ARHAB is the study and use of high altitude balloons to explore near space. Near space is the region of atmosphere between 60,000 feet and the accepted boundary of space at 328,000 feet altitude. These altitudes make near space far more like Earth orbit than the surface of the Earth. Air pressure in near space reaches 99% of a vacuum of better and temperatures drop to a low of -60F or colder. Cosmic radiation is over 100 times greater than at sea level. Hobbiests and citizen scientists alike use balloons to send payloads to near space, recording images and scientific data as they go. ARHAB.org is the primary location for posting and tracking upcoming high altitude balloon missions.
For more information on ARHAB, this Wikipedia entry is a great introduction. Or read the UKHAS Beginners Guide to High Altitude Ballooning to start planning your own flight. The APRS.org Wiki hosts a page dedicated to ARHAB with some additional information.
ARHAB Community
The ARHAB community is active across the world:
In the US, the annual Great Plains Super Launch is the largest gathering of ARHAB enthusiasts.
Tracking and Prediction Sites
Below are some recommended tracking sites:
Admin
The ARHAB.org site is maintained by Astra West W5MAW. Previously the site was maintained by Keith Kaiser WA0TJT.