two months after intense rains began to pound much of South America

two months after intense rains began to pound much of South America, rivers along the northwest coast of Peru remained flooded. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite captured the top image of the flooded river systems on April 1, 2008. Streams and pools of dark blue water dominate what was a tan-pink desert in early February, immediately before the rains began.
The images show the Sechura Desert in northwestern Peru near the border with Ecuador. The large image shows additional flooding extending north into Ecuador. To increase the contrast between muddy water and land, which often look the same in photo-like images, the image was made with both visible and infrared light. This false-color combination colors water black and dark blue and bare or sparsely vegetated earth tan. Plant-covered land is green, and clouds are turquoise and white.
Floods throughout Peru damaged farmland, homes, and transportation networks, reported the United Nations Office for the Coordination of Humanitarian Affairs. On February 28, the government of Peru declared a state of emergency in Piura and Lambayeque, the regions included in this image, and in Tumbes, the region immediately north of the area shown, and in Ucayali, a region in central Peru along the Brazilian border. More than 450,000 people were affected by flooding throughout Peru as of March 12, said the United Nations.

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.^[4] 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.

Effect on agriculture

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 per year to survive.

Agriculture of all nations at least to some extent is dependent on rain. Indian agriculture, for example, (which accounts for 25 percent of the GDP and employs 70 percent of the nation's population) is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy, as evidenced in the numerous droughts in India in the 90s.