Because people in the 17th century had no standards for air quality and thus recorded few observations, it is difficult to compare today's conditions with those before European settlement.

Aside from the disturbing documentation of decline that inevitably emerges to instruct us about our modern treatment of environment, there is little information on this topic to interpret what Smith and his shipmates were able to see during their voyages exploring the Bay. The story which emerges is pretty complex.

Visibility in the spring and summer over the Chesapeake Bay depends on many factors, including the degree to which salt particles have been blown off wave tops, and the temperature differential between water and the overlying air. If the water is cool enough and the air humid, vapor in warm air can cool to its dew point and haze or fog can form. The process is exacerbated in modern times by several groups of pollutant chemicals and the many fine particulates that remain suspended in the atmosphere from our activities.

In 1608, Smith and his crew set out to explore the Chesapeake in an environment largely different from what we experience today. After their trip up the lower Chesapeake Eastern Shore, the discovery barge from Jamestown emerged from the Straits of Limbo (today's Hooper Straits). They named the end of that land Momford's Point. Today it's the tip of the Hooper Island chain. It was, near as can be figured, June 11, 1608. Their account remarks:

"We passed by the straits of Limbo for the western shore: so broad is the Bay here, we could scarce perceive the great high cliffs on the other side."

This view can be exactly duplicated today on a clear morning, and I measured it aboard my yawl Nimble, a vessel of similar size. This distance, at which the sunlit yellow of these cliffs winks above the horizon on a small sailing vessel, is just 12.1 nautical (13.9 land) miles. That's one instance where the lower limit of visibility can be locked in with unusual accuracy.

Even today, that's not all that unusual a range of vision for the Chesapeake in early June. It is a time when the Bay is, in most years, still relatively cool, and frontal passages are frequent enough that the atmosphere is well stirred and thus clearer with minimal haze.

There are a few historic illustrations from much later that help us "look" at visibility around the Bay. Two are of the lower Patapsco River harbor area, which Smith and his mariners visited 144 years earlier. Here, Smith suspected were valuable mineral resources and, on the coastal plain, useful farmland.

Wayne Clark, one of my co-authors in "John Smith's Chesapeake Voyages, 1607-1609," believes they explored this estuary thoroughly, actually looking for Indian settlements. They found none, to their surprise, a situation we now figure was the result of intertribal warfare which left this part of the Chesapeake a no man's land.

It did not remain unpopulated for long. Artist John Moule painted a view of this harbor area in 1752 from an elevated point, looking over grazing animals and scattered trees toward distant farms and a rural landing with one finger pier. In a painting made in the 1840s, the then-established Baltimore is cluttered with sailing vessels and a single, side-wheel steamer. Both views are from Federal Hill.

Today, the Federal Hill looking out at Moule's view, an observer would see Baltimore's redeveloped Inner Harbor, and behind it the city's tall buildings fading off into whatever haze is there on that particular day.

In the late 19th century, observers stationed in a watchtower at the summit of Federal Hill used this vantage point to identify incoming ships. They recorded as a standard of visibility being able to see White Rocks (a geological formation near the Patapsco's mouth). This distance is about 9 nautical (10.4 land) miles. Given that the manufacturing age was well in progress around this already urban estuary, such a distance hardly represented the ideal pre-industrial visibility. It is, however, a pretty good comparison for today's summer atmospheric turbidity, with its dense, photochemical haze from vehicle, power plant and industrial emissions.

There is a buoy in the Patapsco ostensibly marking where Francis Scott Key was detained during the War of 1812 bombardment of Fort McHenry. This battle was fought in the summer of 1814 and during the British attack, Key and his shipmates claimed to have seen the Fort's flag by dawn's early light. (The resulting song is "The Star-Spangled Banner," our national anthem.)

The distance is only about 3.5 nautical (a bit more than 4 land) miles. While this range of visibility is not unusual today, I've often thought it would be instructive for the Park Service, which is custodian of Fort McHenry, to have a mark out at that range to determine how many days the flag would be visible in modern summers.

I stood on the bridge of the EPA's research ship, Peter W. Anderson, in August 1988 with Capt. Dwight Paine, discussing the sightlines in the Patapsco River as we headed into Curtis Bay. Visibility was about 2.6 miles (less than 3 nautical miles). Paine was angry and shook his head, saying that hazes are even seen out at sea today. "There is no way," he said looking ahead of the ship, "that haze like this was here 40 or 50 years ago. This is all from pollution."

In 2001, Maryland's Department of Environment CAMNET program proposed installing a haze camera on Key Bridge pointing essentially at what would have been Key's 1814 view of the Baltimore skyline, The camera's been moved to a nearby location, but real time views are found at

A pre-1799 painting shows George Washington-then a gentleman farmer retired from the presidency-talking to his foreman, with Mount Vernon in the background about a quarter mile to the east.

The far shore of the Potomac is also in the background, receding into the hazy distance. The river turns here, at a point on the Maryland shore called Marshall Hall, so the true extent of visibility can't be discerned. At this distance, about 3.25 nautical (3.75 land) miles, though, the shoreline is pretty well faded into the late summer haze (grain is being harvested, and daisylike flowers are in bloom at the feet of figures in the painting).

The apparent visibility in the painting is not too different from that of today, so hazy conditions seem to have occurred at least sometimes in the late 17th century. What we do not know is how often this happened.

