The arctic and subarctic may be defined in several different ways, depending on whether they are looked at from the point of view of astronomy, botany, physics, climatology, ocean navigation or other specialized field. For engineering, the Arctic and Subarctic are best defined on basis of the prevailing air temperature conditions. Annual precipitation is mostly very light in the Arctic, much of it falling as snow. Considerably heavier precipitation falls in certain parts of the Subarctic that are under the influence of maritime storm paths. The freezing of lakes, rivers and coastal waters in winter can be a major controlling factor in the scheduling and effectiveness of field activities in the Arctic and Subarctic. Waterways that can be used for boats or float-equipped aircraft in the summer become unusable for these vehicles when freezeup starts in the period of September to November. Several weeks are then required before the ice becomes thick enough to support other types of vehicles. .  In many parts of the Arctic and Subarctic, where pressure gradients tend to be weak and temperature inversions are common, surface winds may normally be fairly low. Where pressure gradients are more marked, however, as in areas near seacoasts, and in and near mountains, strong winds may be quite common and wind speeds can attain hurricane velocities.