J., the narrator, despite his confidence, is very often clueless and as unfit for boating down the Thames as his companions, and this quote helps illustrate his uncertainties. Before making the statement about not being able to understand the weather, he first offers context.

George, over breakfast, reads in the paper that the weather is supposed to be "rain, cold, wet to fine," with the occasional thunderstorm. This sets J. off on a rant about how neither weather forecasts nor barometers can be trusted. He tells the story of trusting a weather forecast of rain; deciding, with his party, to stay home; and watching as other people headed out for picnics—only to waste the day indoors when it didn't, after all, rain. Likewise, he has trusted weather forecasts that promised sunny weather, only to find himself in a storm.

J. also finds the barometer unreliable. He notes that when he was staying in a hotel in Oxford, the barometer would change what it indicated if he tapped on it, causing him to lose faith in its reliability.

Therefore, he says,

The weather is a thing that is beyond me altogether. I never can understand it.

Ultimately, the weather seems so fine that J. and the others decide to ignore the forecast of rain. J.'s irritation is a commentary on the changeability of weather in England, where it often goes quickly from rain to shine and back again, defying predictions.