Most of Miami's Rain Falls in Six Months

Miami’s rainfall is not remarkable for its total. It is remarkable for how it arrives. The NOAA climate normal for Miami International Airport over the 1991 to 2020 period is 67.41 inches a year, which is a lot but not extraordinary. The distribution is what shapes what happens to buildings.

The monthly numbers

From the same NOAA normals, in inches:

Month	Normal
January	1.83
February	2.15
March	2.46
April	3.36
May	6.32
June	10.51
July	7.36
August	9.58
September	10.22
October	7.65
November	3.53
December	2.44

May through October comes to 51.64 inches, which is 76.6 percent of the annual total in half the year. January through April together contribute under ten inches.

And within those wet months, the rain does not arrive as drizzle. It arrives as afternoon convective storms and, in some years, as tropical systems, which means the ground has to absorb or shed a great deal of water in a short window rather than over days.

Why this makes drainage problems seasonal

A grade that fails only under intense rainfall looks perfectly adequate for six months of the year. Walk a property in March and the beds look fine, the patio looks flat enough, the downspout looks harmless. Walk it during a June storm and the water tells you a completely different story: a puddle standing at a wall, a stream running along the base of the house, a trench worn into a bed under a valley in the roof.

Which is the practical advice buried in the statistics. If you are trying to understand where water goes on your property, do it during the wet season, in the rain, with an umbrella. Fifteen minutes of watching during an afternoon storm is worth more than any amount of dry-day inspection.

What the water does once it is in the ground

In a clay market, water changes the soil’s volume and the soil pushes on the building. In Miami-Dade the soils barely shrink or swell at all, the USDA survey records a county-wide average linear extensibility under 0.2 percent, so that mechanism is absent. What water does here is move material.

The ground is permeable, the limestone below is full of solution voids, and the water table is only a few feet down. Water repeatedly delivered to one spot at the base of a wall carries fine material out of the soil and down into those voids. Over a wet season it produces a hollow. Over several, it produces a settled footing or a slab spanning a void.

That is the reason the highest-value drainage measure on most Miami properties costs almost nothing: an extension on a downspout that currently discharges at the wall, carrying the water clear of the building. Full treatment in the water and drainage guide.

Hurricane season, and what it adds

The Atlantic hurricane season runs from 1 June to 30 November, overlapping the wet season almost exactly. NOAA’s climatology for the 1991 to 2020 base period gives an average season of 14 named storms, 7 hurricanes, and 3 major hurricanes, with the first named storm typically in the second half of June and the first major hurricane around late August or early September.

For foundations, the relevant part of a tropical system is usually not wind but rainfall volume and, on the coast, surge. A storm delivering the equivalent of two months of rain in a day overwhelms whatever drainage arrangement exists, and it also raises groundwater temporarily, which for waterfront property means more water cycling through the ground behind a seawall than that wall was designed for.

A short seasonal checklist

• Before May, clear whatever roof drainage exists and make sure downspout discharge lands well clear of the building.
• Before May, check that yard drains and any French drain outlets are clear.
• During the wet season, walk the property in the rain at least once and note where water stands.
• On waterfront property, check that seawall weep holes are open. Their job is to relieve pressure from behind the wall, and blocked ones load it in the direction it was least designed for.
• After a major storm, look for new cracks and new depressions rather than assuming everything held.

None of that is expensive, and it addresses the single most common cause of foundation movement in this county.

Sources: NOAA NCEI 1991-2020 Climate Normals, station USW00012839, Miami International Airport; NOAA National Hurricane Center season dates and climatology.