Lima lies in a tropical region but has very little rainfall. The most important influence on the city's weather is the cold Humboldt Current, which runs along most of the Peruvian coast. The current prevents sea-surface water evaporation that would create more clouds and precipitation. The high Andes Mountains lying to the east block most storm systems from the Amazon basin from reaching the coast. The areas to the north and south of the capital arid plains receive little rainfall and support only sparse vegetation.
Temperatures in Lima are mild throughout the year. During the winter, which lasts from June through October, temperature remains in a narrow band between 54 and 68 degrees Fahrenheit. The air is humid and skies are often overcast. On many winter days, a light fog known as the "garua" hangs over the city, as cool air moves in from the ocean and condenses in the atmosphere. Morning mists and light rain are common winter events. The humidity, fog and lack of rainfall combine to leave a slippery gray film of dust and grit on the city's streets and buildings.
A brief spring season runs through November and December. Temperatures increase and the city becomes less humid. Summers begin in January and last through March. There are fewer overcast days, lower humidity and less fog and mist. Temperatures rise and rainfall is very rare and light. An occasional storm arrives from the high slopes of the Andes Mountains. Temperatures in the summer average 75 degrees, with highs averaging about 84 degrees and lows around 66. The brief autumn season brings falling temperatures and rising humidity.
The rise in elevation in the eastern districts of Lima creates a separate "microclimate" in this part of the city. In these neighborhoods, fog and mist are less frequent, temperatures are slightly higher, and the air is less humid. Sunshine increases as the elevation rises further east.
El Nino arrives in some years to disrupt the climate of the South American coast. During an El Nino event, temperatures increase in both winter and summer. Rainfall is more frequent and Lima experiences an occasional thunderstorm.
