The Loo (Hindi: लू, Urdu: لو, Punjabi: ਲੂ) is a strong, hot and dry summer afternoon wind from the west which blows over the western Indo-Gangetic Plain region of North India and Pakistan. It is especially strong in the months of May and June. Due to its very high temperatures (45 °C–50 °C or 115°F-120°F), exposure to it often leads to fatal heatstrokes.
Since it causes extremely low humidity and high temperatures, the Loo also has a severe drying effect on vegetation leading to widespread browning in the areas affected by it during the months of May and June.
Origin and ending of the Loo
The Loo ends in late summer, with the arrival of the Indian monsoon. In some areas of North India and Pakistan, there are brief, but violent, dust storms known as Kali Andhi (or black storms) before the monsoon sets in. The arrival of monsoon clouds in any location is frequently accompanied with cloudbursts, and the sudden transformation of the landscape from brown to green can seem "astonishing" as a result of the ongoing deluge and the abrupt cessation of the Loo.
Dwelling adaptation to the Loo
Since the plains of North India and Pakistan are both very hot and extremely dry during this season, water evaporates quite readily. Although this leads to the drying out of many ponds and lakes, the extreme dryness of the air is also easily exploited to create evaporation-based cooling systems. Windows shielded with fiber-screens of the fragrant khas (ख़स/خس or vetiver) dry-grass that are kept damp with a simple water-pumping mechanism are quite effective as an inexpensive form of air conditioning, and have been in common use throughout the plain portions of the northern Indian subcontinent for centuries. Because evaporation proceeds at a very rapid rate in the extreme dryness, the cooling effect can be quite dramatic and result in dwellings where the interior feels chilly. The water in the screens evaporates very rapidly, however, so it must constantly be replenished from raised tanks or with pumps (that can sometimes be driven by the Loo itself). Any water reservoir used must also be shielded from the Loo and the sun, or it can rapidly be depleted.
Ecological effects of the Loo
Many birds and animals succumb to the Loo in the summer months, especially in deforested areas where the Loo blows unhindered and shelter is unavailable. Certain insect-borne diseases, such as malaria, have historically registered dips during the Loo season as insects populations also plummet during this season. Even prior to the 1897 discovery that mosquitoes transmitted malaria, British officials in India had noticed that the Loo in the plains of Northern India naturally made the region relatively free of the disease.
The Loo in popular culture
Due to the dangerous, and potentially fatal, effects of the loo on vegetation, humans and animals, it is sometimes referred to as an evil wind in popular Indo-Pakistani culture. Avoiding exposure to the Loo is strongly recommended for children and the elderly, as well as pets. Most people attempt to stay indoors as much as possible during afternoons in the Loo-affected months. Heatstrokes are commonly referred to as Loo lagna (लू लगना) being stricken by the Loo  Certain sherbets, which are popularly believed to have a cooling effect on the body and provide some protection against Loo-caused heatstrokes, are widely consumed during the Loo-season. These include sherbets of rose, khus, shahtoot, bel and phalsa. A specific Unani recipe called Rooh Afza combines several of these popularly believed cooling agents, and is sold commercially as a syrup to flavor sherbets, cold milk drinks, ices and cold desserts, such as the popular falooda. Lassi, a yogurt-based drink of North India and Pakistan, is also extremely popular and believed to confer some protection against the Loo. Additionally in the desert state of Rajasthan Kairi ka Panna कैरी का पन्ना (drink of raw/unripe mango) is a very popular way of remaining cool and resilient to Loo.
- S.V.S. Rana (2007), Essentials of Ecology and Environmental Science, Prentice Hall of India, ISBN 81-203-3300-4, "... In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in the months of May and June, usually in the afternoons. It is known as loo. Its temperature invariably ranges between (115°F-120°F) 45 °C and 50 °C (115°F-120°F). People, when exposed to loo ..."
- Barbara Tufty (1987), 1001 Questions Answered About: Hurricanes, Tornadoes and Other Natural Air Disasters, Dover Publications, ISBN 0-486-25455-0, "... This wind dries out crops and makes people fretful by its eerie singing ..."
- Phani Deka (2006), Geography: Physical and Human, New Age International, ISBN 81-224-1912-7, "... In north India, the hot wind of Thar desert that blows during early summer is called Loo ..."
- Andhi (Kali Andhi), WeatherOnline, retrieved 2009-06-21, "... The Kali Andhi, or black storm is a violent, squally dust storm occuring [sic] in late spring in north-western India. The Andhi heralds the imminent arrival of the monsoon ..."
