Definition
A composite weather radar display assembled by stitching together returns from multiple individual NEXRAD radar sites into a single seamless picture covering a wide geographic area. Because the image is built from many separate radar scans taken at slightly different times and angles, it is not a real-time snapshot of one radar but a blended product that may lag actual conditions by several minutes.
Plain English
A wide-area weather picture pieced together from many radar stations, shown as one continuous map. It looks like a single image, but it is really many radar pictures combined, and it is always a few minutes behind what the weather is actually doing right now.
Context Anchor
Seen on electronic flight displays or weather displays that show NEXRAD datalink weather.
Derivation
From the Latin musaicum, meaning 'work of the Muses' — originally artwork made by fitting together many small tiles to form one larger picture. The aviation use keeps that idea exactly: many small radar pieces fitted together into one larger weather image.
Why Pilots Care
Gives pilots a broader view of storm systems and precipitation beyond the range of any single radar site.
Analogy
It is like a large map made from many photos. Each photo covers one area, and the full map appears only after the pieces are put together.
Grounding Statement
Radar sites spaced across the country feed their scans into one unified image so nothing is missed between stations.
Intuition Check
Do not assume a mosaic image is a live radar picture from your airplane. It is a combined, processed image from ground radar sites, and it may not show the weather exactly as it is at this moment.
Example Sentence 1
On the cross-country leg, the pilot used the NEXRAD mosaic image to plan a wide deviation around a line of thunderstorms building over the route.
Example Sentence 2
Pilots use the mosaic image to decide whether to deviate around weather that lies outside any single radar's coverage.