Summer has clearly arrived. It’s not so obvious from the weather, but it sure is evident driving along the Maine Turpike on Fridays and Sundays.

Now is the time when those in the tourist industry are filled with visions of sunny days and occupied rooms.

It’s also the time when we in Maine welcome our seasonal residents. Of the 721,830 housing units counted in Maine in the 2010 census, 118,310 were listed as “seasonal, recreational, occasional use.” That’s more than 16 percent of our total.

Another 18,848 were listed as “vacant, for rent” and “rented, not occupied.”

Presuming that some portion of these units are available for summer rental brings Maine’s total seasonal housing to more than 120,000 units — nearly one in five housing units.

Presuming further that these housing units have the same household size as the Maine average — 2.3 people per household — indicates a seasonal population of 276,000 people.

Even if they’re here for only three months, that’s still a lot of mouths to feed and imaginations to satisfy.

If these visitors have a per capita income 20 percent greater than Maine’s, then — even at only three months’ residency — Maine stands to receive nearly $2.1 billion in consumer spending that is not derived from our year-round population.

Assume higher levels of seasonal rental, longer stays and higher visitor incomes, and the number is even greater. In any case, something over $2 billion will have a substantial impact in a place where the total year-round personal income is approximately $48 billion.

So where are these seasonal homes? Not surprisingly, most are located in coastal areas. York County, Cumberland County, the midcoast region (Knox, Lincoln and Waldo counties) and Hancock County account for 52 percent of the state’s seasonal homes.

And this is the area where seasonal home use is growing. Between 2000 and 2010, the number of seasonal homes in Maine increased by nearly 17,000. Of these, more than 63 percent were located in the six coastal counties noted.

It is impossible to tell from census figures how many of these homes held for seasonal occupancy were newly built and how many were converted from former full-time occupancy, but in either case, their residents — however temporary — provide a substantial economic boost to the areas where they are located.

And it is for just this reason that the greatest impact of seasonal housing may not be in the obvious coastal area, but in the more remote rural areas.

Cumberland County, for instance, has more than 12 percent of the state’s seasonal homes and more than 21 percent of the state’s year-round population.

Franklin County, in contrast, has 6.1 percent of the state’s seasonal homes, but only 2.3 percent of the state’s year-round population.

Relative to the “official” population, therefore, seasonal residents could have a significantly greater impact in Franklin County than in Cumberland County. I say “could” only because impact clearly depends on how long seasonal residents stay and how much money they spend there.

The same relative disparity holds true in nearly all of Maine’s other rural counties as well.

In Washington and Oxford counties, the share of seasonal homes is approximately double the share of year-round population, and in Piscataquis County it is nearly five times as great. In Aroostook and Somerset counties, the differential is not so great.

The point here is that seasonal visitors, whether we recognize the fact or not, constitute an important part of Maine’s economic engine. And in our rural, inland regions, their relative importance is even greater.

What we do to welcome these part-time residents and encourage them to come more often, stay longer and become more involved in their communities will go a long way in determining the economic destinies of these regions that have otherwise been the unfortunate losers in the ongoing process of global economic transformation.

Charles Lawton is senior economist for Planning Decisions, a public policy research firm. He can be reached at:

clawton@maine.rr.comn