The`term MESO is short for mesocyclone, which is generally a radar term. A`mesocyclone is a rotation found within the tower of a strong thunderstorm, and can sometimes lead to the formation of a tornado, or a tornado vortex signature on radar.

When radar detects a mesocyclone, it is good practice to watch for time continuity between volume scans, a diameter of two to six miles, and an extension vertically of over 10,000 feet. These characteristics can only be found in velocity imagery. Nonetheless, the mesocyclone algorithm is fairly good at making sure all of the needed criteria are roughly met. One thing that can be watched from the composite reflectivity radar `image is the time period at which a mesocyclone is present. The longer the period a rotation is sustained within the head of a storm, the better chance there is a mesocyclone is active within that given cell.

One of four key words can be found within the MESO column of the storm attribute table.

• NONE – A mesocyclone is likely not present. Since the radar, through several tilts, can penetrate many layers where a mesocyclone resides in a thunderstorm, it is unlikely that a mesocyclone would go undetected.
• UNCO – "Uncorrelated Shear" – A weak and insignificant mesocyclone may be present. Given the sensitivity of the algorithm used for detecting mesocyclones, there is a possibility that minimal rotation within the cloud needed for a future severe thunderstorm is not even present. Rotation is found in only one radar slice vertically.
• 3DCO – "3D Correlated Shear" – A moderate mesocyclone is possible, and some inner-cloud rotation has been detected on at least two radar slices vertically. A 3DCO needs to be watched for persistence and growth.
• MESO – Well-developed rotation has been detected on at three or more radar slices for the full mesocyclone indication. The thunderstorm it is attributed to may be severe given the mesocyclone is persistent.

Since the algorithm which detects mesocyclones is sensitive, false mesocyclones can at times be detected along frontal boundaries and squall lines, or where a definite wind shift is present, especially when the front edge of a squall line is parallel to the radar beam. Therefore, a mesocyclone on radar is not always an indication a storm is severe.

Radars can also sometimes falsely detect a mesocyclone if a thunderstorm is directly over a radar site. On the flip side of that, a radar cannot detect mesocyclones well, or possibly at all, if a thunderstorm is close to the radar's transmitter. Beams from the radar's transmitter may not enter into the updraft at or near the proper angle to make accuracy possible. A lesser, or possibly absent, mesocyclone classification is displayed as a result.

Mesocyclones should not be considered a formation that is viewable, as tornadoes are. Some cloud features may suggest that a mesocyclone is present in a thunderstorm, such as curved feeder or inflow bands.