_________ is used for filling weather observation balloons.

Explanation

**Step-by-Step Solution:**

1. **Identify the Purpose**: The question asks for a gas used to fill weather observation balloons.

2. **Recall Common Gases Used in Balloons**: Think about the gases that are commonly used in balloons. Two common gases are hydrogen and helium.

3. **Consider the Properties of the Gases**:
- **Hydrogen**: While hydrogen is lighter than air, it is highly flammable and can be dangerous.
- **Helium**: Helium is also lighter than air but is non-flammable, making it a safer option.

4. **Conclusion**: Given that helium is lighter than air and non-flammable, it is the preferred choice for filling weather observation balloons.

5. **Final Answer**: Therefore, the gas used for filling weather observation balloons is **helium**.

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