page_title: Troposphere
section_title: Meridional flow
hierarchical_section_title: Troposphere / Atmospheric flow / Meridional flow
caption_attribution_description: NOAA/NCEP 500 hPa graphic for October 23, 2003
context_page_description: The troposphere is the lowest layer of Earth's atmosphere, and is also where nearly all weather conditions take place. It contains 75% of the atmosphere's mass and 99% of the total mass of water vapour and aerosols. The average height of the troposphere is 18 km in the tropics, 17 km in the middle latitudes, and 6 km in the polar regions in winter.
  The total average height of the troposphere is 13 km.
  The lowest part of the troposphere, where friction with the Earth's surface influences airflow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km deep depending on the landform and time of day. Atop the troposphere is the tropopause, which is the border between the troposphere and stratosphere. The tropopause is an inversion layer, where the air temperature ceases to decrease with height and remains constant through its thickness.
  The word troposphere is derived from the Greek tropos and sphere, reflecting the fact that rotational turbulent mixing plays an important role in the troposphere's structure and behaviour. Most of the phenomena associated with day-to-day weather occur in the troposphere.
context_section_description: When the zonal flow buckles, the atmosphere can flow in a more longitudinal (or meridional) direction, and thus the term "meridional flow" arises. Meridional flow patterns feature strong, amplified troughs of low pressure and ridges of high pressure, with more north-south flow in the general pattern than west-to-east flow.
caption: Meridional Flow pattern of October 23, 2003. Note the amplified troughs and ridges in this 500 hPa height pattern.
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