Winds. The horizontal movement of air is wind. Winds along the ground are made by the bottom limb of a convection cell.

Winds are deflected by the Coriolis effect. This effect is caused by the west-to-east rotation of the Earth. Coriolis is called an effect because moving entities appear to curve, although their path is straight. The appearance of a curve is due to the Earth having rotated beneath the flying or floating object.

If there were no Coriolis effect, winds would blow in a straight line. But Coriolis makes the winds appear to curve as the planet moves beneath them.

Winds can also be deflected by topography; for example, winds may need to skirt around a mountain range.

Coriolis effect. A carousel rotates counterclockwise. In the left panel, a person throws a ball from the outside of the carousel toward the inside. The ball travels down the 12 o'clock line. But while the ball is flying above the carousel, the person on the carousel has moved counterclockwise. The right panel shows what the ball thrower sees if she remains at 12 o'clock but the carousel moves. The ball appears to curve in a clockwise direction, even though it travels in a straight line.

Global wind belts. The bottom limbs of the major convection cells form the global wind belts.

Without Coriolis effect, the winds in the global wind belts would blow north to south or south to north. But due to Coriolis the winds appear to curve as the planet moves beneath them, clockwise in the northern hemisphere and counterclockwise in the southern.

Climate is partly determined by the amount of solar radiation a location receives, which depends on its latitude. Latitude also determines a location's position in the global wind belts, which is another major factor in determining its climate. For example, many of the world's great deserts, such as the Sahara and Sonora, are found at 30° N latitude.

Global wind belts. The global wind belts and the directions the winds move. Blue winds are relatively cold and red winds are relatively warm. H's indicate high pressure zones and L's indicate low pressure zones.

Jet streams. Jet streams are winds that have an enormous influence on weather and climate.

Jet streams are rivers of air suspended in the atmosphere. They are found wherever there are large differences in temperature between two masses of air. The major jet streams are found where the major global convection cells meet: The subtropical jet stream around 30oN and oS and the polar jet stream around 60oN and oS.

In the Northern Hemisphere the location of the polar jet stream vacillates from about 20oN in winter to around 50oN in summer. A location north of the jet stream is beneath polar air and will be cooler than one south of the jet stream where the air has come from warmer regions. Changes in the location of the polar jet stream are responsible for a lot of weather differences between winter and summer in North America.

The polar jet stream is especially stormy because that is where subtropical and polar air meet.

Jet streams. The two major jet streams are where the atmospheric convection cells come together at around 30^oN and ^oS and 60^oN and ^oS.

Proximity to an ocean influences climate. Oceans moderate temperature because large bodies of water can absorb or release a lot of heat without only slight changes in temperature.

Land areas near an ocean have more moderate temperatures than those that are further away. This is especially true if the prevailing winds come off the ocean as they do in coastal California.

The temperature of the ocean also affects the temperature of the nearby land. Warm surface currents can heat the nearby land, like the Gulf Stream warms the United Kingdom, making Scotland and other high-latitude countries much warmer than they would otherwise be.

Fun fact: Although London in the United Kingdom and Calgary, Alberta in Canada are both at 51^o N latitude, Calgary has a climate that is influenced by the continent it sits in so its temperatures are more extreme. London is influenced by the Gulf Streams, which makes the region's climate much warmer and wetter in winter than it should be for being so far north.

London average high (^oC/^oF):
year 15/59; July 23/74; Jan 8/46
Calgary average high (^oC/^oF):
year 10.5/51; July 23/73; Jan -2.8/27

Gulf Stream. Warm water flowing up the western Atlantic travels across the ocean and warms the northern portion of Europe.

Mountains influence climate. Mountains are barriers to weather patterns and also influence climate.

Mountains separate a coastal region from a continent and limit the influence of the ocean. The California Coast Range restricts cooler Pacific air to the coastal area, allowing the Central Valley to become extremely hot. The Sierra Nevada further restricts Pacific air from moving east.

Mountains also influence where rain will fall. As oceanic air rises over a mountain range and cools, it loses its moisture by precipitation. The air on the far side of the mountain is then dry and becomes warm as it descends. The Sierra Nevada collect rain and snow as Pacific air rises over them. On the eastern side of the range is a rain shadow desert, the Great Basin.

Rain shadow. Warm moist air rises, cools and causes precipitation as it goes over a mountain. On the other side the air is dry. The mountain causes a rain shadow.

Local weather. The climate of a region is its weather averaged over decades. Weather reflects atmospheric conditions at that location at that time.

Although climate changes gradually, weather can change almost instantly. Weather is the result of the temperature of the air, moisture in the air, air movements, and whether there is a nearby air mass of different temperature.

Atmospheric convection cells can form anywhere there is uneven heating of the surface. Local and regional winds are due to a variety of factors and there are many types. Two examples of winds follow.

Land and sea breezes. Wind is created by air moving from high pressure to low pressure zones.

The greater the difference in air pressure, the greater the wind speeds. Several situations bring about conditions that generate wind.

The temperature differences between the ocean and nearby land create breezes. In winter, the air over an ocean is warmer and less dense than the air over the nearby land. The air over the ocean rises, sucking air from over the land seaward. Land breezes blow cool air from the land out to the warmer ocean, which moderates land temperature.

In summer, the relatively warm land develops a low pressure. The rising air draws cooler ocean air into the land, creating a sea breeze. Sea breezes low summertime temperatures in coastal areas.

Land and sea breezes. A convection cell forms between warmer land and cooler sea in summer, creating a sea breeze. Convection goes the opposite direction in winter, when the land is cooler than the ocean and a land breeze forms.

Santa Ana winds. The Santa Ana winds of California are famous because they are often associated with devastating wildfires.

Santa Ana winds occur when it is very hot along the Southern California coast. High altitude air in the Great Basin is drawn toward the coast by low pressure. The air moves across the southwestern deserts and is extremely hot. As it races downhill, it causes evaporation to a landscape that has been in its summer drought season. At this time of year, the dry vegetation burns out of control if a fire starts.

Santa Ana winds. Low pressure along the California coast causes air from the high pressure cell in the Great Basin to race westward. The air is hot and dry and can fuel devastating fires in the coastal mountains.