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We derive the physical properties of 580 molecular clouds based on their 12COand 13CO line emission detected in the University of Massachusetts-Stony Brook(UMSB) and Galactic Ring surveys. We provide a range of values of the physicalproperties of molecular clouds, and find a power-law correlation between theirradii and masses, suggesting that the fractal dimension of the ISM is around2.36. This relation, M = (228 +/- 18) R^{2.36+/-0.04}, allows us to derivemasses for an additional 170 GRS molecular clouds not covered by the UMSBsurvey. We derive the Galactic surface mass density of molecular gas andexamine its spatial variations throughout the Galaxy. We find that theazimuthally averaged Galactic surface density of molecular gas peaks betweenGalactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detectedin molecular gas, the Galactic surface density of molecular gas is enhancedalong the positions of the Scutum-Crux and Sagittarius arms. This may indicatethat molecular clouds form in spiral arms and are disrupted in the inter-armspace. Last, we find that the CO excitation temperature of molecular cloudsdecreases away from the Galactic center, suggesting a possible decline in thestar formation rate with Galactocentric radius. There is a marginallysignificant enhancement in the CO excitation temperature of molecular clouds ata Galactocentric radius of about 6 kpc, which in the longitude range of the GRScorresponds to the Sagittarius arm. This temperature increase could beassociated with massive star formation in the Sagittarius spiral arm.

Physical Properties and Galactic Distribution of Molecular Clouds identified in the Galactic Ring Survey