spread from areas of initial ignition can occur. For example, in the incendiary raid on Tokyo of March 9, 1945, an area of 8 square miles of the most highly combustile area of the city was seeded by bombers. The fire spread over 16 square miles in 6 hours, completely destroying it. Fires that spread rapidly along a front driven by high natural winds are called conflagrations. It is important to note, however, that fire spread of this magnitude (factor of 2) was a relatively uncommon experience during the World War II incendiary raids in Europe and Japan. The most frequent experience was that the area completely destroyed by fire was equal to or less than the areas initially seeded with bombs.

EFFECTS OF MEGATON WEAPONS

Since weapons today are likely to be in the megaton class, the absolute areas of fire damage in Hiroshima and Nagasaki are not of great significance, but the fact that many situations are likely to be encountered in different targets that would reduce the areas destroyed by factors of from 3 to 13 over what would be predicted by the most pessimistic assumptions, is of great importance for realistic evaluations of the effects of thermonuclear attacks.

For megaton weapons, the thermal energy is released over a longer time than for kiloton weapons so that fuels that would require 2-3 cal/cm2 to be ignited by a 20-kiloton explosion would require 4 to 5 cal/cm2 for one of 10 megatons. Thus under very dry conditions and with unlimited visibility, an airburst 1-megaton weapon could produce primary ignitions out to a distance of 10-11 miles and a 10-metgaton one out to 25 miles or over areas of 380 and 2,000 miles, respectively.

However, if these weapons were surface burst under more normal conditions of fuel moisture and atmospheric visibility, these areas could be reduced to less than 200 and 1,600 square miles, respectively. Under conditions of recent rain, irregular target geometry, hilly terrain and poor visibility, the maximum estimates could be reduced by a factor of 10 or more.

It is important to point out that, if an enemy chooses to surface burst his weapons in order to cover large areas with high levels of fallout radiation, he cannot at the same time achieve the maximum area of primary ignition that would result from the same weapon, airburst, because part of the thermal energy is absorbed in the ground and in debris from the crater which mixes with the fireball. Also, the area of shadows cast by hills, buildings, and so forth, would be greater so that fewer potential sources of primary ignition would be exposed to direct thermal radiation.

IMPLICATIONS OF MASS FIRE FOR FIRE PROTECTION AND CIVIL DEFENSE ORGANIZATIONS

One of the first conclusions to be drawn from the World War II experience with mass urban fires is that, in the areas severely damaged by blast, firefighting is virtually impossible. In the first place, a large percentage of unsheltered firefighting personnel in such areas would be killed or injured and their equipment destroyed by the blast. Even if such facilities were protected by blast shelters, the debris in the streets

would make it impossible to get to the fires. Furthermore, the large numbers of fires and their rapid development in a matter of minutes would completely overwhelm the normal capacity of the firefighting services and the heat would rapidly reach such high levels that personnel in the open could not live. Furthermore, the many breaks in the water system would reduce the available water to negligible amounts in a short time.

Any additional firefighting equipment and personnel that might be provided for emergency use following a thermonuclear attack should be located well outside blast damage radii, peripheral to likely targets and provided either with water supplies independent of the city system or ample firefighting chemicals. Their function would be to fight the spread of fire at the periphery of blast damage.

Similar considerations apply to emergency rescue, first aid and medical teams and to all sorts of emergency supplies of food, medicines and portable emergency hospitals. Also, all such facilities should be as highly dispersed as is economically feasible and practical.

Another serious problem is that for surface burst weapons, deposition of fallout would be taking place during just the period when such emergency services are most urgently needed. Such emergency units should be provided with fallout shelters and radiating monitoring equipment. Also, the feasibility of shielded vehicles designed for the specific tasks of such units should be investigated.

The mass fire problem has another important consequence for shelter programs. Any shelter designed to withstand appreciable blast pressures or located in an area where mass fire is probable, should be designed so that it can be completely sealed off from the outside air and a recirculating air purification and cooling system capable of operating for the duration of the fire should be provided. The reason is that during an intense mass fire, the air reaches high temperatures and becomes contaminated with carbon monoxide and dioxide as well as heavy smoke so that it would not provide a viable atmosphere for the shelter occupants. Many occupants of bomb shelters were found dead after the great Hamburg fire storm, apparently killed by asphyxiation, carbon monoxide poisoning or heat, who were otherwise uninjured and could have survived if the shelters had not been dependent on the outside air supply for ventilation. It also goes without saying that such shelters should be adequately insulated from conducted heat, that is, several feet underground.

