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One of the lessons that we all learnt living in colder climates as a child was how to keep warm by wearing layers of cloths. 
Layers of cloths trap air between each layer. Cold air on the outside will be able to make the outer layer air cold, but the body will keep the inner layer air warm. By wearing many layers the body is kept warm by the layer of trapped warm air.
Thai people often have never experienced cold weather. Being constantly cold will be a problem for any Thai national. Thai people will even find mild summer days and warm spring days cold.
Before she arrives already have
Socks
Scarves
Hats
Gloves
House Coats
Extra bedding. i.e blankets or quilts
Hot water bottle.
When you girlfriend arrives, we strongly suggest taking her out shopping, buying different layers of cloths and explain how this will keep her warm. Even in spring or summer make sure you have warm winter type coats.
Primark is a fashionable low cost high street retailer in the UK and Ireland
Ideas for layers Tops
T shirt, shirt and cardigan or jumper, winter coat
Ideas for layers Skirts and Jeans
If you girlfriend wears a skirt, then suggest also wearing tights, socks and warm boots.
If you girlfriend wears trousers or jeans then suggest wearing tights and socks.
House Heating
Even on a warm day, expect that your girlfriend will require the central heating on and the windows closed.
Keeping your girlfriend warm
Keeping your girlfriend warm will make her stay with you more enjoyable
Every woman likes to feel protected and loved by her man, she will feel safe and comfortable with you and will really appreciate the time and effort you have given her to make her feel warm.