AT PRECISELY 8:14 a.m. Eastern Daylight Time on June 21 the sun reached the northernmost point in the sky on its annual journey through the heavens. It was the summer soltice, the longest day of the year in the northern hemisphere.

Of more consequence to those of us who keep an eye on the weather (and who doesn't keep an eye on the weather?) the summer solstice marks the astronomical beginning of summer and sets the climatological stage for the hot season. Inexorably, the latitudes that were the domains of polar air during the winter (remember?) are being invaded by warm humid air from the Gulf of Mexico and the Caribbean as well as being tested by a few forays of hot dry air from Mexico's interior.

With the exception of the extreme northern districts, the mountains, and the northern coastal areas, few regions of the United States are immune from the enervating and oppressive effects of hot weather. But the hot season atmosphere can do more than merely spread blankets of discomfort; it can kill.

It is difficult to obtain precise statistics on hot weather fatalities. A hint is provided in the records of the National Weather Service which reveal, for example, that during a seventeen-year period ending in 1967, more than 8000 persons in the United States succumbed to excessive heat and solar radiation. Few among the natural hazards take a greater average toll. But these are the direct casualties. It is virtually impossible to document the number of deaths encouraged by heat or solar radiation. How many diseased or aging hearts, for example, surrendered in that period to the stress of heat waves? Washington the Tropical

The summer season in Washington, for climatological record purposes, extends from June through August, and rivals any other subject as a topic of discussion in the National Capital area. Long-term residents seem to take a perverse pride in boasting of the heat and humidity that settle over our town. Curiously, the image of our climate as an impossibly tropical one is entrenched in the minds of people all over the world who have never spent any time here.

Just how bad are our summers? Many of our elected officials certainly seem loath to spend the summer here. His contemporaries noted that Abraham Lincoln muttered obscenities about the heat, and sought relief on summer nights at the then-bucolic Soldiers' Home. British Embassy officials have noted that as recently as fifty years ago, staffers assigned to Washington during the summer received extra pay for serving under tropical conditions.

But these and thousands of other references and comments about our summer are mainly subjective and therefore suspect. The only way to assess our climate precisely is to mine out some of the facts and figures in the well-kept record books of National Oceanic and Atmospheric Administration climatologists. These data are impersonal, highly revealing, and permit no tricks of memory. They also show that summers here are really not the sources of discomfort that they are in many other cities of the Southeast. Washington, perhaps, deserves a better climatological image than it has acquired.

If you're a newcomer to the Washington area, or if you have a short weather memory, you will find it useful to skim some of the climate and weather statistics as we approach the hot season. Detailed weather records have been kept in this area of rmore than 100 years - an ample time period of reveal the normal and anomalous moods of our weather.

A scan of the statistics that tell the story of Washington summers past can suggest the probabilities of the summer weather yet to come, if you bear in mind that "normal" weather is made up of extremes. The chances are that what happened before will happen again.Bear in mind also that Washington, like other large urban areas, creates a "heat island," a region of noticeably higher temperatures, particularly at night, that distinguishes the built-up part of the metropolis from the less-developed suburban and ex-urban areas of the west and northwest.

A couple of hot spells are worth recalling: August 5-9, 1918, and July 18-22, 1930. These hot spells, like most others in Washington, were caused by a strong northerly drift of air from the subtropics that accompanied a westward extension of the Bermuda high-pressure area inland over the South Atlantic states.

The five-day period, August 5-9, 1918, smothered the District with excessive heat and humidity, and many residents were prostrated. During that period (remember, no air conditioning then), the average high temperature for each day was 99-6 degrees Fahrenheit. The nights brought little relief; the average nighttime low was 76.5 degrees, accompanied by oppressively high humidities. On August 6, most of the heat illnesses were reported, when the official temperature at the old Weather Bureau building at 24th and M Streets NW climbed to 105.5 degrees. A thunderstorm on the evening of the next day finally cracked the spell.

The other hot spell of notable duration, July 18-22, 1930, was marked by an average daily high temperature of 101 degrees, with lows at night remaining in the mid-70s. In the afternoon of July 20, the mercury touched 106 degrees - the highest temperature ever recorded here. Lower humidities, however, made this hot spell more bearable than the 1918 siege.

One of the yardsticks for measuring a summer is to note the number of days in which the maximum hits 90 degrees or more. During a normal June-August period, Washington records about twenty-one days with such highs. Against this average, the dry summer of 1966 was a painful standout, with nearly sixty days reaching highs of 90 degrees or more. Julys Hot and Cold

July is the warmest month, with the average daily high reaching 87 degrees, and overnight lows about 69 degrees. The highest July reading was the aforementioned all-time high for Washington of 106 degrees in 1930. The record low for July is 52 degrees in 1895. An interesting contrast in July statistics is a reading of 54 degrees recorded at 4 p.m., July 18, 1891, when a major storm system swept into town with heavy rain and chilly temperatures from the North Atlantic. July, normally, hs eleven days when maximum temperatures reach 90 degrees or higher.

