Availability is the most essential attribute of qualities of cloud services. Most popular public cloud services claim availability commitments with corresponding penalties in their SLAs. However, lack of clarity in availability commitment and penalty make it hard for consumers to understand the SLAs well and furthermore, to compare different cloud providers under different contexts, which would even become an obstacle for enterprise consumers to embrace public clouds. We present a cloud SLA availability commitment framework including availability calculation and penalty calculation services, and compare SLAs of well-known public IaaS cloud providers with investigation of their merits and defects. We also present a business model for cloud providers to find the optimal penalty degree for their SLAs, which will help in defining availability based SLA of cloud services.