The Spanish word 'niebla' meaning 'fog' comes directly from the Latin word 'nebula', which meant 'mist' or 'fog'. The evolution from Latin to Spanish shows a typical sound change where the Latin 'e' became the Spanish diphthong 'ie', a common phonological development in Spanish words.

A related Spanish word is 'nebuloso' (meaning 'cloudy' or 'nebulous'), which maintains a form closer to the original Latin word and is often used in more formal or scientific contexts.

English speakers might recognize the connection to several related words that also come from Latin 'nebula', such as 'nebula' itself (a cloud of gas and dust in space), 'nebulous' (cloudy, vague, or ill-defined), and 'nebulizer' (a device that turns liquid into a fine spray). All these words share the core meaning of a cloud-like substance or mistiness.