Map from Google - Croatia

In Croatia, there are three types of climate:
- the climate of Istria, the Adriatic coast and the islands, which is mild (especially in the southern part), and rainy;
- the climate of the Dinaric Alps, which is cold and snowy in winter, and cool in summer with thunderstorms in the afternoon;
- the continental climate of the interior plains.

Croatia is a rainy country: it's moderately rainy in the plains and inland valleys, and definitely rainy along the coast and the western slopes of the mountain ranges. The rainfall pattern is Mediterranean in the coastal strip, with a minimum in summer and a maximum in autumn and winter, while in the interior plains, precipitation is frequent throughout the year, and in winter it often occurs in the form of snow, but is more abundant in summer, when it often occurs in the form of thunderstorm.
Here are the average temperatures of Rijeka.
Average temperatures - Rijeka
Min (°C)336913161919151174
Max (°C)91012162125282824191310

The sea temperature is warm enough for swimming in the summer, especially in July and August, but also in September.
Sea temperature - Rijeka
Sea (°C)121111131722242523191614

In the southern part of the coast, the sea temperature is a bit warmer in winter, while in summer it is quite the same.

In the Adriatic coast the climate is Mediterranean. The northern coast, in Istria and cities such as Pula (Pola in Italian) and Rijeka (Fiume), has fairly mild winters, with an average in January around 5 °C, but with sudden drops in temperature when the Bora blows, a fierce wind coming from the icy Russian plains. Bora brings rare and not abundant snowfalls, because the cold air comes from the continental dry area. Here summers are warm and sunny, sometimes hot, but with the breeze that tempers the heat; sometimes there can be cool days with rains and even a bit of Bora. Further south, the climate becomes progressively milder, so that in Dalmatia, where Split (Spalato) is located, and in the large islands of the Dubrovnik (Ragusa) region, the average in January rises to around 7/8 °C and even more in the most sheltered areas. Even here, however, outbreaks of cold air are possible in winter, with a wind similar to Bora, especially at the outlet of the valleys, where the air is channeled between the mountains, but usually without frosts.
Average temperatures - Dubrovnik
Min (°C)76911151922221915118
Max (°C)121214172125282925211713

Precipitation is abundant in the entire Adriatic coast, where between 1,000 and 1,500 millimetres of rain per year fall, especially in autumn and winter. There are, however, some relatively sheltered areas, in the bays and along the coast which are protected by the islands, such as the one where Zadar (Zara) is located, which receives about 800 mm of rain per year, because it is protected by three lined up islands, over which a part of the rains is discharged. Summer is dry and sunny throughout the coastal strip, with occasional rains or thunderstorms, most likely in Istria. Only in the northernmost zone, where Rijeka is located, the rains are quite frequent and abundant even in July and August.
Here is the average precipitation in Rijeka, where 1,500 mm of rain per year fall.
Average precipitation - Rijeka
Prec. (mm)135115105110100110801001651751851551540

In Dubrovnik, in the southern part of the coast, rainfall amounts to about 1,000 mm per year. As you can see, the rains in summer become quite rare.
Here is the average precipitation.
Average precipitation - Dubrovnik
Prec. (mm)95901009075502560801101401251035


In the area of the Dinaric Alps, the climate becomes more continental, but it remains rainy due to its proximity to the sea, especially in the slopes exposed to the west and south.
For example, in Gospic, located less than 20 kilometres away from the coast, and 550 metres above sea level, the average temperature in January is -1 °C, while in July and August it's 18 °C.
In this area, the rains are abundant all year round (therefore also the winter snowfall), with a relative minimum in summer. During winter, cold waves from the north or east can occur. In spring, pleasant days alternate with sudden returns of cold, with possible snowfalls even in April. During summer, some afternoon thunderstorms are possible, and the nights are still cool, but sometimes the days can be hot.
The Plitvice Lakes National Park is located in this area, about 550/600 metres above sea level.

In the northern and eastern plains, the climate is continental, and the rainfall pattern is very different from that of the Mediterranean: summer is the rainiest season, mainly because of the thunderstorms occurring in the afternoon, while in winter, although precipitation is frequent (and occurs often in the form of snow), it is not abundant.
In Zagreb, the capital, the average temperature in January is -0.5 °C, and in July it's 20.5 °C.
Average temperatures - Zagreb
Min (°C)-4-31591314141062-2
Max (°C)36111621252726221694

Rainfall is still quite abundant, 860 millimetres per year, with a minimum in February of 42 mm, and a peak in June, of about 99 mm. The rains remain quite abundant even in autumn.
Here is the average precipitation in Zagreb.
Average precipitation - Zagreb
Prec. (mm)5040506080100809085708565855

The situation is similar across the northern plains, except in the extreme east, on the border with Serbia, in cities like Osijek and Vukovar, where the autumn rains are much poorer, therefore the annual precipitation drops below 700 mm.
For the rest, the climate in this whole strip of flat northern land is similar. Winter is cold, often a cold drizzle or snow falls, days can be dull and foggy, but sometimes the cold air masses from Russia can lower the temperature to about -20 °C. Summer is warm or even hot, with pleasant days alternating with hot and sultry days, when the maximum temperatures can reach 35 °C, and some chance of thunderstorms in the afternoon.


When to go

For swimming and sunbathing, the best months are July and August. Recall that the northernmost part of the coast (Istria, Rijeka), can sometimes experience thunderstorms, or rainy days even in summer, which are rarer along the coast of Dalmatia.
In general, the autumn rains (which are heavy in the coastal area) start from the middle of September. In autumn, the southerly winds prevail.
The rest of Croatia (inland regions, including the capital) can be visited from May to September. The sun often shines, but during the afternoon, thunderstorms are possible (on average one day every 3 or 4). In July and August it can be hot at times, especially in the plains and in inland valleys, so those who suffer from the heat may prefer May, June and September.

What to pack

In winter: in inland areas: warm clothes, hat, down jacket, scarf, gloves, raincoat or umbrella. On the coast: warm clothes, coat, jacket, raincoat or umbrella; hat, scarf and gloves for the days with Bora.
In summer: light clothes, T-shirts, but also long pants, light jacket and a sweatshirt for the evening and cooler days, especially in inland areas and Istria; umbrella or raincoat, especially in inland areas and Istria.