What does the tiny cloud Olivia draws at the end of the story suggest?

Every evening at seven, Olivia walked to the edge of her grandfather's cotton field outside Lubbock and checked the rain gauge. It was a clear plastic tube tied to a fence post, with lines marked on the side. She wrote the number in a small green notebook that her grandfather had kept for more than forty years.

This summer, the number was almost always zero.

"Why do we even write it down?" Olivia asked one night in July, holding up a page full of zeros. "Nothing ever changes."

Her grandfather took the notebook and turned back through the years. He stopped at a page from long before she was born. That year, too, was a column of zeros, all the way through August. Then, in September, the numbers jumped: two inches, three inches, half an inch.

"Dry years end," he said. "But you only know how they end if somebody keeps writing."

Olivia did not answer. She was not sure she believed him, but she kept checking the gauge every night.

Weeks later, in early September, she woke to a sound she had almost forgotten: rain drumming on the tin roof. It rained all night and into the morning. When it finally stopped, Olivia ran outside in her socks, her feet sinking into the soft red mud.

The water in the tube had risen past the line for two inches.

She carried the notebook into the kitchen, where her grandfather was pouring his coffee. Carefully, she wrote the number. Then, in the margin beside it, she drew a tiny cloud.