| Summary: | A new, more accurate formula for calculating the net longwave radiation flux<i>LW</i> ↑↓ has been devised for the Baltic Sea region. To this end,the following sets of simultaneously measured data regarding the longwave radiation of the sea andthe atmosphere were used: the temperatures of the sea surface and its contiguous air layer,the water vapour pressure in the air above the water, and the cloud cover.These data were gathered during numerous research cruises in the Baltic in 2000-03 and were supplemented by satellitedata from Karlsson (2001) characterising the cloud cover over the whole Baltic. The formulaestablished for <i>LW</i> ↑↓ can be written in the form of three alternative equations,differing with respect to their cloud cover functions:<br><i>LW</i> ↑↓ =</font><table><tr><td>0.985σ<i>T</i><sup>4</sup><sub><i>s</i></sub> - σ<i>T</i><sup>4</sup><sub><i>a</i></sub> (0.685+0.00452<i>e</i>)</td><td><font size="+6">{</font></td><td>(1 + <i>d</i><sub><i> </i></sub>n<sup>2</sup>) average for all cloud types (Z1)<br>(1 + <i>d</i><sub><i>i</i></sub>n<sup>2</sup>) separately for low-, mid- and high-level clouds (Z2)<br>(1 + <i>d</i><sub><i>i</i></sub>n<sup>ϒi</sup>) separately for low-, mid- and high-level clouds (Z3)<br></td></tr></table><font face="Times New Roman">where σ - Stefan-Boltzmann constant; <i>T</i><sub><i>s</i></sub> - sea surface temperature [K]; <i>T</i><sub><i>a</i></sub> - air temperature [K]; <i>e</i> - water vapour pressure [mbar]; <i>n</i> - total cloud amount [0 - 1]; <i>d</i> - mean empirical dimensionless coefficient, determined for all cloud types or for particular months (see Tables 3 and 4); <i>d</i><sub><i>a</i></sub> - empirical coefficient determined for the quadratic function: <i>d</i><sub><i>1</i></sub> = 0.39 for low-level clouds, <i>d</i><sub><i>2</i></sub> = 0.305 for mid-level clouds, <i>d</i><sub><i>3</i></sub> = 0.22 for high-level clouds; <i>d</i><sub><i>i</i></sub> - empirical coefficient determined as follows: <i>d</i><sub><i>1</i></sub> = 0.39 for low-level clouds when γ<sub>1</sub> = 1.3, <i>d</i><sub><i>2</i></sub> = 0.29 for mid-level clouds when γ<sub>2</sub> = 1.1; <i>d</i><sub><i>3</i></sub> = 0.17 for high-level clouds when γ<sub>3</sub> = 0.96. The improved accuracy of this formula (RMSE ≅ 10 W m<sup>-2</sup>) is due chiefly to the establishment of functions and coefficients characterising the cloud cover over the Baltic in particular months of the year and their incorporation into it.
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