Frequently, during winter, as cold polar air behind mid-latitude cyclone moves over warm ocean, the air mass becomes destabilized through warming at the ocean surface. Distinctive cloud patterns form in this cold air outbreak.

Cloud streets: Cloud streets are narrow plumes of cumuliform clouds that develop in strong straight low-level, and cold winds flows across warmer waters. They are typically observed in an unstable layer that is 1-3 km thick with small wind shear and a low level temperature inversion. The cloud streets align with the direction of the wind. Cloud top heights are low at first, then increase farther offshore as the marine inversion is lifted by sensible heat transfer into the boundary layer. The streets become wider as the convection becomes deeper. Precipitation is generally absent. As a rule of thumb, the spacing between cloud streets is approximately 5 times the cloud top height.

Closed cells: Typically found in subsiding air. As a result, the unstable air near the ocean is capped by a subsidence inversion, causing the cumulus to flatten out and become stratocumulus. Closed cells are often associated with anticyclonic flow near the surface (e.g. in the southeastern quadrant of anticyclones in the N.H.) Closed cells usually indicate surface winds less than 25 knots. Sometimes brighter, thicker strands are observed within the closed cells. These are often associated with light precipitation and reduced visibilities.

Open cells: The cloud patterns typically have cloud-free centers with a cloudy periphery. Open cells often form when there is strong instability below the marine inversion, as in cold advection situations to the rear of an oceanic cold front. In weaker flow situations, they form circular or elliptical rings, while in strong flow, open arc lines are most likely. Open cells usually indicate surface winds of greater than 25 knots. In areas of open cell convection, the diameter of the cells is approximately 15 times the cloud top heights. In a mid-latidue cylcone over warm water, they can occur in the cyclonic low-level flow or ahead of the upper-level trough. Open cell cloud formation is a regular occurrence on the back side of a low pressure system in the mid-latitudes. In the northern hemisphere on the back side of the low pressure center, cold air will be drawn in from the north, and on the front side, warm air will be drawn up from latitudes closer to the equator. This cold air advection occurs over warmer waters and open cell cloud formations often result. (Closed and open cells in a satellite image)

The jet stream is often 1-3 degrees poleward of the boundary between closed and open cells.

Jet stream in relation to open and closed cells

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