Good question! The weather in the Swedish mountains changes quickly. As Abisko and Nikkaloukta are situated north of the polar circle the evenings and nights are often quite cool. Rough weather with hard winds and heavy rains happens, why it is important to bring warm clothes. As a general rule we recommend you to bring clothing that will keep you warm and comfortable in a few degrees (centigrade), rain and heavy wind. Weather is usually not this bad but if you make sure to be comfortable in these conditions you will not be disappointed and be able to enjoy the week despite bad weather conditions.
A good summer day in Lapland the temperature can get up to 20 degrees Celsius, usually around 15. In these conditions we usually prefer walking in a t-shirt and pants.