SOURCE

// 生成字母和数字数组
function generateText() {
  const letters = [];
  const numbers = [];

  const a = "a".charCodeAt(0);

  for (let i = 0; i < 26; i++) {
    letters.push(String.fromCharCode(a + i));
    
    if (i < 9) {
      numbers.push(i + 1);
    }
  };

  return [...letters, ...numbers];
};

// 从生成的数组中随机取出一个字符
function randomText() {
  const texts = generateText();
  const text = texts[Math.floor(Math.random() * texts.length)];
  return text;
};

function rainEffect() {
  const cloudEle = document.querySelector(".cloud");
  const textEle = document.createElement("div");

  textEle.innerText = randomText();
  textEle.classList.add("text");

  const left = Math.floor(Math.random() * 310);
  const size = Math.random() * 1.5;
  const duration = Math.random();
  const styleSheets = {
    left: `${left}px`,
    fontSize: `${0.5 + size}em`,
    animationDuration: `${1 + duration}s`,
  };
  Object.assign(textEle.style, styleSheets);

  cloudEle.appendChild(textEle);
  setTimeout(() => {
    cloudEle.removeChild(textEle);
  }, 2000);
};

// 每隔20ms创建一个雨滴元素
setInterval(() => rainEffect(), 20);

  <div class="container">
    <div class="cloud">
      <!-- <div class="text">a</div> -->
      <!-- <div class="text">b</div> -->
      <!-- <div class="text">c</div> -->
      <!-- 雨滴将会在这里出现 -->
    </div>
  </div>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

:root {
  --body-color: #181c1f;
  --primary-color: #ffffff;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  background-color: var(--body-color);
}

.container {
  width: 100%;
  height: 400px;
  display: flex;
  justify-content: center;
  border-bottom: 1px solid rgba(255, 255, 255, .1);
  /* 添加一个从下往上线性渐变的镜像效果，增加视觉层次感 */
  -webkit-box-reflect: below 1px linear-gradient(transparent, transparent, transparent, transparent, #0005);
}

.cloud {
  position: relative;
  top: 50px;
  z-index: 100;

  /* 横向云朵 */
  width: 320px;
  height: 100px;
  background-color: var(--primary-color);
  border-radius: 100px;

  /* drop-shadow函数将阴影效果应用于投影图像 */
  filter: drop-shadow(0 0 30px var(--primary-color));
}
.cloud::before {
  content: "";
  /* 左侧小云朵 */
  width: 110px;
  height: 110px;
  background-color: var(--primary-color);
  border-radius: 50%;
  position: absolute;
  top: -50px;
  left: 40px;

  /* 右侧大云朵 */
  box-shadow: 90px 0 0 30px var(--primary-color);
}

.cloud .text {
  position: absolute;
  top: 40px;
  height: 20px;
  line-height: 20px;

  text-transform: uppercase;
  color: var(--primary-color);
  /* 为文字添加阴影，看上去发光，增加视觉效果 */
  text-shadow: 0 0 5px var(--primary-color), 0 0 15px var(--primary-color), 0 0 30px var(--primary-color);
  transform-origin: bottom;
  animation: animate 2s linear forwards;
}

@keyframes animate {
  0% {
    transform: translateX(0);
  }

  70% {
    transform: translateY(290px);
  }

  100% {
    transform: translateY(290px);
  }
}