Nublado vs. encoberto
In Portuguese, both nublado and encoberto describe weather conditions involving clouds, but they describe different levels of cloud coverage. Generally, nublado is the broad term for cloudy, while encoberto specifically means overcast or fully covered.
Nublado
A1Nublado is the most general word for "cloudy". It implies the presence of clouds, but the intensity can vary. A sky that is nublado might still have gaps where the blue sky or the sun peeks through, or it might have distinct, puffy clouds. It is the opposite of a clear sky (céu limpo).
Hoje o dia está nublado, então não está muito quente.
(Today the day is cloudy, so it is not very hot.)
A previsão disse que a tarde será parcialmente nublada.
(The forecast said the afternoon will be partially cloudy.)
Eu gosto de caminhar na praia quando o tempo está nublado.
(I like walking on the beach when the weather is cloudy.)
Apesar de estar nublado, a luz do sol ainda é forte.
(Despite being cloudy, the sunlight is still strong.)
O céu ficou nublado de repente antes da tempestade.
(The sky got cloudy suddenly before the storm.)
Encoberto
B1Encoberto literally means "covered". In meteorology, it translates to "overcast". It describes a sky that is completely hidden by a thick, uniform layer of clouds. When the sky is encoberto, there are no breaks in the clouds, no blue sky is visible, and the sun is completely blocked out, resulting in a gray atmosphere.
Não conseguimos ver o eclipse porque o céu estava totalmente encoberto.
(We could not see the eclipse because the sky was totally overcast.)
O dia amanheceu cinza e encoberto.
(The day dawned gray and overcast.)
Geralmente chove quando o céu fica encoberto desse jeito.
(It usually rains when the sky gets overcast like this.)
Os voos foram cancelados porque o céu estava encoberto e com neblina.
(The flights were cancelled because the sky was overcast and foggy.)
Para amanhã, esperamos um céu encoberto o dia todo.
(For tomorrow, we expect an overcast sky all day long.)
Summary
To summarize, use nublado for a general cloudy day where you might see individual clouds or patches of sky. Use encoberto for an overcast day where the sky is a solid sheet of gray and the sun is not visible at all.