Map from Google - Costa_de_la_Luz

The Costa de la Luz, Spain's southwestern coast, overlooking the Atlantic Ocean, has beautiful beaches, hotels and golf courses, and areas that lend themselves to surfing and diving. Here the climate is Mediterranean, with mild and relatively rainy winters, and warm and sunny summers. However, there are some differences that are worth considering: the western part is hot in summer, with highs around 30 degrees in July and August, and a greater likelihood of hot days, while the eastern part, and in particular the south-eastern tip, overlooking the Straits of Gibraltar, is mild and windy.
In winter, very cold days on the Costa de la Luz are rare, but sometimes you might experience short cold periods, in which the minimum temperatures can reach zero degrees.

Let's start from the west. In the westernmost part, the mouth of the Guadiana River, which marks the border with Portugal (where we find the Algarve, see Faro), is divided into branches that create islands: Isla Canela, Isla del Moral and Isla Cristina. Further to the east, there are the towns of La Antilla and Islantilla. Further east, in the area of the mouth of Rio Piedras, we find the coastal towns of El Rompido and El Portil.

Isla Cristina from the plane

We are in the province of Huelva: the capital is located more to the east, near the confluence of the Tinto and Odiel rivers, a short distance from the coast (on the Huelva coastline we find Punta Umbría). Here the average temperature of January is 11 °C, that of July and August is 25.5 °C. In Huelva, in summer daytime temperatures are already 2/3 degrees higher than on the coast, and at times it can be very hot (though more rarely than in the interior of Andalusia), with highs around 35/37 degrees.
Here are the average temperatures of Huelva.
Average temperatures - Huelva
HuelvaJanFebMarAprMayJunJulAugSepOctNovDec
HuelvaJFMAMJJASOND
Min (°C)67911131719191814108
Max (°C)161821222529323230252017

The rains in Huelva are not abundant, since they amount to 530 millimetres per year, and follow a Mediterranean pattern, in fact most of them fall from October to April, while in summer it almost never rains. The rainiest month is December with 105 mm.
Here is the average precipitation.
Average precipitation - Huelva
HuelvaJanFebMarAprMayJunJulAugSepOctNovDecYear
HuelvaJFMAMJJASONDY
Prec. (mm)7050405030735256585105530
Days75564100267852

While on the coast the summer temperatures remain typically acceptable because of the sea breeze, in the interior we find a particularly hot region, the Guadalquivir valley, the area that in summer becomes the hottest in Europe (see Seville).
Continuing to the east, around the beach resort of Matalascañas we find the Doñana National Park, a UNESCO World Heritage Site, rich in biodiversity, with many species of birds that find refuge in the numerous lagoons. On the coast, facing south-west, there are wide beaches with dunes.

The temperature of the sea in the western part of the Costa de la Luz remains very cool even in summer, since the waters of the Atlantic never heat up by much, as we can see from the temperature of the sea at Isla Canela.
Sea temperature - Isla Canela
Isla CanelaJanFebMarAprMayJunJulAugSepOctNovDec
Isla CanelaJFMAMJJASOND
Sea (°C)161516161719202121201817

Beyond the mouth of the Guadalquivir we enter the province of Cádiz, where summer tends to become oddly less warm, as you head south. Here we find places like Chipiona and Rota, where there's an airport. In Rota the summer temperature is still quite high, however, compared to the western part of the coast and in Huelva, very hot days are less frequent, while sometimes there can be quite cool days, with highs around 24/26 degrees, even in July and August, because of the east wind (Levante), which in Rota is intermittent and blows only at certain times, but becomes more frequent and intense as you head southeast.
Here are the average temperatures of Rota.
Average temperatures - Rota
RotaJanFebMarAprMayJunJulAugSepOctNovDec
RotaJFMAMJJASOND
Min (°C)781011141819201815118
Max (°C)161820212428303028252017

To the southeast of Rota there are a number of cities very close to each other, but separated by bays and branches of rivers: El Puerto de Santa María, Cádiz, located in the homonymous bay, the nearby cities of Puerto Real and San Fernando (the latter is an island, even though it was artificially connected to the mainland), and finally Chiclana de la Frontera. The beaches in this part of the coast are Playa de la Barrosa and Playa de Sancti Petri.
Here is the average temperature of Cádiz, which tends to be mild because it is surrounded by the sea. Even here in summer the wind from the east can blow, causing some pretty cool periods.
Here are the average temperatures.
Average temperatures - Cadiz
CadizJanFebMarAprMayJunJulAugSepOctNovDec
CadizJFMAMJJASOND
Min (°C)101112131619212220171311
Max (°C)161718192225282826232017

Cádiz from above

Continuing along the coast towards the south-east we find places like Roche, Conil de la Frontera, El Palmar, Zahora and Cape Trafalgar, beyond which we find Barbate. From this area we enter the Strait of Gibraltar area, which in summer is particularly subject to the cool Levante, and thus has a particularly mild microclimate, even cool in the southern tip where we find Tarifa.
Here are the average temperatures of Tarifa.
Average temperatures - Tarifa
TarifaJanFebMarAprMayJunJulAugSepOctNovDec
TarifaJFMAMJJASOND
Min (°C)111112131518202019171412
Max (°C)151516171922242523211816

The temperature of the sea in the province of Cádiz is similar to that of the province of Huelva, and remains cool even in summer, however, it's slightly warmer, and reaches 22 degrees in August and September, as can be seen for example in the temperature of the sea at Conil de la Frontera.
Sea temperature - Conil
ConilJanFebMarAprMayJunJulAugSepOctNovDec
ConilJFMAMJJASOND
Sea (°C)161616161820212222201917

When to go

The best period to visit the cities of Costa de la Luz, for walks and hikes, corresponds to the months of April, May and October, especially if you plan an excursion in the interior of Andalusia, which in summer is often torrid.
For a beach holiday the best time is instead the summer, from June to mid-September. It is advisable to bring a sweatshirt for cool and windy evenings, especially in June and September, but for safety, even in July and August, especially in the eastern part, that is the province of Cádiz.

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