It is assumed that sight distances around the Bay and its watershed were much greater at the time of European contact than today. For example, in the Shenandoah, rising atmospheric contaminant levels in our lifetime-photochemical haze-have caused dramatic decreases that impair or outright terminate the long vistas once enjoyed by visitors. Thomas Jefferson knew this region well and wrote about it in 1784-1789, lamenting the loss of Virginia's forests after the time of settlement. He showed a remarkable understanding of local weather mechanisms:

"These settlements formed a zone in which, though every point was not cleared of its forest, yet a good proportion was cleared and cultivated. The cultivated earth, as the sun advances above the horizon in the morning, acquires from it an intense heat which is retained and increased through the warm parts of the day. The air resting on it becomes warm in proportion, and rises."

This phenomena produces what's called "thermals." They often result in clouds forming during hot days-usually puffy "fair weather" cumulus clouds. Jefferson is close to understanding the "sea breeze effect":

"[The breeze] from the east is strong, as passing over the ocean, wherein there is no obstacle to its motion. It is probable that this easterly breeze forces itself far into, or perhaps beyond, the zone which produces it. This zone is, by the increase of population, continually widening into the interior country."

Jefferson later speculated about climatic changes from deforestation and agricultural land use. "The Eastern and South-eastern breezes come on generally in the afternoon. They have advanced [with clearing of the land] into the country very sensibly within the memory of people now living. They formerly did not penetrate far above Williamsburgh. They are now frequent at Richmond and every now and then reach the mountains. ...As the lands become more cleared, it is probable they will extend still further westward."

He also stated: "We know too little of the operations of nature in the physical world to assign causes with any degree of confidence." But then he wrote that people are "willing always, however, to guess at what we do not know."

Jefferson also discussed atmospheric clarity elsewhere in the watershed. At Monticello, his home at the edge of the Blue Ridge mountain range, he made use of its elevation and vantage to make many observations about weather, and even climate from 1741 to 1769. Jefferson even postulated a warming climate:

"A change in our climate however is taking place very sensibly. Both heats and colds are becoming much more moderate within the memory even of the middle-aged. Snows are less frequent and less deep. They do not often lie, below the mountains (He compares them with the Shenandoah and other valleys.) more than one, two, or three days and very rarely a week. They are remembered to have been formerly frequent, deep and of long continuance."

(This is a cautionary tale for our personal observations today and the tendency to think we personally have experienced climate change. Don't think I doubt global warming for a minute. Watch a couple of more decades, though, and after a run of colder years, people will claim global warming is either a chimera, or has been reversed by management actions. I suspect neither will be true in our foreseeable future.)

Jefferson went to discuss the mirage-like "looming"-stretching, distortion or even inversion-occurring in the images of distant objects, which arises from vertical temperature differences in the atmosphere. His observation of sight differences is only incidental to his discussion, but he matter-of-factly states that from Monticello in the mid-18th century:

"The Blue ridge of mountains comes into view, in the North East, at about 100 miles distance, and passes by [Monticello] within 20 miles and goes off to the South-west". He discusses the looming phenomenon being "seen at 50 miles...and on objects at least 40 or 50 miles distant."

This corroborates MDE estimates that natural visibility in this region is about 90 miles. The current visible range is more likely 14-24 miles. Wouldn't we like to have visibilities like that today?

Occasionally we do. It's rare, but possible to see the Washington Monument's obelisk in the District of Columbia from Hog Wallow in the Blue Ridge Mountains, a distance of 77 miles.

What's going on here? Observations at Shenandoah National Park help to illustrate this loss of visible range.

Shenandoah was created by Civilian Conservation Corps workers between 1933 and 1942 out of the wilderness, as well as almost 5,000 private homes within the park's perimeter which were razed and their inhabitants displaced. The Skyline Drive was built from 1931-39 at a cost of $5 million, a tidy sum for the time.

One of the great draws of the park is its dramatic vistas. In 1941, my parents visited the new scenic roadway, and experienced these dramatic views quite unimpeded.

When I visited in 2003, more than 60 years later, the views were not as good. Park Service data show that in the last 50 years-and even in the last 20-the decrease in visibility has been dramatic. Two thirds of the loss is directly assignable to human causes, and most of that to coal-fired power plants.

There are dramatic instances of clarity, but the proportion of summer days with impaired visibility are distributed as follows:

48 miles [77 km] 10%
16 miles [25 km] 50%
Less than 7 miles [14 km] 11%

Sulfur dioxide residuals from fossil fuel combustion play a significant role in reducing air clarity, by virtue of forming smog and generalized haze.

In August 2003, a widespread electric power blackout occurred in the northeastern United States and southeastern Canada, shutting down a large number of power plants, including those in Ohio, West Virginia and Pennsylvania.

University of Maryland scientist Lackson Marufu and colleagues took measurements from aircraft and found that atmospheric sulfur dioxide dropped 90 percent, resulting in a 50 percent reduction in smog during the blackout. Skies quickly became dramatically bluer and visibility increased by a stunning 20 miles.

Reduced visibility costs money, let alone the health problems all of us suffer to one extent or another. Smoke haze in Southeast Asia was estimated, in 1997-98, to have cost agriculture, transportation and tourism more than $9 billion. Any further questions?