Read the passage below and answer the question.

On a summer night in a large Indian city, the air often stays warm long after sunset, while a village thirty kilometres away cools down quickly. Scientists call this the urban heat island effect. A city can be several degrees warmer than the countryside around it, and the difference is usually greatest at night.

The main cause is the material a city is built from. Concrete, brick and tar soak up sunlight during the day and store it as heat. After sunset they give this heat back slowly, so the streets go on warming the air for hours. Open fields store far less heat and lose it quickly. The shape of a city makes matters worse. Tall buildings standing close together trap warm air between them, and vehicles and air conditioners add heat of their own.

Trees work against this in two ways. Their shade keeps the sun off roads and walls, and the water that evaporates from their leaves cools the air around them, the way an earthen pot cools the water inside it. That is why a lane lined with old neem trees can feel cooler than a bare street only a few minutes' walk away.

Most of the remedies are neither new nor costly. Cities can plant trees along their roads, protect the parks they still have, and paint roofs white so that they reflect sunlight instead of absorbing it. These steps will not end the summer, but they can help the many families who have no air conditioner to sleep through a hot night.

Question: According to the passage, why do the streets of a city go on warming the air for hours after sunset?