How does the moisture index change across Oklahoma?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: The Thornthwaite Moisture Index (TMI) is a single number that describes how wet or dry a climate is by weighing rainfall against the water the climate can evaporate. Oklahoma grows drier from east to west, and state evaporation data shows the western half losing roughly 10 to 16 more inches of water each warm season than the east. Drier climates generally drive seasonal soil drying deeper into clay.

What is the Thornthwaite Moisture Index?

The TMI compares a location's yearly precipitation with its potential evapotranspiration, the amount of water that would evaporate from soil and pass through plants if water were always available.

• A positive TMI means rainfall exceeds that demand (a moist climate).
• A value near zero means they are roughly balanced.
• A negative TMI means demand exceeds rainfall (a dry or semi-arid climate).

Foundation engineers use the TMI in slab design methods for expansive clay, such as those used for post-tensioned slabs, to estimate how far soil moisture will change under and around a slab. The idea connects directly to the active zone, the layer of soil whose moisture changes with the seasons.

Climate classes

Thornthwaite's 1948 method computes the index from a yearly moisture balance as TMI = (100R − 60D)/PE, where R is water surplus (runoff), D is moisture deficit and PE is potential evapotranspiration ⁵. An ODOT-sponsored study by Oklahoma State University and the University of Oklahoma used these class limits ⁵:

The same study notes that TMI is widely used in geotechnical and pavement engineering, and that it has been correlated with equilibrium soil suction (how tightly dry soil holds water) below the zone of seasonal moisture change ⁵.

How does Oklahoma's climate change from east to west?

No verified city-by-city TMI table was found for this page. The ODOT-sponsored study instead mapped TMI statewide with three calculation methods and found that the zero TMI line runs across central Oklahoma, so the climate there ranges from moist sub-humid to dry sub-humid ⁵. The study describes Oklahoma's climate overall as ranging from humid to semi-arid ⁵. Other Oklahoma climate data shows the same east-to-west drying trend.

Evaporation increases to the west

OSU Extension fact sheet BAE-1529 lists average May to October evaporation from shallow water bodies by climate division ¹:

The fact sheet notes that losses increase moving west and can be higher during drought ¹. That difference of about 10 to 16 inches over six months reflects much higher evaporative demand in western Oklahoma ¹.

Climate controls drying depth

An Oklahoma State University study using Mesonet soil-suction data estimated the depth of seasonal moisture change at about 0.5 to 4 meters across the state, depending on soil properties and climate, with the shallowest soil most sensitive to climate ². A drier climate (lower TMI) pushes the soil toward drier conditions more of the year.

Drought can override the averages

Averages hide extreme years. A 1961 study applying Thornthwaite's classification found desert (arid) conditions somewhere in Oklahoma in 1933 to 1937, 1954 and 1956; in 1956 they covered the whole Panhandle and part of northwestern Oklahoma ⁶. In August 2012, about 97% of Oklahoma was in extreme or exceptional drought, and north-central Oklahoma had its driest May-to-early-August since 1921 ³. In May 2015, by contrast, the statewide average rainfall of 14.40 inches set the record for the wettest month in Oklahoma history ⁴. Swings like these matter as much as the long-term index for how much clay moves; see drought and wet years.

What does this mean for each region?

• Tulsa and northeast Oklahoma sit at the wetter, lower-evaporation end of the state ¹. See Tulsa metro.
• Oklahoma City lies in central Oklahoma, near the zero TMI line between moist and dry sub-humid climates ⁵. See OKC metro.
• Southwest Oklahoma and the Panhandle have the highest evaporation ¹. See southwest Lawton and Red Plains and Panhandle High Plains.

Related

• Active zone and drought depth
• Drought and wet years
• Shrink-swell and active zone
• Post-tensioned slabs
• Mesonet for foundation pros