Map from Google - Rimini

In Rimini, and in the neighboring towns of the Adriatic Coast, Riccione and Cattolica, the climate is semi-continental, with fairly cold winters and hot, sunny summers, a little stuffy, but tempered by the breeze. The city is located outside of the Po Valley and near the sea, but on the Adriatic Coast (which in winter is colder than that of the Tyrrhenian Sea), and far enough to the north to be affected, during cold periods, by the Bora, a cold gusty wind that blows from the plains of Eastern Europe, especially in the cold half of the year. The city, however, is also sensitive to the Sirocco, or more precisely to the Garbino, a mild southwest wind that descends from the mountains.
Here are the average temperatures.
Average temperatures - Rimini
Rimini J F M A M J J A S O N D
Min (°C) 1 1 4 7 12 16 19 18 15 11 6 2
Max (°C) 8 9 13 17 22 26 29 29 25 20 13 9

Precipitation is moderate, given that it amounts to 645 millimetres per year; the rains are well distributed throughout the year, even though there is a maximum in autumn. However, summer is the season in which the rains are more rare; in this season, and in late spring, they can occur in the form of thunderstorms.
Here is the average precipitation.
Average precipitation - Rimini
Rimini J F M A M J J A S O N D Year
Prec. (mm) 40 45 50 55 50 55 45 50 70 75 65 45 645
Days 6 6 6 7 7 6 4 6 7 8 7 7 76

The winter, from December to February, is cold enough, although it is less cold than in the Po Valley. However, the wind can increase the feeling of cold, especially when the Bora blows. Snowfalls are fairly common, in the sense that you can see some flakes in most years, however, they are more rare than in the interior of Romagna, and generally are not abundant. In any case, there can be cold spells with temperatures around zero degrees even during the day, a cold wind blowing from the sea, and brief snowfalls, even in the form of showers or thunderstorms. Frosts at night are quite common, but usually not remarkable, except during periods of most intense cold, when temperatures can fall to -10 °C and beyond.
Rainy days are not lacking, although winter is the (relatively) driest season.
When the south wind blows, temperatures become milder, and by day they can reach and sometimes exceed 15 degrees.
In periods of high pressure, fog can form.

Spring in Rimini is a quite unstable season, especially at the beginning, in March and in the first half of April, when the first mild days alternate with the return of the cold and the Bora. However, as the weeks pass, the weather becomes nicer, especially from mid-April to late May, when there are many sunny days, with mild or pleasantly warm temperatures during the day.
However, Atlantic disturbances able to bring rainfall are frequent.

The summer, from June to August, is hot and humid, and generally sunny. During the day, the sea breeze usually makes the heat bearable, while the nights are sultry because of the humidity of the sea. The hottest periods occur when the south wind blows, because in these cases the tempering effect of the sea disappears, therefore the temperature may reach 35/37 °C.
Sometimes, even in summer, some Atlantic disturbances able to bring a rainy and cool weather can pass, perhaps accompanied by gusts of Bora, especially in June and late August; or thunderstorms may break out in the afternoon and evening, but for long periods, the sun shines and the weather is hot.

Autumn offers several nice days in September and sometimes in early October, and then quickly becomes gray and rainy enough. The first cold days usually occur in November. Overall autumn is the wettest season of the year.

The sunshine amount in Rimini is not high from mid-October to February, when sunny days are rare. However, the sun is not lacking in spring, and it is very common in summer, except for the few rainy days or when sunny days are interrupted by thunderstorms in the afternoon.

Rimini, beach and umbrellas

The sea in Rimini is warm enough for swimming from July to September, while in June it is still a bit cool.
Here are the average sea temperatures.
Average sea temperatures - Rimini
Rimini J F M A M J J A S O N D
Sea (°C) 12 11 11 13 18 22 25 26 23 20 16 14

When to go

The best time to go to Rimini and the Adriatic coast for beach holidays, is summer, from June to August, with a preference for July and August, as it is the hottest and sunniest period, and also when the sea is warmest.