Description:
<jats:p>We present a galactic survey which to date consists of 47,000 positions covering −3° < <jats:italic>l</jats:italic> < 122°, −1° < <jats:italic>b</jats:italic> < 1°, observed in the J= 1→ 0 line of <jats:sup>13</jats:sup>CO to an rms noise level of 0.15 K in 0.68 km s<jats:sup>−1</jats:sup> channels, using the 7 m antenna at Crawford Hill. Maps made from the survey data show a clear difference between spiral arm and interarm regions. The signature of spiral structure on kiloparsec scales is the presence in galactic survey data of voids in <jats:italic>l, b, v</jats:italic> space which contain many times fewer Giant Molecular Clouds (GMCs) than do adjacent regions of similar size. The difference between arm and interarm regions in the inner galaxy is manifested only in the GMCs — small clouds are present throughout. These results are based on catalogs of clouds and their estimated sizes in <jats:sup>13</jats:sup>CO. We suggest that GMCs are formed as interstellar gas enters a spiral arm, and that they break up into small molecular or atomic clouds as the gas leaves the arm.</jats:p>