- Cloudburst washes away water mills in Himachal, Headlines India, 2006-07-24, retrieved 2009-06-21, "... A cloudburst in a remote Himachal Pradesh mountainous valley washed away several water-run mills, a bridge, a road and a large tract of farmland, but there were no reports of casualties ..."
- John Strachey (1894), India, K. Paul, Trench, Trübner & Co, "... At last, when the heat has become greater than ever, the clouds begin to collect, and there comes down a deluge, almost always accompanied by thunder and lightning ... nothing can be more wonderful than the change which comes almost instantaneously over the whole face of nature ... within a time that may be measured by hours rather than days, the country that was like a desert begins to look like a garden. The rapidity of the progress of vegetation is astonishing ..."
- Sheikh Mohamad Ikram, Thomas George Percival Spear (1955), The cultural heritage of Pakistan, Oxford University Press, "... He introduced the system of cooling water with the help of saltpetre, and invented the Khas-ki-Tatti, which has for centuries remained the Indian method of air-conditioning ..."
- William Rice M. Holroyd (1873), Tashil-ul-Kalam, or Hindustani made easy, Oxford University, "... Khas, vulgarly khas-khas, a kind of grass from which screens or tattis are made, for use in the hot season. The tatti is placed in a doorway and kept perpetually wet; and the hot dry westerly wind, after being allowed to pass through it, becomes quite cold ..."
- "Indian summer too hot for Cheetahs from Singapore", India Gazette, 2009-05-06, retrieved 2009-06-25, "... Authorities in the Shakkarbaug Zoo ... removed a pair of Cheetahs ... from public view as the animals could not bear the intense heat wave ... have been lodged in a ... temperature-controlled environment ... As many as 30 Cheetahs brought by various zoos in India ... died as they could not adjust to the harsh Indian climate during summer months ..."
- Allan McArdle, J. N. Panda (1968), A Handbook for Poultry Officers in India, UNICEF, "... bird losses because they won't go out in the sun to drink, and then die in heat waves ..."
- Charles Frederick Oldham (1871), What is malaria? And why is it most intense in hot climates? An enquiry into the nature and cause of the so-called marsh poison, with remarks on the principles to be observed for the preservation of health in tropical climates and malarious districts, Lewis, "... In the plains of upper India, sleeping out of doors is attended with but little danger, even for Europeans, in the dry, hot months; although, at other seasons, fever would be a probable result ... far outweigh all risk from "malaria." ..."
- Ahmed Ali (1994), Twilight in Delhi, New Directions Publishing, ISBN 0-8112-1267-X, "... The temperature rose higher and higher ... the loo began to moan, blowing drearily through the hopeless streets. The leaves of the henna became sered and wan ... even birds were not immune ... many pigeons died ... Men shut themselves up inside the houses or in shops. At noon the city seemed deserted and dead ..."
- Salman Rushdie (2007), Shame, Picador, ISBN 0-312-27093-3, "... I'll be fair: nobody likes the Loo, that hot afternoon breath-that-chokes. We pull down our shutters, hang damp cloths over the windows, try to sleep ... The Loo is an evil wind ..."
- Manohar Malgonkar (1972), The Devil's Wind, H. Hamilton, "... A searing wind came over the white-hot desert, the wind that we call the loo, but a loo which had the special, abrasive rasp of the Devil's Wind ..."
- Gopal Singh (1976), A geography of India, Atma Ram, "... The hot dusty wind known as 'Loo' may persist for a few days and make outdoor life difficult. Since the visibility becomes poor, air services are sometimes suspended ..."
- Ruth S. Freed, Stanley A. Freed (1993), Ghosts: life and death in North India, American Museum of Natural History, ISBN 0-295-97303-X, "... It is also known as heat stroke and loo lagna (affected by the loo - a hot wind) ..."
- C.P. Khare (2004), Indian herbal remedies, Springer, ISBN 3-540-01026-2, "... Sharbat Khas is a cooling and refreshing drink of Unani medicine ..."
- H. Panda (2004), Handbook on Ayurvedic Medicines with Formulae, Processes and Their Uses, National Institute of Industrial Research, ISBN 81-86623-63-9, "... When we keep in mind the many qualities Rooh Afza's ingredients described above, it is easy to understand why it has been found to be an exceptionally appropriate summer drink ..."
- Roshan Lal Anand (1933), The Milk Supply of Lahore in 1930, Board of Economic Inquiry, Lahore, Punjab, "... There are many among the lower ranks who do without milk or tea during winter, but lassi is almost the last thing a Punjabi would forego during summer ... the "national drink" of the Punjabi ..."