The New Orleans metropolitan area is virtually surrounded by water. Lake Pontchartrain, some 610 square miles in area, borders the city on the north and is connected to the Gulf of Mexico through Lake Borgne on the east. In other directions there are bayous, lakes, and marshy delta land. The proximity of the Gulf of Mexico also has a great influence on the climate. Elevations in the city vary from a few feet below to a few feet above mean sea level. A massive levee system surrounding the city and along the Mississippi River offers protection against flooding from the river and tidal surges. The New Orleans International Airport is located 12 miles west of downtown New Orleans, between the Mississippi River and Lake Pontchartrain.
The climate of the city can best be described as humid with the surrounding water modifying the temperature and decreasing the range between the extremes. Almost daily sporadic afternoon thunderstorms from mid–June through September keep the temperature from rising much above 90 degrees. From about mid–November to mid–March, the area is subjected alternately to the southerly flow of warm tropical air and to the northerly flow of cold continental air in periods of varying lengths. The usual track of winter storms is to the north of New Orleans, but occasionally one moves this far south, bringing large and rather sudden drops in temperature. However, the cold spells seldom last over three or four days. The lowest temperatures observed are below 10 degrees. In about two–thirds of the years, the lowest temperature is about 24 degrees or warmer. The lowest temperatures in some years are entirely above freezing.
During the winter and spring, the cold Mississippi River water enhances the formation of river fogs, particularly when light southerly winds bring warm, moist air into the area from the Gulf of Mexico. The nearby lakes and marshes also contribute to fog formation. Even so, the fog usually does not seriously affect automobile traffic except for brief periods. However, air travel will be suspended for several hours and river traffic, at times, will be unable to move between New Orleans and the Gulf for several days.
Rather frequent and sometimes very heavy rains are typical for this area. There are an average of 120 days of measurable rain per year and an annual average accumulation of over 60 inches. A fairly definite rainy period occurs from mid–December to mid–March. Precipitation during this period is most likely to be steady rain for two to three day periods. April, May, October, and November are generally dry, but there have been some extremely heavy showers in those months. The greatest 24–hour amounts have exceeded 14 inches. Snowfall is rather infrequent and light. However, on rare occasions, snowstorms have produced accumulations over 8 inches.
While thunder occurs with most of the showers in the area, thunderstorms with damaging winds are infrequent. Hail of a damaging nature seldom occurs, and tornadoes are extremely rare. However, waterspouts are observed quite often on nearby lakes. Hurricanes have effected the area.
The lower Mississippi River floods result from runoff upstream. If the water level in the river becomes dangerously high, the spillways upriver can be opened to divert the floodwaters. Rainfall in the New Orleans area is pumped into the surrounding lakes and bayous. Local street and minor urban flooding of short duration result from occasional downpours.
Air pollution is not a serious problem. The area is not highly industrialized, and long periods of air stagnation are rare.
Based on the 1951–1980 period, the average first occurrence of 32 degrees Fahrenheit in the fall is December 5 and the average last occurrence in the spring is February 20.