Page 57 - (DK) Ocean - The Definitive Visual Guide

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LONG-HAUL SAILING
Winds can blow with a consistent
strength and direction over large areas of
ocean. Consequently, on long-haul sailing
trips, the same basic sail settings can
often be used for days on end.

air ascends
from cyclone Pressure-system Winds
warm air rising
air descends into In any area of ocean where air sinks—often at subtropical latitudes—a zone of
anticyclone
high atmospheric pressure, or anticyclone, develops. Where warm air rises, areas
low pressure
at center central area of of low pressure, called cyclones or depressions, occur. These often develop near
high pressure the equator and subpolar latitudes. Cyclones and anticyclones create linked,
cold air sinks circulating wind patterns, which continually move and change. In the Northern
Hemisphere, there is a clockwise movement of air
CYCLONES AND around an anticyclone, and a counterclockwise motion
ANTICYCLONES around a cyclone. This pattern is reversed in the
air spirals Air moves from an area of Southern Hemisphere. Local pressure systems can affect
around central high pressure toward one
area of low of low pressure, but the the general pattern of prevailing winds. In particular,
pressure air moving from Coriolis effect modifies this, cyclones move swiftly over the ocean and can produce
cold air flows high to low pressure
toward area of low deflected by Coriolis producing circular winds. rapid changes in wind strength and direction.
pressure effect to form spiral
warm air cools
at high altitude cold air sinks
Coastal Breezes DAY AND NIGHT
Land heats up faster
Local winds, called onshore and offshore than water during the day.
air heats up
breezes, are generated near coasts, especially in and rises over cool air Warm air rises over the land
sunny climes. Onshore breezes—sometimes land drawn in and draws in cold air from
called sea breezes—develop during the day. the sea. At night, the land
cools more quickly,
These are caused by the land heating up more reversing the airflow.
quickly than the sea, as both absorb solar
radiation. This occurs because the sea absorbs
large quantities of heat energy with only a ONSHORE BREEZE
small rise in temperature, whereas the same amount of heat energy is
likely to cause the land temperature to rise sharply (see p.31). cold air sinks air heats up
cool air drawn and rises
As the land warms up, it heats the air above it, causing the air to rise. seaward over ocean INTRODUCTION
Cooler air then blows in from the sea to take its place. In the evening,
BREEZY COAST and at night, the opposite effect occurs. At nightfall, the land quickly
On warm coasts, there is often a noticeable
drop in temperature from midday as a cool sea cools down, but the sea remains warm and continues to heat the air
breeze blows in off the water. The breeze above it. As this warm air rises, it sucks the cooler air off the land, and
typically reverses in the evening and at night. so generates an offshore breeze. This is sometimes called a “land breeze.” OFFSHORE BREEZE