Give a brief description of a daydream and what caused it by reality?

Walter Mitty's daydream which opens the story is a good example of how he translates reality into fantasy. 

"We're going through!” The Commander’s voice was like thin ice breaking. He wore his full-dress uniform, with the heavily braided white cap pulled down rakishly over one cold gray eye. “We can’t make it, sir. It’s spoiling for a hurricane, if you ask me.” “I’m not asking you, Lieutenant Berg,” said the Commander. 

This daydream is especially interesting for several reasons. One is that it shows how Mitty puts himself in roles that are appropriate to his age. In order to have reached the rank of Commander he would have to be middle-aged. Many young people indulge in fantasies regularly, but they fantasize about being football or baseball heroes or, in the case of girls, perhaps as Cinderella types or lead singers with rock groups. Mitty has no youthful fantasies anymore, which shows he is something of a realist even in his daydreams.

The reality that triggers his daydream of piloting a powerful hydroplane through hurricane weather is in the fact that he sees a storm coming and the fact that he knows he is going to have to drive through wet weather to get himself and his wife to Waterbury for their shopping trip and back again to their country home. At the end of the story the storm will finally hit. He will be standing with his back against the wall of a drugstore smoking a cigarette. The cigarette and his standing against a wall will trigger the idea that he is standing bravely before a firing squad.

“To hell with the handkerchief,” said Walter Mitty scornfully. He took one last drag on his cigarette and snapped it away. Then, with that faint, fleeting smile playing about his lips, he faced the firing squad; erect and motionless, proud and disdainful, Walter Mitty the Undefeated, inscrutable to the last. 

What snaps him out of his fantasy role as Commander Mitty piloting the hydroplane is his wife's voice--the voice of reason, reality, caution, and authority.

“Not so fast! You’re driving too fast!” said Mrs. Mitty. “What are you driving so fast for?”

She is one of those women who repeats everything.

“You were up to fifty-five,” she said. “You know I don’t like to go more than forty. You were up to fifty-five.” 

When Mitty was mentally giving orders to increase the power of the hydroplane engines, he unconsciously pressed a little harder on the car's accelerator and was soon going fifty-five miles an hour. His wife probably had her eye on the speedometer and didn't squawk until the needle reached 55. He was courting danger, not only in his imaginary hydroplane, but in his automobile. 

Thurber's story just gives a glimpse into Walter Mitty's mind. He has many other fantasies, but the one triggered by the approaching storm clouds and one triggered by the rain and sleet when they hit put a frame around the story by giving it a sense of a beginning and an end.