what is the wind speed when brenda morehead was in the united states?
CREATE TABLE "junior_under_20_women" (
  "rank" real,
  "fastest_time_s" real,
  "wind_m_s" real,
  "athlete" text,
  "nation" text,
  "date" text,
  "location" text
);
SELECT MIN("wind_m_s") FROM "junior_under_20_women" WHERE "nation"='united states' AND "athlete"='brenda morehead';