1. A computer implemented weather based method of estimating solar irradiance, the method comprising: accepting a text description of a weather condition for a given time in a computing system; classifying by a computing system the text description of a weather condition for a given time into a classification string according to the text description of a weather condition for a given time by a computing system; providing, in a computing system, a dummy variable representing the respective classification string match; providing a coefficient for each dummy variable in a computing system; accepting in a computing system a theoretical clear sky global horizontal irradiance, a coefficient for temperature, ambient temperature and a dew point; subtracting the dew point from the ambient temperature to provide a Cloud Formation Level (CFL) value in a computing system; providing a coefficient for the CFL; determining an estimated global horizontal irradiance by a computing system to develop a solar irradiance estimation model for evaluating a solar resource dependent project, wherein the estimated global horizontal irradiance is the theoretical clear sky global horizontal irradiance multiplied by the total of the sum of the series of coefficients for respective dummy variables multiplied by the dummy variable representing the respective classification string match added to the coefficient for temperature multiplied by the ambient temperature added to the CFL multiplied by a coefficient for the CFL.