El Nino is defined by prolonged warming in the Pacific Ocean sea surface temperatures when compared with the average value. The U.S NOAA definition is a 3-month average warming of at least 0.5 C (0.9 F) in a specific area of the east-central tropical Pacific Ocean; other organizations define the term slightly differently. Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years. The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Nino "conditions"; when its duration is longer, it is classified as an El Nino "episode. When El Nino conditions last for many months, extensive ocean warming and the reduction in easterly trade winds limits upwelling of cold nutrient-rich deep water, and its economic impact to local fishing for an international market can be serious. More generally, El Nino can affect commodity prices and the macro-economy of different countries. It can constrain the supply of rain-driven agricultural commodities; reduce agricultural output, construction, and services activities; create food-price and generalized inflation; and may trigger social unrest in commodity-dependent poor countries that primarily rely on imported food"