There are several ways off making this happen including frontal lifting, and outflow from other thunderstorms. In the mid-latitudes during the summer months, the most common type of stimulation for the aspiring air parcel to begin it's journey to stormhood, is the uneven heating of the earth's surface. Air in one area becomes warmer in relationship to the air around it and becomes upwardly mobile. When the temperature of the air surrounding the parcel and the dew point temperature are equal, we have condensation. If cooling continues, we arrive at the cloud condensation level, and we have a cloud.
All thunderstorms start off as a cumulus cloud, the fleece white cloud of summer days.
However when the conditions are right, this normally docile cloud can continue to rise, and as the day gets hotter, form into a cloud known as the cumulus congetus, or Towering Cu. When the top of the cloud gets way on up there it starts to freeze, and we see the tops of the cloud go from a cauliflower shape to a more fuzzy shape as ice crystals form. Don't think for a minute freezing at the top of a cloud is the same temperature that water freezes at the earth's surface. At any rate, ice forms at the top of the cloud and we can have all kinds of interesting phenomenon such as hail, rain, lightning, and down burst. Wind shear is change in wind speed with direction and height.