Map from Google - Norway

Norway is a Nordic country, but it is affected by the Gulf Stream, so its climate is less cold than you might think, especially along the west coast. However, the country is very extended in latitude, and it has a rugged coastline, plateaus and mountain ranges, so there are different climates to be taken into account.
The ocean, whose surface temperature remains a few degrees above freezing even in winter, tempers the winter weather, while it remains cool in summer, therefore it reduces the annual temperature range along the coast. On the contrary, in the interior this mild influence is felt little, because in Norway there are almost no plains, hence, highlands and mountain ranges are able to hinder the penetration of mild currents coming from the sea.
The following map shows the climate zones in Norway.

Norway climate zones

The west coast has a cool, wet climate, characterized by the frequent passage of weather fronts, so that clouds, rain and wind dominate throughout the year.
During winter, the temperature remains slightly above freezing (0 °C or 32 °F) in the southern part, so that Bergen, the second largest city in the country, has an average temperature in January and February around 2 °C (35.5 °F). Precipitation does not always occur in the form of snow, but it often occurs in the form of a continuous and cold drizzle.
Here are the average temperatures in Bergen.
Average temperatures - Bergen
BergenJanFebMarAprMayJunJulAugSepOctNovDec
BergenJFMAMJJASOND
Min (°C)0-11371012129731
Max (°C)446914171817141175
Min (°F)323034374550545448453734
Max (°F)393943485763646357524541

The temperature of the sea at Bergen is cold, but it's mild for the latitude, as we can see from the following table.
Sea temperature - Bergen
BergenJanFebMarAprMayJunJulAugSepOctNovDec
BergenJFMAMJJASOND
Sea (°C)76668111415131198
Sea (°F)454343434652575955524846

Moving to the north, the temperature gradually decreases, but the average daily temperature reaches the freezing point only around the 65th parallel, just below the Arctic Circle. It's remarkable that the Lofoten and Vesteralen Islands, located above the Arctic Circle itself, even in winter can sometimes experience rain instead of snow. Continuing to the north, the temperature goes down to a few degrees below freezing, and snow becomes frequent even along the coast, but at this point we enter in the climate area of the northern coast (as shown in the map above).
The sea, which in winter makes the air milder, during summer cools down the air, so that in July and August, maximum temperatures along the west coast are around 15 °C (59 °F), and minimum temperatures around 10 °C (50 °F). However, if you move a few kilometres (or miles) inland, you can experience an increase in daytime temperatures, which can rise by a few degrees, and this happens in Bergen and other towns located not directly on the ocean coast, but inside a fjord.
Precipitation is quite common along the western slope. In the southern area, it is also abundant: in Bergen the average annual precipitation is about 2,200 millimetres (87 inches), but in cities located at the foot of the hills it can exceed 3,000 mm (118 in).
Here is the average precipitation in Bergen.
Average precipitation - Bergen
BergenJanFebMarAprMayJunJulAugSepOctNovDecYear
BergenJFMAMJJASONDY
Prec. (mm)1901501701151051301501902852702602352250
Prec.(in)7.55.96.74.54.15.15.97.511.210.610.29.388.6
Days211719171716181923242222235

Continuing to the north along the coast, precipitation is still frequent, but it becomes less abundant, and drops below 1,000 mm (40 in) in the coast to the north of Trondheim, although it can still reach 2,000 mm (79 in) in the slopes near the coast, where there are hills directly exposed to the westerly winds.

