This is the 11724th most frequent Korean word.
낮으니
"낮으니" means "because [something] is low" or "since [something] is low."
낮으니 is used to indicate a causal relationship, implying that some condition ('온도가 낮다', the temperature being low) leads to a recommended action.
온도가 낮으니 따뜻한 옷을 입으세요.
Since the temperature is low, please wear warm clothes.
낮으니 here shows a consequence stemming from the shallowness (low state) of the water, suggesting an action based on the condition.
물이 낮으니 다리 밑으로 건너갑시다.
Since the water is shallow, let's cross beneath the bridge.
낮으니 in this context connects the mild condition of sunlight to a favorable circumstance for having a picnic.