Where two masses of air from different sources and with different characteristics meet, Boundary lines are formed where the cooler and heavier air runs under the warmer and Lighter air and compels it to rise. As in the previous case the expansion and cooling of this air results in the formation of Clouds spreads as a more or less continuous of stratiform clouds.
Air may be forced to rise up the sides of mountains and high grounds towards which the wind is blowing..The consequent expansion and cooling of the air again leads to the formation of stratus type cloud if the ascent of the air is sufficient to cause condensation.
Rain,Snow and Hail
If the cooling process which gives rise to cloud formation is continued the droplets of which the cloud grow in size until they become too heavy to remain suspended in the air and they therefore fall to the ground as very fine rain or drizzle. If the droplets are held aloft byby upward currents of air they grow by agglomeration And eventually fall as raindrops. When very strong up-draughts exists as in large cumulus type clouds, the drops become very large before falling as heavy showers of rain or , under certain conditions, as hail. The latter is caused by very strong up-draughts carrying the raindrops above the freezing level, where they turn into continually growing balls of ice which falls to the ground as hailstones. Thunderstorms are usually accompanied by hail because they are also caused by strong updraughts extending to great heights.
If the temperatures at which condensation takes place is below freezing points , the droplets form as ice crystals, which grow and become snow flakes. Snowflakes often melts and change to raindrops before reaching the ground ; as a rough guide , snow rather than rain is likely if the air temperature at ground or sea level is below 3º C (37◦F).
EQUITORIAL Zones: [within 3º or 4º of the equator].
In this zone seasons are...