Atmospheric Dynamics: Convection, Pressure, and Stability

Atmospheric Dynamics: Vertical Movements and Stability

Convection

Vertical movements occurring in the troposphere, called convection, are caused by variations in temperature, humidity, or atmospheric pressure.

• Thermal Convection: These movements are caused by the contrast in air temperature. Air near the surface tends to rise, forming thermals of rising air, while higher up, the air tends to cool and descend.
• Moisture Convection: These are caused by the presence of water vapor in the air. Water vapor can be measured in two ways:

1. Absolute Humidity: The amount of water vapor in a determined volume of air, expressed in g/m³.
2. Relative Humidity: The amount of water vapor in 1m³ of air, expressed as a percentage, in relation to the maximum amount it can hold at a given temperature.

For clouds to form, condensation nuclei must exist in the atmosphere. If there are many of these nuclei, condensation may begin even earlier. If there are too few, the air may not condense. Clouds are made up of millions of tiny water droplets or ice crystals suspended in the air. Ice is formed in the highest part of the cloud tops.

Vertical Movements Due to Atmospheric Pressure

Atmospheric pressure, the pressure exerted by a column of air above the Earth's surface, is measured with a barometer. The standard value is 1 atm. The pressure of a site varies as a function of humidity and air temperature. Isobars are lines connecting geographical points of equal pressure.

• Anticyclone: An area surrounded by high-pressure isobars, where pressure decreases from the center outward.
• Storms: An area of low-pressure isobars, where pressure increases from the center outward. When a less dense air mass in contact with the Earth's surface begins to rise, pushed by rising thermal currents, it creates a vacuum that is filled with colder air.
• Anticyclone Formation: When a cold air mass aloft tends to drop, it hits the ground. In the contact zone, a large amount of air accumulates, and the wind tends to move from the center to the outside.

Vertical Gradients

Vertical gradients refer to the difference in temperature between two points at a difference in altitude of 100m.

1. Vertical Temperature Gradient (VTG): Represents the vertical variation of air temperature at rest, usually 0.65ºC/100m.
2. Inversion: Airspace in which temperature increases with height, i.e., a negative VTG.
3. Dry Adiabatic Gradient (DAG): The value of this is 1ºC/100m for air carrying water in vapor form. This gradient is dynamic, as it affects an air mass that is conducting a vertical movement while being in balance with the surrounding air.
4. Moist Adiabatic Gradient (MAG): Occurs when an upward-moving air mass (DAG) reaches its dew point, condensing the water vapor it contains and forming a cloud. The MAG value depends on the amount of initial water vapor; the greater the amount, the lower the MAG.

Atmospheric Stability and Instability

• Unstable Conditions: Occur when there is an upward movement of an air mass whose internal temperature varies according to the dynamic gradient (DAG). For ascent to be possible, the relationship VTG > DAG must hold.
• Stable Conditions: The opposite of unstable conditions. A cold air mass, upon heating, dries. On the surface, it will create an anticyclone due to increased atmospheric pressure in that area. There are two types:

1. VTG is positive and smaller than DAG: No vertical movement occurs.
2. VTG is negative (VTG < 0): Clouds form at ground level (fog).