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Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. An area of rotation, 4-10 kms wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
If you are inside, Stay inside with doors and windows shut and stay away of these windows and exterior walls. Go to small interior room or stairwell on lowest floor of building (bathrooms are often best). If available, crouch under large/heavy furniture. Protect your head with cushion/mattress. If you are outside, seek shelter in building (not car/RV) immediately. If none available, lie flat in low dry spot (ravine/ditch) or under low bridge Keep alert of flooding. As a last resort, hang on tightly to the base of a shrub or small tree. If you are in a car/RV, do not stay in vehicle and do not drive to outrun it. Most people actually drive into the path. If, possible, run to nearby solid building. If none available, lie flat in ditch/ ravine and watch for flooding. Protect your head. Do not go under bridge overpasses, they are not safe locations as winds funnel into these areas and off little protection.