Is Zakat obligatory on a fluctuating sum of money in a bank account, ranging between 35,000 and zero?

Zakat is obligatory on wealth under two conditions: reaching the nisab (minimum threshold) and that it does not fall below it for a full hawl (lunar year). If it reaches the nisab and a hawl passes over it, the amount of Zakat is one-quarter of one-tenth (2.5%), otherwise there is no Zakat. The nisab is equivalent to 85 grams of gold or 595 grams of silver.