How do we measure (or empirically calculate) the amount of entropy in the atmosphere and oceans? Here's a paper on the entropy budget1, where moist entropy is defined in equation (8) as


  $$
s = (1-q_t)(C_{pd} \ln T - R_d \ln p_d) + q_t C_l \ln T + \frac{q_v L_v}{T} - q_v R_v \ln \mathcal{H} 
$$
  
  In this expression, $q_v$ is the speciﬁc humidity for water vapor, $C_{pd}$ is the speciﬁc heat at constant pressure of dry air, $C_l$ is the speciﬁc heat of liquid water, $T$ is the temperature of moist air, $R_d$ and $R_v$ are the gas constants of dry air and water vapor, $p_d$ is the partial pressure of dry air, $\mathcal{H} = e/e_s$ is the relative humidity, $e$ is the water vapor pressure, and $e_s$ is the saturation vapor pressure.


I'm just curious: is entropy directly measured, or indirectly measured from other sources? If we measured it, what would be the spatial-temporal resolution of such a measurement?



1 Pauluis, Olivier, and Isaac M. Held. "Entropy Budget of an Atmosphere in Radiative–Convective Equilibrium. Part I: Maximum Work and Frictional Dissipation." Journal of the Atmospheric Sciences 59: 125-139.
There is a convention for moist air entropy built from dry air entropy being 0 at T = 273.15K and P = 101325Pa, and liquid water entropy being 0 at its triple point. See the page numbered 47 of this NIST document:

http://www.nist.gov/calibrations/upload/5241.pdf

Entropy is not measured directly.  

Entropy $S$ is defined as $dS \equiv dQ_{rev}/T$

Therefore, entropy can measured by measuring heat transfer and temperature. 

See Calculation of Entropy Change in Some Basic Processes for more information.