The regularities of the near-surface quasihomogeneous layer (NQL) formation during the daytime have been studied. The experiments were carried out on the 18th cruise of the R/V Akademik Vernadskiy (JASIN-78 international program) with an upward-floating sonde. According to the experimental data, the thickness of the NQL in the midday hours under stable meteorological and radiation conditions practically does not change compared with the morning and evening. The possibility of describing the NQL by means of an integral model taking into account the volume absorption of solar radiation was considered. A comparison of the experimental and theoretical values of the mixed layer depth has been carried out according to the data of measurements in four different regions of the Atlantic Ocean.