Trends in Fog Frequency and Duration in the Southeast United States

1. Trends in Fog Frequency and Duration in the Southeast United States Gloria Forthun, Michael Johnson, William Schmitz, and Jim Blume Southeast Regional Climate Center

2. Questions • Are the number of fog event days changing in the southeast? • Are the number of fog event days changing seasonally in the southeast? • Are the lengths of fog events changing in the southeast?

3. Definition of Dense Fog and Data Source • Dense fog is defined as horizontal visibility of ¼ mile or less (NWS) • TD3280 dataset of hourly observations for first-order stations

4. Assumptions • A fog event day occurs when there is at least one hour that reports dense fog. • If 12 or more hours of reports are missing in a day, it is excluded. • If a fog hour has precipitation, it is excluded. • If a time period has a set pattern of reporting, it is excluded. • If 50% or more of a year is missing, it is excluded. • Years with missing days are prorated. • Stations with 1/3 or more missing years are excluded.

5. Methodology • Variance from inter-annual variability is a much larger component of the total variance than the variance explained by long term trends  • To isolate any trend signal a temporal smoothing approach is used to reduce the degrees of freedom for significance testing • A t-test of the difference between the first 15 years of the data and the last 15 years of the data is used • Simple linear regressions of the averages of the independent samples of 5 year non-overlapping periods and the running means of overlapping 5-year moving windows are used • If 2 or more years missing then 5 year period is excluded.

7. Results from T-Test

8. Results from Linear Regression

13. Future Work • Examine the seasonal variability of fog event days for trends • Examine the duration of fog events for trends • Examine other factors that may influence fog trends

14. Thank you. Special thanks to Mike Palecki and Justin Glenn