The Dependence of Emissivity on Sand Moisture Content

1. The Dependence of Emissivity on Sand Moisture Content By: Emily Teske Meteorology Department of Marine and Environmental Systems Florida Institute of Technology

2. Objective • Measure the Emissivity of Wet, Damp, and Dry sand • Compare these measured emissivities to previous studies • Dry = 0.90 and Wet = 0.94-0.96

3. Emissivity • Measure of how efficiently an object radiates energy compared to a blackbody (=1) at the same temperature and a given wavelength • Emissivity = 1 means that object radiates all energy absorbed • The smaller the emissivity value the more energy absorbed

4. Motivation • No known published values for damp sand • Emissivity important for the surface energy budget (skin temperature) • Important for remote-sensing • Biological impact (flora and fauna of the dune environment)

5. Sampling Methods • Use two-meter dipstick and handheld IR thermometer to measure the temperature of the sand. • Temperature of wet, damp, and dry sand was taken Image From: http://www.az-instrument.com.tw/en/productsinfo/135.html

6. IR Thermometer gives temperature at the surface. Temp Probes Measure temperature 1cm below the surface.

7. What the IR Measures • IR thermometer measures the irradiance (I) of the surface for a given wavelength window • Uses the Stefan-Boltzmann Equation to output a temperature • Where ε is the pre-set emissivity of the IR thermometer (=1) • The temperature may be wrong depending on what the actual emissivity of the surface is

8. How to get Emissivity • Use the temperature probes to get a temperature not dependent on a known emissivity • Use the irradiance (I) from the IR thermometer and temperature (T) from temperature probes to calculate an actual emissivity (ε) Irradiance Measured by IR Temp measured By Probes

9. Data Analysis • Temperature correction was needed since two-meter dipstick temperature probes were slightly under the surface (~ 1 cm) • Additional Correction was needed for cloud cover • The adjusted temperatures were then used to calculate emissivity

10. Before sunrise Peak solar Our Measurements From: Tang et al 1999

11. Location of temp probes ΔT

12. Temperature Correction • A temperature soil profile model was used to adjust temperatures • The ΔT equation below was used • ΔT is the difference between the surface temperature and the temperature at depth , z, below the surface • A is the diurnal amplitude (°C) of the temperature • A=30 for dry sand, 15 for damp sand, and 10 for wet sand

14. 4.45 °C • The measured temperature was 49.9°C and emissivity 0.963. • The adjusted temperature is 54.3°C lowering emissivity to 0.910

15. Cloud Correction • assumed that the full A (amplitude) of the temperature correction would be too large in the presence of clouds • A ratio of the actual-theoretical solar radiation was taken • This ratio was then multiplied by the ΔT value obtained from plot

16. ΔT = 5.25°C New ΔT = 4.6°C Avg = 825 Wm-2 Avg = 688 Wm-2

17. Cloud Correction Example • ΔT without cloud correction = 5.25°C • Actual-theoretical ratio found to be 0.83 • New ΔT = 4.36° • The adjusted temperature would then be 57.4°C rather than 58.25°C (ε= 0.914,0.890)

18. Wet Damp Dry Wet Damp Dry

19. Conclusion • Measured emissivity values were slightly higher than published values • Temperature corrections improved emissivity estimates • The damp sand emissivity fell between that of wet and dry sand as expected • Other factors to be considered: sand moisture content, sand grain size/type, color

20. Questions? Next: Jeremy Fimat