Twentieth century trends in dew point temperature throughout the Upper Midwest Jesse Wartman

Why are dew-point temperatures important? Important in forecasting Used for specific and relative humidities Regulate transpiration and evaporation processes Key factors in the surface energy and hydrological budgets Water vapor is the strongest contribution to the greenhouse effect

Past Studies Gaffen and Ross (1999) Increase through spring and fall Robinson (1998) Dew points were lower in the Upper Midwest during the winter Robinson (2000) Increase of 0.9°F per 100 years over U.S. Increase over spring and fall

Possible Errors Non-uniform stations Technology and instrument changes 1950s - sling psychrometers 1960s – dial hygrothermometers Most recent – HO-83 dial hygrothermometer (NWS)

Hypotheses Dew-point temperatures have experienced significant, positive trends over the past 44 years in the Upper Midwest. Those dew-point temperature tendencies are no different from trends that have been found across the rest of the United States in past studies.

Data and Methods Hourly obs through IEM from the NCDC Monthly Seasonal Winter - December, January, & February Spring - March, April & May Summer - June, July & August Fall - September, October, & November Yearly Decadal

Observation Stations Legend of Cities St. Louis Kansas City Omaha Des Moines Sioux Falls Minneapolis

Missing Data Sioux Falls – 0% Minneapolis – 18% - data gap – decadal averages were substituted in, no significant change so left out Omaha – 30.11% - no data before 1974 Des Moines – 1.33% - data gap Kansas City – 27.27% - no data before 1973 St. Louis – 27.27% - no data before 1973

Significance Significant if P-value < 0.05 Semi-significant if P-value < 0.1 P-Value – calculated in JMP, observed significance probability from t-ratios T-ratio – tests hypothesis that each parameter is zero, ratio of the parameter estimate to its standard error

RESULTS Monthly Climatic Trends Seasonal climatic trends Yearly average climatic trends Decadal climatic trends Twenty-two year climatic trends Extreme days Precipitable water

Monthly Climatic Trends P-Value < 0.05 – P-value < 0.1

Seasonal Climatic Trends T d (°F)

Seasonal Climatic Trends T d (°F)

Seasonal Climatic Trends T d (°F)

Seasonal Climatic Trends T d (°F)

Yearly Average Climatic Trends T d (°F)

Yearly Climatic Trends Averages Kansas City:.128* Sioux Falls:.1* Minneapolis:.083* Des Moines:.077* Omaha:.064 St. Louis:.0084 Total ~ 7.54°F per 100 years 0.9°F per 100 years (Robinson, 2000)

Decadal Climatic Trends T d (°F) T d (°F)

Decadal Climatic Trends T d (°F) T d (°F)

Twenty-two year trends Omaha: -64°F * St. Louis: -13°F Des Moines: -5 °F Minneapolis: -4°F Kansas City: -3°F Sioux Falls: 4°F Omaha: 12°F * St. Louis: 13°F * Des Moines: 11°F * Minneapolis: 13°F * Kansas City: 14°F Sioux Falls: 11°F

Twenty-two year trends

Pacific Decadal Oscillation (PDO) Monthly temperature anomaly (°C) Years Monthly values for the PDO index (Jan Oct. 2006) (Mantua, 2000)

Extreme days: dew points over 70°F

Extreme days: dew points over 75°F

Precipitable Water Surface Vapor Pressure: P o = 1mb*e {1.81+(17.27*D)/(D+237.3)} Precipitable Water: h = P o /(ρ w *g)

Precipitable Water

Implications

Implications Precipitation Heat stress Crop production Soil errosion Water supplies Human health Heat waves

Concluding Remarks Shift from negative trend to positive trend Positive overall trend Hypotheses Dew-point temperatures have experienced significant, positive trends over the past 44 years in the Upper Midwest. -> TRUE Those dew-point temperature tendencies are no different from trends that have been found across the rest of the United States in past studies. -> FALSE 7.54°F over 100 years in Upper Midwest 0.9°F over 100 years in United States

Future Work Modelling studies Future increases? Increase in greenhouse gases PDO El Ni ñ o Atlantic Oscillation Other factors

Acknowledgements Daryl E. Herzmann Eugene S. Takle Jon Hobbs

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