomena, while others may arise spontaneously in response to changing climatic conditions, potentially wreaking havoc. Undercurrents are part of the larger system of air circulation and patterns which creates global weather, and explains why storm systems move in the way that they do, and how the weather of various regions on Earth is created.
In bodies of water, multiple undercurrents can sometimes be found under surface currents. In the open ocean, undercurrents tend to remain very stable, making them easy to study. The largest undercurrent system is that which drives the thermohaline circulation system of the ocean, which brings upwelling lightweight, warm water from the equator to the poles, where it slowly becomes colder and more dense, sinking to the bottom and then flowing back towards the equator, where it will upwell again thousands of years later.
Surface ocean and air currents actually interact with each other; you can look at a chart of ambient air currents mapped over ambient ocean currents for an illustration. By studying the pattern of surface currents in an area, scientists can sometimes make predictions about the undercurrents, also sometimes known as sub-surface currents. These predictions are usually based on a pattern of recorded undercurrents in the region, along with a consideration of the factors that may affect these currents.
Swimmers have to be especially careful of undertows, undercurrents which run in the opposite direction of surface currents. An undertow can sometimes be much stronger than a surface current, and very unexpected. Beaches with known undertows and rip currents usually post signage to warn people to be careful in the water. Some areas have well known undertows; along the coast of North America, for example, a powerful undercurrent runs parallel to the shoreline. This undertow sometimes switches directions, making it hard to recover people and cargo lost close to the shore.