How are the different probability levels described in UKCP09, and why?

The IPCC have developed qualitative terms to describe the % ranges.

In detail

The IPCC have allocated qualitative terms to these quantitative projections:

• Virtually Certain > 99%
• Very Likely > 90%
• Likely > 66%
• About as likely as not 33 to 66%
• Unlikely < 33%
• Very Unlikely < 10%
• Extremely Unlikely <1% 

In UKCP09 we have adopted this philosophy and assigned qualitative terms to different cumulative probabilities for the UKCP09 reports. Stakeholder consultation resulted in the presentation of 3 probability levels in the reports:

• Very likely to be greater than/very unlikely to be less than = 10%
• Central Estimate = 50%
• Very likely to be less than/very unlikely to be greater than= 90%
• Range between 10% and 90% = wider range