The state of Rio Grande do Sul, has a lot of rainfall problems, the largest being related to the hailstorm. The motivation of this work is based on the difficulty of the predictability of the event because this may happen in micro or mesoscale. A good analysis of this with remote sensing and in situ data is very important to calculate the indexes of instability, which have a great relationship not only to hail but with other phenomena. With the values found for these cases may be established thresholds for the same using computational techniques helping to the forecast. The cases chosen were those that had the greatest social and economic impact in the last six years and that occurred in the state of Rio Grande do Sul. In order to calculate the indexes we used the data survey of the Salgado Filho International Airport in Porto Alegre. The indexes are calculated: SWEAT index (Severe Weather Threat) CAPE (Convective Available Potential Energy), K (index K), TTI (Total Totals Index), LI (Survey Index) and SI (Showalter Index).