10/06/2025

Wind Patterns and Pressures in the Southern and Northern Hemispheres.

• General Concepts of Wind Patterns

  • Wind patterns vary significantly between the Northern and Southern Hemispheres depending on the season.

  • In the Southern Hemisphere summer, the wind is on the south side of the line (equator).

  • High latitudes experience significant differences in temperature, impacting seasonal weather patterns.

High Pressure Areas.

• High Pressure in Central Asia (Siberian High)

  • Central Asia experiences extreme cold in winter, influenced by high-pressure systems.

  • The phrase high pressure refers to thermal high pressure and polar high pressure phenomena.

  • The Siberian High dominates the coldest areas in the Northern Hemisphere during winter.

• Characteristics of High Pressure

  • High pressure leads to subsiding air, which diverges outward.

  • This outward flow of cold, dry air contributes to a dry winter in Asia.

  • Despite some snowfall, moisture is scarce due to the lack of incoming humid air.

Seasonal Winds in Asia.

• Winter Trade Winds

  • During winter, trade winds flow clockwise, leading to dry conditions in Southern Asia as air moves from the cold Arctic surroundings.

  • The Himalayas act as a barrier, preventing cold air from reaching Southern Asia, contributing to dryer air conditions.

• Summer Conditions

  • The Intertropical Convergence Zone (ITCZ) shifts northward, drawing in moisture from warm equatorial ocean areas.

  • Equatorial Westerlies replace trade winds during summer, characterized by their moist, unstable nature.

  • This results in significant rainfall, creating a typical monsoon system in South and Southeast Asia.

Characteristics of the Monsoon System.

• Summer Monsoon Patterns

  • The onset of summer monsoons is marked by a sudden increase in rainfall, often described as a burst of the summer monsoon.

  • Rainfall is frequent during summer, tapering off in the fall.

  • High precipitation totals are observed, especially in Central India.

• Winter Rainfall Patterns in Africa

  • Sub-Saharan Africa has a monsoon system influenced by eastward winds from equatorial waters, leading to high moisture and unstable winds.

  • Precipitation is concentrated near the coast, diminishing rapidly inland due to the presence of the Sahara Desert.

Local Winds: Sea Breeze and Land Breeze.

• Local Wind Systems

  • Sea and land breezes are local phenomena similar to monsoons but occurring on a smaller scale.

  • Daytime Sea Breeze: Occurs as land heats up quicker than water, drawing cool air from the sea.

  • Warmer air rises over the land, leading to cloud formation and possible thunderstorms.

• Nighttime Land Breeze

  • At night, land cools rapidly, creating colder air which flows back towards the sea as a land breeze.

  • A katabatic wind is a type of cold downslope wind, noticeable particularly in valleys at night.

Winds in Mountainous Regions.

• Chinook Wind

  • On the leeward side of mountains, chinook winds are warm due to the release of latent heat during condensation as air descends.

  • Known as snow eaters, these winds can rapidly diminish snowpacks by sublimating snow into vapor.

• Desert Regions

  • The Mojave Desert experiences high temperatures due to compressional heating from downslope winds.

  • This process increases temperatures without adding moisture, substantially lowering relative humidity (e.g., from 15% to 3%).

Ocean Currents and Global Circulation

• Relation to Climate

  • Global circulation patterns drive surface ocean currents, which play a significant role in determining coastal climates.

  • All major ocean currents have warm and cold characteristics depending on their origins and directions.

• Descriptions of Key Ocean Currents

  • Warm Currents: Gulf Stream, North Atlantic Drift, Brazil Current, East Australian Current, Kuroshio Current.

  • Cold Currents: California Current (North Pacific), Peru Current (South America), Benguela Current (Southern Africa)

• Effect of Currents on Climate:

  • Cold currents adjacent to the coast can lead to arid conditions (e.g., California, Canary current) because of their cooling effect on air, preventing precipitation.

  • Conversely, warm currents contribute to humid coastal conditions (e.g., Gulf Stream leading to mild weather in Scandinavia).

Conclusion

• Understanding weather patterns and ocean currents is critical for comprehending regional climates and weather phenomena such as monsoons and local wind systems.

• Awareness of the relationship between ocean currents and climate will assist in predicting potential weather occurrences and conditions in various parts of the world.