During a four-week period at midsummer in which reports of major crime around the city dropped by almost 5% from the same period last year, a sharp increase in rape complaints was recorded.

Like most other major crimes – except robbery, which is up 0.6% so far this year, and grand larceny, which is holding steady from last year – rape is down for the year, by 4.9%. But in the 28-day period that ended August 21, the number of rape reports was 31% higher than for the same period in 2004.

The summer jump in rape complaints is most apparent in Manhattan, where in the four weeks from July 25 to August 21 there were 36 rape complaints, compared to last year’s 24; in Brooklyn, where there were 46 complaints this year and only 34 last year, and in Queens, where rape complaints jumped to 48 this year from 29 last year.

Law enforcement officials also reported that 13% of rapes in the five boroughs in the 28-day period that ended August 21 were classified as “stranger rapes,” as compared to the year-to-date average of 8.5%.

Historically, significantly more rapes occur in July and August than in other months, a professor of security management at John Jay College of Criminal Justice, Robert Mc-Crie, said. He has studied national statistics from the FBI’s Uniform Crime Reports going back to 1968.

In 2003, Mr. McCrie said, 9.5% of all rapes in America occurred in July and 9.6% in August, while only 7.1% of rapes happened in December. In 2002, 9.6% of rapes took place in July and 9.5% in August, compared to only 6.9% in December.

The rise in rape during the two hottest months of the year conforms to the tendency of violent crimes to increase when the weather is warmer, Mr. McCrie said. The phenomenon, he said, is caused by a combination of factors, such as people being outside more often, the altered effects of alcohol consumption in warmer weather, and a general change in behavior.

In the New York area, this summer has been “significantly hotter” than last, a meteorologist, Frank Lombardo, said.

“The heat was compressed into the first week of August and the last two weeks of July,” Mr. Lombardo, who is with a consulting company, WeatherWorks, based in Hackettstown, N.J., said last week.

In 2004, the average high in Central Park in July was 81 degrees, and there were no days above 90 degrees. This July, the average high was 85 degrees, and eight days had temperatures above 90, including two when the temperature exceeded 95 degrees. Between August 1 and 24 of last year, the average high was 80 degrees, and the hottest day was 89 degrees, and in the same period of this year the average high was 88.5 degrees, and seven days had temperatures of at least 95 degrees.

In the period from July 25 to August 21, while murders as well as rapes exceeded the total for the comparable period last year, fewer forcible robberies and assaults were reported.

“It’s inadvisable to try to interpret the significance of data from just one 28-day period,” the New York Police Department’s deputy commissioner of public information, Paul Browne, warned.

“Having said that,” he went on, “the single largest category of rapes, by far, reported in the 28-day period was ‘acquaintance rape,’ meaning that the victim knew the attacker. The next-largest category also included attacks in which the victim and attacker knew each other – namely ‘domestic rape,’ meaning the victim was a relative, usually the spouse or common-in-law spouse of the attacker.”

Given that acquaintance rape and domestic rape accounted for 91% of all rapes year, Mr. Browne said, the recent spike in rape complaints “may indicate a greater willingness of rape victims to come forward even when they know the attacker.”

Or it may indicate nothing more than the nature of statistics to “return to the mean,” a professor of police studies at John Jay College, Eli Silverman, said.

Rape has gone down considerably this year, and a single “uptick” does not necessarily indicate a significant trend, he said, pointing out a drastic jump in subway crime a few months ago that faded as quickly as it emerged.

Thanks to the Police Department’s CompStat system, which records crime statistics by precinct and tracks trends, “these figures come in so quickly” that almost as soon as there is one of these upticks, the department focuses on the problem and seeks to resolve it, Mr. Silverman said.

“It’s important that it’s called to their attention, and now that it’s on their radar,” Mr. Silverman said, “I’m sure the commissioner and other high-ranking people will put more resources in to knock the numbers back down.”

“Resources constantly shift according to conditions,” a Police Department spokesman, Jason Post, said, “and through different monitoring mechanisms, we take note of trends and upticks and respond accordingly. This is something we take very seriously in our Special Victims Unit, and we have a domestic violence officer in every precinct, and procedures in place to deal with rape.”