It's a little cooler in August, but not much. The highs hover around 85 degrees while the daily lows weigh in at around 68 degrees. The highest August temperature of record was 105.5 degrees in 1918; the lowest August reading was 49 degrees in 1890. Normally there are six August days when the high reaches 90 degrees or more.

During summer nights, the average relative humidity simmers over 80 per cent, but during the afternoons it customarily falls to around 50 per cent. Into Every Life, Some Rain

Nearly a third of all summer months in Washington's history have had five inches or more of rain. At the other extreme, a little less than ten per cent of the summer months have had less than one inch. Less than ten per cent of all hours in the summer have rainfall at any time during the hour. Bearing in mind that outdoor activity in the Washington area is mainly planned for afternoon or evening hours, it is possibly useful to note that the probability of rainfall in summer during the hours from 2 p.m. to midnight is about twenty-five per cent.

Most of Washington's summer rainfall (which totals a little over twelve inches, if there is normal precipitation during June, July and August), is produced from heavily congested cumulus clouds or from "thunderheads" (cumulonimbus), and are of the showery variety, falling in scattered, checkerboard fashion. The showers are usually of the local air mass type, or the result of a frontal or squall-line system moving through the area. Although these transient showers or thunderstorms are often accompanied by brief strong and gusty winds, summertime, on the average is the time of year with the least wind. Afternoon velocities average seven or eight miles an hour, and the period from late evening until early morning has an average wind of less than five miles an hour.

Some summer thunderstorms are of the severe type, produced by cumulonimbus clouds that may reach 40,000 to over 50,000 feet above th surface. These towering storm cells can produce a variety of destructive products: violent shifting winds, hailstones, torrential rains, and lightning.

In the late summer, tropical storms, including full-grown hurricanes, occasionally swirl northeastward along the Atlantic coast. These storms, born over tropical waters, often yield a large portion of rainfall for this region at that time of the year. Tropical storms provide about twenty per cent of August's rainfall. Washington, fortunately, is so situated that it hardly ever is punished by the full sustained winds or, of course, by the tidal action of a hurricane. Hurricanes have usually lost some of their punch by the time they reach our region. If they approach over land from the southwest, they start to lose their tropical characteristics before they arrive, and their winds have moderated.

Summer weather in Washington often seems to linger into September, but the end of that month, the average daily high temperature is almost 10 degrees lower than at the beginning. Keeping Your Cool

All the record show one thing - that extremes of heat have occurred in the past and will occur again in the future. It no more than simply playing safe, therefore, to examine how the human body responds to excessive heat and humidity.

The body continually works to maintain a balance between heat it produces internally and heat it loses to the outside environment. The human body is, in effect, an automatic heating and cooling system. The food we eat provides the main source of heat. Body heat is generated and stoked by the oxidative processes of food ingestion. Heat is also gained from the surrounding environment by conduction, convection and radiation. But the body cannot keep storing heat; it is essential for it to get rid of excessive heat, and there are four mechanisms that do the job: evaporation (cooling when sweat evaporates), radiation, convection and conduction. Heat is also lost in a small way through excrets and unevaporated sweat.

The body possesses "thermostatic" equipment that makes it possible for man to maintain the balance of heat gain and loss, and to adapt to varying conditions of weather and climate. The hypothalamus, next to the pituitary gland at the rear part of the brain where it joins the spinal column, functions somewhat like the thermostat on the living room wall. This sensitive heat center reacts to heat or cold, alerting the neighboring pituitary gland to send the alarm to the body's entire glandular system.

Studies of heat syndrome and its victims indicate that it finds victims in all age groups. Other things being equal, however, the severity of the disorder increases with age. Heat cramps in a teenager may be heat exhaustion in someone about 40 years old, and heat stroke in a person over 60. For people whose hearts are generally under par, climatic stress is an added and sometimes dangerous burden.

Sunburn is not ordinarily classed with heat syndrome disorders, but it remains of pertinent mention because the burn injury to the skin by ultraviolet radiation sometimes significantly retards the skin's ability to shed excess heat.

The job of forecasting the weather, including anomalously hot or humid conditions, is carried out by the National Weather Service, a major component of the National Oceanic and Atmospheric Administration, Department of Commerce. The Weather Service, however, does not provide such key alert words as "watch" or "warning" in relation to hot weather forecasts as they do with respect to forecasts of hurricanes or tornadoes. It is up to each of us to assess our own individual limits and capabilities in adapting to the heat of summer.

The Weather Service, joined by the Public Health Service, Red Cross, American Medical Association, and other organizations, offers the accompanying heat wave safety rules and information on heat syndrome. Those who would be forewarned and forearmed would be wise to study the charts as we move into the season of the heat wave.