Understanding Vertical Temperature Gradients and Atmospheric Stability
Vertical Temperature Gradients
Vertical Gradients: The difference in temperature between two points located at an altitude difference of 100 meters.
The Vertical Temperature Gradient (GTV) represents the vertical variation in air temperature under static or resting conditions. Its value varies with height, latitude, and season.
Inversion: An airspace where the temperature increases with altitude instead of decreasing, i.e., when the GTV is negative. Example: Winter.
Dry Adiabatic Lapse Rate (GAS): This is dynamic. Rising air can be considered a “closed system” or adiabatic, with no heat exchange with the surrounding air.
Saturated or Moist Adiabatic Lapse Rate (GAH): When rising air reaches its dew point, water vapor condenses, forming a cloud. The air mass continues to rise, but at the GAH. This rate depends on the initial amount of water vapor; the greater the amount, the lower the GAH.
Conditions of Atmospheric Stability and Instability
Conditions of Instability
These conditions exist when an air mass moves upward, and its internal temperature varies according to the dynamic gradient GAS, while the vertical temperature variations correspond to the GTV. Upward movement is possible when GTV > GAS. Storms may occur, but rain is not guaranteed.
Conditions of Stability or Subsidence
These conditions reverse convection, as a cold air mass, denser at a given height, descends towards the surface.
- When the GTV is positive and less than the GAS (0 < GTV < 1): There are no vertical movements.
- When the GTV is negative (GTV < 0)
More intense subsidence usually occurs in winter, with calm winds, long nights, and very cold air. Pollution can become trapped.
Atmospheric Dynamics and Circulation
Atmospheric dynamics involve horizontal wind circulation. This path is rarely rectilinear due to the influence of the Coriolis effect. This effect is a consequence of the Earth’s rotation and counterclockwise movement. The Coriolis force is greatest at the poles and progressively decreases, vanishing at the Equator.
Wind flows from cyclones to storms in a radial direction, following the pressure gradient. Deflection by the Coriolis force results in a clockwise turn around high-pressure systems and a counterclockwise turn around storms in the Northern Hemisphere (opposite in the Southern Hemisphere).
General Circulation of the Atmosphere
Hot equatorial air rises due to contact with the Earth’s surface, resulting in equatorial storms. At the poles, low temperatures cause cold air to sink, leading to the settlement of a permanent polar anticyclone. The Coriolis force produces a wind shift to the right in the Northern Hemisphere (left in the Southern Hemisphere), resulting in three types of cells:
Hadley Cell
This cell is characterized by high energy due to the vertical incidence of the sun. Equatorial storms cause warm air to rise to the tropopause, then move towards both poles as horizontal wind. The Coriolis effect causes deviation. Around 30 degrees North or South, the air creates a zone of subtropical anticyclones, which, when they settle on a continent, cause the largest deserts in the world. The subtropical anticyclone of the Azores influences the climate. Sometimes during the summer, the continental anticyclone of the Sahara exerts its influence.
The cell is closed due to the trade winds, surface winds that blow from these subtropical anticyclones to the Equator, where both hemispheres converge, causing the Intertropical Convergence Zone (ITCZ).
Polar Cell
Located approximately at 60 degrees latitude, where air rises again, forming subpolar storms.
Ferrel Cell
Located between the previous two, it is formed by the action of surface winds from the west (southwest in the Northern Hemisphere and northwest in the Southern Hemisphere), blowing from the desert to areas of polar storms.