Frontal (or Cyclonic) Rain is caused by cyclonic activity and it occurs along the fronts of the cyclone. It is formed when two masses of air of different temperature, humidity and density meets. For example meeting of moisture laden warm tropical wind with a polar air mass. A layer separating air masses is called the front. Front are of two types , the warm front and the cold front. At the warm front, the warm lighter wind rises gently over the heavier cold air, which being heavy stays close to the ground. As the warm air rises, it cools, and the moisture present in it condenses to form clouds altostratus clouds. This rain falls steadily for a few hours to a few days.
Effect on Agriculture
At the cold front, the cold air forces the warm air to rise aggressively causing its moisture to condense quickly, which results in the formation of cumulonimbus clouds. The rain from this cloud is usually heavy and of short duration.
Falling raindrops are often depicted in cartoons or anime as “teardrop-shaped” round at the bottom and narrowing towards the top but this is incorrect. Only drops of water dripping from some sources are tear-shaped at the moment of formation. Small raindrops are nearly spherical. Larger ones become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes. The shape of raindrops was studied by Philipp Lenard in 1898. He found that small raindrops (less than about 2 mm diameter) are approximately spherical. As they get larger (to about 5 mm diameter) they become more dough nut-shaped. Beyond about 5 mm they become unstable and fragment. On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water.
Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm drizzle impacts at about 2 m/s, while large 5 mm drops impact at around 9 m/s. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater. See droplet’s sound
Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain.
Precipitation, especially rain, has a dramatic effect on agriculture. All plants need at least some water to survive, therefore rain (being the most effective means of watering) is important to agriculture. While a regular rain pattern is usually vital to healthy plants, too much or too little rainfall can be harmful, even devastating to crops. Drought can kill crops in massive numbers, while overly wet weather can cause disease and harmful fungus. Plants need varying amounts of rainfall to survive. For example, cacti need small amounts of water while tropical plants may need up to hundreds of inches of rain to survive.