Terahertz (THz) communication has been envisioned as a key technology for providing ultra-wide bandwidth and high-speed data transmission in future networks. However, THz band signal suffers from frequency-dependent path-loss, molecular absorption and line-of-sight (LoS) blockage. Thus, coverage modelling for THz networks has essential difference from low-band networks. In this letter, considering blockage, directional antennas and molecular absorption, we develop an analytical model for coverage probability analysis in outdoor THz networks based on stochastic geometry. Accuracy of the proposed model is validated through Monte-Carlo simulations. Based on the proposed model, we further uncover the impact of nodes density, transmission distance and atmosphere on network coverage and available transmission bandwidth. Numerical results reveal that denser nodes, longer transmission distance, lower latitude and more humid seasons result in lower coverage probability and narrower transmission windows.