/i/ formants

low F1, high F2

/a/ formants

high F1, low F2

/u/ formants

low F1, low F2

F1 related to

tongue height (high, low)

F2 related

tongue advancement/retraction (forward/back)

High vowels

low F1

Low vowels

high F1

degree of mouth opening

small - low F1
large - high F1

front vowels

high F2

back vowels

low F2

more retracted tongue

lower F2

F3 relates to

back constriction - high F3
front constriction - low F3

general raising of F1 as

pharyngeal cavity size is decreased & lip aperture is increased

generally consistent lowering of F1 as

front resonating cavity is enlarged because of tongue retraction (& lip protrusion)