Iceland will play Nigeria in Volgograd on Friday, with the temperatures exceeding 86 degrees Fahrenheit in the city.
"We expect to win. It's hot, and it will be easier for Nigeria to play in such conditions than for us. But we'll think about the weather when we draw up our set piece,"Hallgrimsson told reporters.
"Their [Nigeria's] strength is in the physique… They are quick, all of them are good athletes… If you watch their games throughout the last year, you can see the improvement in tactical awareness… They have players of the highest quality, so this is a massively good team with a lot of individual skills," Hallgrimsson said.
The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the matches being played in 11 cities across Russia.