Understanding Precipitation, Temperature, and Atmospheric Circulation
Understanding Weather Elements
Types of Precipitation
- Rain: Droplets of liquid water that originate from the condensation of water vapor or ice crystals melting during their fall. Most rain, except in tropical areas, is produced by the latter mechanism.
- Snow: Formed by the growth of ice crystals, often aggregating with smaller crystals. Snowflakes are highly branched aggregates, resulting in a slow rate of fall. Slush can be partially melted snow or frozen rain droplets.
- Hail: Solid ice balls formed only in large convective storms. Strong updrafts drag large raindrops to the top of the cloud where they freeze, repeating the process and creating a concentric layered structure.
Temperature Gradient
The temperature in the troposphere generally decreases with height, with an average vertical temperature gradient (VTG) of 6.5°C/km. However, the actual gradient varies significantly depending on location and time and is often non-uniform.
Anticyclones and Storms
- Anticyclone: High-pressure zones.
- Storm: A low-pressure area. Warm air rises, replaced by cold air from the surroundings, creating inward-blowing winds. In an anticyclone, cold air descends and flows outwards.
Wind in storms is convergent, while in anticyclones, it is divergent. When both are close, surface wind blows from the anticyclone to the storm. The path of the wind is usually curved due to topography and the Coriolis effect.
Coriolis Effect
The Coriolis effect is a force caused by the Earth’s rotation. It is stronger at the poles, where rotation is faster compared to the equator. Air moving in the northern hemisphere deflects to the right, while in the southern hemisphere, it deflects to the left.
Key Areas of Atmospheric Circulation
In equatorial regions, storms form due to intense heating from vertical sun rays. In polar regions, intense cold creates a polar anticyclone. Theoretically, surface wind flows from polar anticyclones to equatorial storms. However, the Coriolis effect causes transport through three types of cells:
- Convective Cells: A closed set of moves backward, and side winds.
- Hadley Cell: Driven by solar energy. Equatorial storms cause heated air to rise to the tropopause and move towards the poles (horizontal wind). The Coriolis effect deflects these winds, contributing to subtropical anticyclones in desert regions. The Azores anticyclone and the Sahara anticyclone influence the climate.
- Polar Cell: Surface wind from the polar cyclone releases polar air, reaching 60° latitude, where it rises to form subpolar storms.
- Ferrel Cell: Located between the Hadley and Polar cells, receiving surface winds from the west (Westerlies) blowing from subtropical deserts to polar regions.