A World of Weather, Edition 3: Chapter 6 Introduction

CHAPTER 6

THE ROLE OF WATER IN WEATHER The evidence in Figure 6.1 is circumstantial but convincing. This photograph of the alleged iceberg that sunk the mighty Titanic on April 12, 1912, off Canada's Nova Scotia was taken the morning after by the chief steward of the Mackay Bennett, one of the first ships to reach the scene. What makes the evidence so compelling is that, at the time he took the photograph, the steward had not yet heard the news that the Titanic had gone down. Reportedly, he took the photograph because he noticed a smear of red paint along the base of the iceberg, indicating a recent close encounter with a ship.

From a meteorological viewpoint, this historical photograph provides direct evidence that water can simultaneously exist in all three phases - liquid, solid (ice) and gas (water vapor) - essentially in the same place at the same time. Water vapor is invisible to the naked eye (and the steward's camera), but clouds in the background of the iceberg attest to its presence. Indeed, a cloud is a vast collection of tiny water drops (or ice crystals if the cloud is sufficiently cold). Clouds become visible when invisible water vapor condenses onto tiny airborne particles called condensation nuclei, or, in the case of very cold clouds, when water vapor "deposits" onto ice nuclei, which are small airborne particles that encourage the production of ice crystals. The simultaneous existence of the three phases of water in the atmosphere sets the stage for the Bergeron-Findeisen process, an in-cloud, give-and-take between water, ice and water vapor that creates much of the precious precipitation that falls to Earth (more on this process in the next chapter).

In Figure 6.1, much of the water vapor that condensed to make cloud droplets (or deposited to make ice crystals) likely evaporated from the ocean. Meanwhile, the iceberg likely came from Greenland, having broken off, or "calved," from a great glacier and then drifted southward in the cold Labrador Current. The tracings of the journeys of water, ice, and water vapor as they change phase and shuttle back and forth between the earth and the atmosphere is called the hydrologic cycle - a maze of possible paths that water molecules can take on their many varied voyages (see Figure 6.2).