The north coast is a bit colder than the western one, and has a subpolar oceanic climate, although a branch of the Gulf Stream arrives here as well. The result is that the open sea does not freeze even at these latitude, and this does not happen anywhere else in the world. The following table shows the temperature of the sea at the North Cape.
Sea temperature - North Cape
North CapeJanFebMarAprMayJunJulAugSepOctNovDec
North CapeJFMAMJJASOND
Sea (°C)5444579109766
Sea (°F)413939394145485048454343

The average winter temperatures, however, are below freezing, and tend to decrease as we move towards the east: the average in January and February ranges from -2 °C to -6 °C (28 °F to 21 °F), from the west to the east. At North Cape, in winter the temperatures are similar to those of Oslo, even though here the winter is longer. The northern coast is close enough to the Pole, therefore is subject to polar winds, which can be furious, especially in winter and spring.
Average temperatures - North Cape
North CapeJanFebMarAprMayJunJulAugSepOctNovDec
North CapeJFMAMJJASOND
Min (°C)-6-6-4-2258852-2-4
Max (°C)-2-2-11591312951-1
Min (°F)212125283641464641362825
Max (°F)282830344148555448413430

In summer, July and August, which have an average temperature around 10 °C (50 °F), are the mildest and also the quietest months, with the lowest risk of wind. Near the Arctic Circle, you can see the midnight sun around the summer solstice (21 June), and for a period much longer as you head north. At the North Cape, the sun never sets from mid-May to late July.
At North Cape, precipitation amounts to about 800 mm (31.5 in) per year. Summer is the least rainy season, although there are still 9/10 days with rain per month. Here is the average precipitation.
Average precipitation - North Cape
North CapeJanFebMarAprMayJunJulAugSepOctNovDecYear
North CapeJFMAMJJASONDY
Prec. (mm)9575656040404550608580100795
Prec.(in)3.732.62.41.61.61.822.43.33.13.931.3
Days18151514111091014181719170

Even here on the north coast, within the fjords there's a decrease in the winter temperatures (so that in the deepest fjords the sea can freeze during winter), and an increase in the summer temperatures: in fact here this phenomenon is even more pronounced than on the west coast. For example, in Lakselv, located in the southern part of a fjord, more than 120 kilometres (75 miles) away from the ocean coast, the average in January is -10 °C (14 °F), while to the north, on the ocean coast, it is just -4 °C (25 °F).

The south-east coast of Norway, on the Skagerrak strait that separates the country from Denmark, is less influenced by the ocean than the west coast, therefore it is less humid and less rainy than the area of Bergen, a bit colder in winter (with average temperatures just below freezing), and warmer in summer, with highs around 20 °C (68 °F), so the stay in the latter season is usually pleasant, and you can even hope to see some sunshine.
Average temperatures - Kristiansand
KristiansandJanFebMarAprMayJunJulAugSepOctNovDec
KristiansandJFMAMJJASOND
Min (°C)-5-5-21691110850-3
Max (°C)124914192019161163
Min (°F)232328344348525046413227
Max (°F)343639485766686661524337

In Kristiansand, precipitation amounts to 1,300 mm (51 in) per year, so it is quite abundant, but it's still much lower than in Bergen.
Average precipitation - Kristiansand
KristiansandJanFebMarAprMayJunJulAugSepOctNovDecYear
KristiansandJFMAMJJASONDY
Prec. (mm)1208085608575901201401651651151300
Prec.(in)4.73.13.32.43.333.54.75.56.56.54.551.2
Days191516121412131718192019194

The fjords of Norway, which we have already mentioned, have different microclimates depending on shape and size, and also on slope exposure, but generally, as you penetrate inland, they become less rainy throughout the year, and more continental, that is, colder in winter and warmer and sunnier in summer. For example, the city of Trondheim, located inside a large and deep fjord that opens into the western coast, has an average temperature of -3.5 °C (25.5 °F) in January, and of 14 °C (57 °F) in July, and therefore it can be included in the continental climate zone, while in Kristiansund, on the west coast and at the same latitude, the average goes from 1 °C (34 °F) in January, to 13 °C (55.5 °F) in July.
Average temperatures - Trondheim
TrondheimJanFebMarAprMayJunJulAugSepOctNovDec
TrondheimJFMAMJJASOND
Min (°C)-7-6-3059101073-2-5
Max (°C)01481417181814942
Min (°F)192127324148505045372823
Max (°F)323439465763646457483936

