A large portion of the United States is known to have problematic expansive clay soil. These expansive clay soils can cause damage to major infrastructures such as roads and lightly loaded residential buildings. The shrinking or swelling potential of unsaturated expansive clay soils requires an understanding of unsaturated soil mechanics, such as matric suction profile and the site’s environmental condition, such as climate. In unsaturated soil engineering, the most used climatic parameter is Thornthwaite Moisture Index (TMI). Since its inception, there have been several versions of TMI models in the literature. Historically, TMI is used to predict suction parameters such as edge moisture variation length, the depth to equilibrium suction, and equilibrium suction. Currently, TMI is used in Post-Tension Institute’s Slab-on-grade Design Manual (DC 10.1-08) to estimate edge moisture variation length and equilibrium suction, and Australian Standard Residential Slabs and Footing (AS2870-2011) to predict the climatic zone and the depth to suction change. However, there is no clear-cut guidance on which version of TMI models to use, how the variables within TMI should be collected, the length of the study period for determination of TMI, or assumptions and compromises associated with TMI estimation methods. In this thesis, broad-scale study and comparison of the original TMI (1948) to the newer TMI models for the contiguous United States are conducted as well as in-depth analysis of the variables within TMI, using National Oceanic and Atmospheric Administration’s (NOAA) dataset and Geographic Information System (GIS). The results of the study, the recommendations for the state of practice for TMI and further research are discussed.