Tuvalu experiences the effects of El Niño and La Niña caused by changes in ocean temperatures in the equatorial and central Pacific. El Niño effects increase the chances of tropical storms and cyclones, while La Niña effects increase the chances of drought. Typically the islands of Tuvalu receive between 200 to 400 mm (8 to 16 in) of rainfall per month. However, in 2011 a weak La Niña effect caused a drought by cooling the surface of the sea around Tuvalu. A state of emergency was declared on 28 September 2011; with rationing of fresh-water on the islands of Funafuti and Nukulaelae. Households on Funafuti and Nukulaelae were restricted to two buckets of fresh water per day (40 litres).