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The vertical structure of the ISM in the Milky Way is complex. A cross-sectional cut of the Galaxy would reveal a stratified distribution of material ranging from the cold ($T\simeq10$ K) dense molecular clouds, which are closely confined to the Galactic disk, to very hot ($T\simeq10^6$ K) diffuse gas extending into the Galactic halo with a scale height ($h$) of perhaps some 5 kpc. Between these extremes lie cold H I (the so-called ``standard clouds''; $T\simeq100$ K, $h\simeq100$ pc), warm H I clouds ($T\simeq10^{3-4}$ K, $h\simeq500$ pc), warm ionized H ($T\simeq10^{3-4}$ K, $h\simeq1$ kpc), and hot gas ($T\simeq10^5$ K,$h=$ 2--5 kpc). In order to understand the dynamical processes which produce and maintain this vertical structure --- and to understand the interrelationships among these distinct classes of interstellar gas --- we must first investigate the physical properties of the observed phases of the ISM. Accurate measurements of temperatures, compositions, ionization states, densities, etc., for individual interstellar clouds are required for the development of a unified model of the ISM.
The UV spectral region is rich in absorption-line diagnostics which allow the study of interstellar matter over a wide range of physical conditions --- from the cold H I clouds, which produce strong absorption from many singly-ionized metals, to the hot 10$^5$ K gas seen in absorption from such ions as C$^{3+}$ and N$^{4+}$. In this poster I will present results from a multiyear program of UV spectroscopy with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope whose goal is to help characterise the properties of individual interstellar clouds in the Galactic disk and halo. I will concentrate on results for three lines-of-sight extending into the lower halo towards the early-type, high-latitude stars HD 93521, HD 149881, HD 215733 ($z$-distances $\simeq$ 1500 pc) and discuss the physical conditions in the interstellar clouds and the implications for understanding the origin of the structure of the ISM in the Galatic halo.
