Drift card

From Glossary of Meteorology

drift card

A card, such as is used in a drift bottle, encased in a buoyant, waterproof envelope and released in the same manner as a drift bottle, for the purpose of estimating the surface current in a body of water.

Cheaper and lighter than bottles, drift cards are especially suited to dropping in large quantities from aircraft, and it is assumed that the card, having less freeboard than a bottle, is less affected by wind.