Large green areas have a cooling influence on their surrounding built-up area, thus reducing the stress produced by the heat island. Traverses made on clear nights with light wind show that Chapultepec Park (,~500ha) in Mexico City is 2-3 ยฐC cooler with respect to its boundaries and its influence reaches a distance about the same as its width (2kin).The largest thermal contrast occurs at the end of the cooling period. For a recent period of four years, mean monthly minimum temperature differences between a climatological station located in the park and the Tacubaya Observatory ( ~ 700 m south of its southern limits) reach 4.0 ยฐC at the end of the dry season in April, whereas during the wet months they are only 1 ยฐC cooler (in July). On sunny mornings the park heats up more slowly than the built-up section at Tacubaya; but two hours after midday there is no significant difference in temperature as shown by mean maximum temperatures. At this time, the canopy layer is well mixed and Tacubaya being downwind ([rom N or NE) is then under the cooling influence of the park.But east and north of the park toward the densely built-up area (where the heat island is located), mean maximum temperatures at the park station are 2-3 ยฐC cooler.The increased roughness of the generally high trees in the park reduces the low-level wind speed increasing the intensity of turbulence. Both these effects are likely to favour the initiation of small-scale convection over the vegetated area. To test this hypothesis, rainfall data for the two stations for a period of four years was examined. Chapultepec/Tacubaya 24-hour rainfall ratios (r) were grouped in six classes. Results show that precipitation was equal or less in the park in 51% of the cases (442). However, in the other half of the cases, rainfall was larger (by 80%) in the park than in Tacubaya Observatory for all intensities. The ratio increased to 140% for intensities between 5 and 10mm/24h.