Storms can do a lot of damage to polytunnels and glasshouses. Wind-pressure increases with about the square of its speed, so doubling windspeed quadruples its force. But there’s plenty you can do to prevent it.
Buy the strongest-framed structure you can afford. Secure it well into the ground, following the supplier’s instructions for your conditions. Be careful where you place it; solid walls or buildings nearby can increase windspeed and cause damaging turbulence.
Check the cladding. Polythene should be secure at the sides and around doors and vents, and any holes or tears cleaned with isopropanol, dried and taped inside and out with clear cold shock resistant repair tape. Secure loose glasshouse panes and replace cracked or broken ones and missing or displaced glazing clips (you did keep those spare clips, didn’t you?). If glazing clips slide hold them in place with silicone mastic. Tighten loose bolts.
Store away any empty pots, buckets and bins that a storm could pick up and hurl through the greenhouse. Close and secure all doors and windows when a storm is brewing.
If you do have storm damage, stay away until the wind abates to avoid injury. Check all cladding, doors and vents again, and do repairs as soon as possible before the next storm. A damaged greenhouse is more prone to further damage, as any opening lets in the wind to burst it like a balloon. Keep glasshouse assembly instructions for reference if they have the pane sizes, and consider keeping pre-cut spare panes in store if replacement glass is hard to get quickly.
Shelter (hedges, plastic webbing etc.) should be partly permeable to filter and slow down the wind, not block it and cause damaging eddies. If shading is a problem, shelter works nearly as well behind a greenhouse as in front.