- var burnCount = 40

#ui
  .fire
    - for (var i = 1; i <= burnCount; i++)
      .burn.heat
    - for (var j = 1; j <= burnCount; j++)
      .burn

$fireSize: 500;
$burnSize: 100;
$burnCount: 40;

body {
  background: #000;
  height: 100vh;
  overflow: hidden;
}
#ui {
  position: absolute;
  top: 50%;
  left: 50%;
  margin-top: #{-$fireSize / 4}px;
  transform: translate(-50%, -50%);
  transition: 100ms;
  
  .fire {
    position: relative;
    width: #{$fireSize}px;
    height: #{$fireSize}px;
    background: #ff9900;
    filter: blur(20px) contrast(30);
    border: #{$fireSize/2}px solid #000;
    border-bottom-color: transparent;
    border-radius: 40%;
    box-sizing: border-box;
    transform: scale(0.4, 1);
    
    .burn {
      position: absolute;
      top: #{$fireSize}px;
      left: #{-$burnSize / 2}px;
      width: #{$burnSize}px;
      height: #{$burnSize}px;
      background: #000;
      border-radius: 100%;
      
      @for $i from 1 through $burnCount * 2 {
        &.heat:nth-child(#{$i}) {
          height: #{random(10)}px;
        }
        &:nth-child(#{$i}) {
          height: #{random($burnSize / 2)}px;
          margin-left: #{random($fireSize) - $fireSize / 2}px;
          animation: burning #{random(2000) + 1000}ms -3000ms infinite linear;
        }
      }
    }
  }
}

@keyframes burning {
  0% {
    transform: translateY(0);
  }
  100% {
    transform: translateY(-#{$fireSize+$burnSize}px);
  }
}