The report summarizes experimental results on the mechanisms and kinetics of hydrogen-air flammable gas cloud formation and evolution due to foreseeable (less than 10-3 kg/sec) hydrogen leaks into confined spaces with different shapes, sizes and boundary conditions. The goals were - 1) to obtain qualitative information on the basic gas-dynamic patterns of flammable cloud formation at different leak velocities (between 9,35 and 905 m/sec) for a fixed leak flowrate and 2) to collect quantitative data on spatial and temporal characteristics of the revealed patterns. Data acquisition was performed using a spatially distributed, reconfigurable net of 24 hydrogen gauges with short response time. This experimental innovation permits to study spatial features of flammable cloud evolution in detail, which previously was attainable only from CFD computations. Two qualitatively different gas- dynamic patterns were documented for the same leak flowrate. In one limiting case (sufficiently low speed of leak), the overall gas-dynamic pattern can be described by the well-known "filling box" model. In another limited case (high velocity of leak), it is proposed to describe the peculiarities of gas-dynamic behavior of flammable cloud by the term of a "fading up box" model. From the safety view point, the ?fading up box? case is more hazardous than the "filling box" case. Differences in macroscopic and kinetic behavior, which are essential for safety provision, are presented. Empirical non-dimensional criterion for discrimination of the two revealed basic patterns for hydrogen leaks into confined spaces with comparable length scale is proposed. The importance of the revealed "fading up box" gas-dynamic pattern is discussed for development of an advanced hydrogen gauges system design and safety criteria.