The opening of King's speech uses metaphors to compare the promises of freedom made in the Declaration of Independence, the Constitution and the Emancipation Proclamation and the failure of these documents to procure those freedoms for all. He then turns to a metaphor familiar to all--the weather.
Quote: "This sweltering summer of the Negro's legitimate discontent will not pass until there is an invigorating autumn of freedom and equality."
Metaphor: King compares the legitimate anger of African-Americans to sweltering summer heat and freedom and equality to invigorating autumn.
Analysis: Anyone who's visited Washington D.C. in August has a keen understanding of what a "sweltering summer" produces--frustration, suffering, restlessness and a longing for relief. The hundreds of thousands in attendance would have clearly understood the implications of the need for relief from a sweltering summer day and the need for legislation that would procure rights for minorities; relief that began to arrive with the Civil Rights Act of 1964.
Quote: "I have a dream that one day even the state of Mississippi, a state sweltering with the heat of injustice, sweltering with the heat of oppression, will be transformed into an oasis of freedom and justice."
Metaphor: King compares injustice and oppression to sweltering heat and freedom and justice to an oasis.
Analysis: King repeats the sweltering heat metaphor toward the end of the speech, referring specifically to Mississippi, a state where some of the worst offenses against blacks had been carried out. By specifying states in the south (he also mentions Georgia, Tennessee, Alabama, and the South in general) and mentioning the oasis that awaits even these places, King magnifies his message of hope to those suffering the most.
Quote: "The whirlwinds of revolt will continue to shake the foundations of our nation until the bright day of justice emerges."
Metaphor: King compares what the Civil Rights movement will produce if their demands are not met to a rapidly rotating, destructive vertical column of air. He compares the day when these rights are procured to a "bright day of justice."
Analysis: Whereas King's first weather metaphor involves a natural progression of events--summer to fall--his second weather metaphor involves violence, destruction, and an inevitable end to the violence and destruction. Martin Luther Jr., it must be noted, is not promoting violence but summarizing the feelings of frustration that have enveloped the throngs of minorities to whom the aforementioned promises of the Declaration of Independence and other American documents had not been fulfilled.
King's use of weather metaphors emphasizes the reality of the movement--that it's a force that cannot be controlled and that must manifest itself through the acquisition of equal rights.