Explain why helium weather balloons, which are used to measure atmospheric conditions at high altitude, are?

Atmospheric pressure is higher at sea level, so as the balloons ascend, and pressure decreases, the helium atoms expand.

Overfilling at sea level would cause balloons to burst at higher altitudes.

Because as they gain altitude the reduced air pressure on the outside of the balloon allows the helium to expand.  If the balloon were filled to its capacity then the balloon would burst at high altitude. Helium balloons are capable of reaching 100,000 feet or more. Even at a trifling 30,000 feet the air pressure is a mere fraction of that at sea level.

Only a guess, but the meteorological folks sending up the balloons probably want them (the balloons not the meteorological folks) floating at a certain altitude.  Too much helium would take the balloons too high and therefore the readings would be mostly useless.  Just a guess though.

If that is the case it's because the volume of air pushing against the balloon and consequently the helium inside is much greater at sea level.  Once the balloon ascends into the atmosphere the conditions are less dense allowing for expansion.  If the balloon were to be filled to near capacity it would potentially burst from the expansion of the gas, which wouldn't be good for the instruments being carried or the data it was supposed to collect.  Hope that helps.

the reasons are given below:
1. Helium is the lightest inert gas in the periodic table.
2. As we rise above sea level the atmospheric pressure starts falling by the law for ideal gases.
3. The volume increases as we rise above sea level by the law for ideal gases.
4. So as we rise in the height the pressure will fall (very low) and the volume of the gas will increase (by several times).
5. If the gas is lighter it will make us go at higher and higher altitude.
6. So we fill the balloons low in volume with Helium just sufficient to make them lift the attached instruments.
7. When they reach the height of few kilometers the volume will expand by few times of original and still being lighter than the surrounding air it will continue to rise up to very high altitude.
8. When the pressure of the gas inside the balloon becomes exact with the outside then the rise will stop and balloon will just float.
9. By the variations in the temperature in that layer of atmosphere beacuse of day chages, the balloon will loose some of its Helium and after few days in air it will start to fall down slowly.
10. Scientists can locate such balloons with GPS attached to them and collect them to gather data recorded by them.