Sift together first 4 ingredients in a bowl, and set aside.
Beat egg whites, cream of tartar, warm water, vanilla extract, and salt in a large bowl at high speed of a mixer until foamy. Gradually add 1 cup sugar, 2 tablespoons at a time, beating mixture until stiff peaks form. Sift flour mixture over egg white mixture, 1/4 cup at a time, and fold flour mixture in gently.
Spoon batter into an ungreased 10-inch tube pan, spreading evenly. Break air pockets by cutting through batter with a knife. Bake at 350° for 45 minutes or until cake springs back when lightly touched. Invert pan; let cool completely. Loosen cake from sides of pan using a narrow spatula; revert cake onto plate.