A number of precautionary measures could greatly reduce the probability of primary ignitions in urban areas. Since combustible trash, such as scrap paper, excelsior and punky or rotten wood, are the fuels most easily ignited by thermal radiation, rigorously enforced regulations requiring that such trash be kept picked up and stored in tightly covered metal containers could greatly reduce the chance of primary fires outside the area of secondary ignitions resulting from blast damage. Also the proper care of exposed wood surfaces by painting is important. These procedures have the added merit of being good fire-prevention practice under normal conditions as well as improving the general appearance of a city. Surveys of potential sources of primary ignitions in typical U.S. cities have shown that the numbers of such sources can be as low as 1,100 to 1,600 per square mile in wellkept residential areas, whereas there can be as many as 11,000 to

15,000 per square mile in slum residential and wholesale warehouse areas at thermal energies of 5 to 9 cal. per square mile. cm.

Another precautionary measure that was tried during World War II was to clear firebreaks in cities. The net experience in Hiroshima and Nagasaki and in other Japanese cities that suffered mass incendiary raids was that firebreaks inside areas where primary and secondary fires were densely ignited over large areas had little effect on the development of mass fire storms or conflagrations, but in some cases both natural and artificial firebreaks of sufficient width helped to limit fire spread over small portions of the fire perimeter.

One of the chief factors influencing fire spread was in the degree of built-upness or the ratio of roof area to ground area. A survey of 11 Japanese cities indicated that for residential areas with 45 percent built-upness, 72 percent of the exposed areas burned. With 30.6 percent built-upness, 46 percent burned, and with 15.5 percent builtupness, 20 percent burned.

While it would not be economically feasible to decrease the builtupness in areas already built, it might be possible in many cases to regulate the maximum built-upness in new urban areas being developed. In this connection, the average U.S. city, particularly in residential areas, is less built up and less combustible than Japanese cities, so that fire damage here should be a smaller fraction of the exposed

area.

Mr. ROBACK. What is the criterion of "built-upness"?

Mr. HILL. It is the ratio of roof area to the total ground area. In other words, it is roughly the fraction of the area that is covered by buildings.

Mr. ROBACK. Regardless of height?

Mr. HILL. Regardless of height.

FOREST FIRES IN THERMONUCLEAR WAR

The problem of estimating the total area of forest and grassland that might be burned over in a thermonuclear attack on the United States and the effect of its long-term consequences on postattack recovery is fraught with many uncertainties. Forest and brush fires could be ignited by spread from fires started in urban and military targets or by the overlap of the area of primary ignition from weapon explosions on forested areas. Also in any large attack, there would probably be a number of gross bombing errors which could ignite wildland areas primarily.

It is even conceivable that an enemy might choose to allocate weapons specifically to the task of starting wildland fires, but in view of the importance to him of reducing our retaliatory capability to a minimum, this appears rather unlikely. It is true that the Japanese made a feeble, unsuccessful attempt to do this in World War II with balloondelivered incendiaries, but this was the only means that they could devise to carry any form of attack to the U.S. heartland and probably was never expected to accomplish more than a nuisance value.

A little over one-third the area of continental United States and Hawaii or about 1 million square miles is forest, brush, and grass land. Alaska contributes an additional 416,000 square miles. A little over one-fourth of this area is not utilized to grow sawtimber or

other forest products, but serves to protect watershed areas from soil erosion, to reduce flood danger and to replenish ground water supplies.

ANNUAL LOSSES FROM FOREST FIRES

In order to have some yardstick by which to measure the impact of possible forest fire damage from a nuclear attack, it is of interest to look briefly at forest fire experience in the past.

