Sur vs. mediodía vs. sud

The Spanish language has different words that can be used to denote the compass point 'South'. These are 'sur', 'mediodía', and 'sud'. Each word comes with its unique context of usage.

Sur

'Sur' is the most commonly used term for the cardinal point 'South' in Spanish. It is utilized in a variety of contexts, including geography, meteorology, and navigation among others.
Argentina está en el sur de América.
(Argentina is in the south of America.)
La brújula señala al sur.
(The compass points south.)

Mediodía

'Mediodía' typically refers to 'midday' or 'noon'. However, it can also be used metaphorically to indicate 'South', especially in a literary or poetic context, as it is the direction where the sun is at its highest point at noon.
El sol está en su punto más alto en el mediodía.
(The sun is at its highest point at midday.)
El mediodía marca la dirección a seguir.
(The South marks the direction to follow.)

Sud

'Sud' is used mostly in scientific contexts or in some regional dialects of Spanish. In regular conversation or writing, it may come across as formal or old-fashioned.
El viento viene del sud.
(The wind comes from the south.)
El sud es el hemisferio más templado.
(The South is the warmest hemisphere.)

Summary

'Sur', 'mediodía', and 'sud' are all related to the concept of 'South'. While 'sur' is the general term for South and 'sud' leans more towards scientific usage or certain regional dialects, 'mediodía' can describe midday or noon, but also acts as a poetic term for South.