It is important to understand that the cooperative effort of government and private organizations to reduce the annual fire damage to our forests began slowly during the first decade of this century and was making real progress by the late thirties. By 1959, 94.7 percent of the forest lands in the continental United States had organized fire protection. The effect of this was to reduce the average annual burned forest area from 65,000 square miles for the 11 years from 1926 to 1936 to 5,340 square miles for the 3 years from 1957 to 1959. Table H-3 summarizes the forest fire experience in the continental United States for the period 1926-59.

TABLE H-3.—Forest area burned annually and numbers of fires in continental United States

The 11-year period from 1926-36 is of interest because there is no obvious trend of decreasing annual burned areas because of improved fire protection such as are shown from 1937 to the present. The range of fluctuation of annual burned areas is probably mainly due to the effects of variation in weather on burning conditions from year to year. This also applies to the fluctuation in the annual number of forest fires. For example, the lowest annual burned area of 38,000 square miles was in 1926, while each of the years, 1930 and 1931, accounted for 81,000 square miles. The range was 42 percent above the average and 25 percent below. For the annual number of fires, 1926 was the lowest with nearly 92,000 and 1936 was the worst with over 226,000, the range being 40 percent above and 43 percent below average.

Similarly, the average area burned per fire was 261 acres or about four-tenths of a square mile and the range was from 191 acres in 1936 to 343 acres in 1929 or 27 percent below average and 31 percent above.

The above figures suggest that for a nationwide nuclear attack of given magnitude on specified targets, variations in the total forest area burned, because of variation in annual fire hazards, could be expected to be about a factor of two from the best to the worst years.

CATASTROPHIC FOREST FIRES

Probably one of the principal reasons why some people have expressed the opinion that a very large fraction of our forested areas would be burned in the event of a nuclear attack is that they are familiar, either through direct experience or study, with the destruction resulting from a number of great catastrophic forest fires in the past. They envision large factors of fire spread from each of a large number of megaton weapons detonated, in all parts of the United States, under the worst burning conditions possible.

The term "catastrophic forest fire" is usually reserved for fires which spread over areas of 150 square miles or more causing great property damage in terms of timber and buildings destroyed and frequently resulting in loss of lives. Since 1825 there have been 12 great catastrophic forest fires. The greatest of these burned over an area of 5,900 square miles in northern Michigan and Wisconsin in October 1871. Many towns and farms were destroyed and 1,638 lives were lost. A large part of the burned area was valuable virgin forest. In the period from 1825 to 1910 there were eight great fires resulting in burned-over areas varying from 250 square miles to 5,900 square miles each. Since 1910 there have been four great fires which burned over from 156 square miles to 469 square miles per fire. The most recent of these were the fires in Maine and New Hampshire in October 1957 and the Malvern Hill fire in Florida in 1956. The former burned over 375 square miles, destroyed much property in Bar Harbor and took 16 lives.

These great fires are truly terrifying in their intensity, rate of spread, and the violence of the fire-generated winds which blow down large trees in advance of the flames and spread flaming brands to spot new fires 5 to 6 miles ahead of the fire front.

The Tillamook fire in Oregon during August 1933 burned 486 square miles of virgin Douglas-fir. The speed with which a forest fire can spread in heavy fuels under the most hazardous conditions is well illustrated by this fire. From August 14 at 1 p.m. until the early morning of August 24 the fire had burned about 63 square miles and it appeared that it might be brought under control soon. Thus, for over 10 days it had burned at an average rate of about 6 square miles a day. On the 24th, the humidity dropped rapidly to 26 percent and hot gale-force winds from the east sprang up. During the next 20 hours of August 24 and 25 the fire burned over an additional 420 square miles, or at a rate of 21 square miles per hour along a 15mile front. The fire was stopped only by the fact that the wind ceased and a thick, wet blanket of fog drifted in from the ocean.

CONDITIONS FOR CATASTROPHIC FIRES

It is important to realize, however, that very special conditions are necessary to make such great conflagrations possible. First, the stage is usually set for such fires by an abnormally dry year or possibly two or three such years in succession. Then a hot period of several weeks without rain, immediately preceding the fire, followed by hot, dry winds approaching gale strength, which drive the relative humidity down to 20 or 30 percent, and a large area of fairly dense forest fuel