In Trondheim, about 900 mm (35.5 in) of rain or snow per year fall; even here the wettest season is autumn. Here is average precipitation.
Average precipitation - Trondheim
TrondheimJanFebMarAprMayJunJulAugSepOctNovDecYear
TrondheimJFMAMJJASONDY
Prec. (mm)65505550557095851151057085890
Prec.(in)2.622.222.22.83.73.34.54.12.83.335
Days181616161517191822221819216

The interior of Norway has a continental climate, with cold and snowy winters, and mild summers. Typically the interior is covered with hills and mountains, and the temperature decreases with altitude, so that at high elevation we find a mountain climate. The snowline in Norway is quite low, about 1,500 metres (5,000 feet).
The coldest area of the country is the Norwegian part of Lapland, which corresponds to the southern part of the Finnmark County. Here the winter is really frigid, in fact the temperature can drop to -50 °C (-58 °F) in the worst moments. Summer is short but intense: in the three months, during which the temperature rises above freezing even at night, the vegetation explodes, and the days are very long. Sometimes it can even get hot, so that the daytime temperature can reach 30 °C (86 °F), while nigths remain cool or even cold.
Average temperatures - Karasjok
KarasjokJanFebMarAprMayJunJulAugSepOctNovDec
KarasjokJFMAMJJASOND
Min (°C)-23-22-17-9-15862-5-14-21
Max (°C)-12-10-41715181592-5-10
Min (°F)-9-81163041464336237-6
Max (°F)101425344559645948362314

In this area, precipitation is much scarcer than in the rest of Norway, both for the distance from the sea and for the cold, so that it does not reach 400 mm (16 in) per year, although in summer, when temperatures rise, precipitation, which in this season occurs in the form of rain, becomes more abundant. Here is average precipitation in Karasjok.
Average precipitation - Karasjok
KarasjokJanFebMarAprMayJunJulAugSepOctNovDecYear
KarasjokJFMAMJJASONDY
Prec. (mm)201515152540706040352015365
Prec.(in)0.80.60.60.611.62.82.41.61.40.80.614.4
Days6555681111987687

Even in the centre-east of Norway, on the border with Sweden, ie the northern part of Hedmark and the eastern part of Sør-Trøndelag, very marked continental features are found, because of the distance from the sea, but also of the position on the eastern side, and of altitude, as this area is occupied by a plateau, between 500 and 1,000 metres (1,600 and 3,300 feet) above sea level. Here, during winter the temperature can drop below -40 °C (-40 °F).

Sognefjord

The area of Oslo, along the south-eastern part of Norway, has a Baltic (slightly continental) climate: winters are cold, with temperatures a few degrees below freezing in the winter months, and frequent snowfall. During winter, mild air masses from the Atlantic ocean can sometimes penetrate even here, so that rain may fall instead of snow, while on other occasions, cold spells are possible, due to air masses coming from Russia, which may lower the temperature to around -25 °C (-13 °F). The average temperature hovers around freezing in November and March, and is mild in the three summer months (June, July and August), with highs around 20/22 °C (68/72 °F). Rainfall is moderate, but well distributed throughout the year, with a maximum between summer and autumn, and a minimum in spring. Like other cities of the country, Oslo is located in a fjord, more than 80 kilometres (50 miles) deep.
Average temperatures - Oslo
OsloJanFebMarAprMayJunJulAugSepOctNovDec
OsloJFMAMJJASOND
Min (°C)-7-7-31711121184-2-6
Max (°C)-2-149162022201593-1
Min (°F)191927344552545246392821
Max (°F)283039486168726859483730

The sea temperature in the Oslo fjord reaches 17 °C (63 °F) in July and August.
Sea temperature - Oslo
OsloJanFebMarAprMayJunJulAugSepOctNovDec
OsloJFMAMJJASOND
Sea (°C)544610141717151296
Sea (°F)413939435057636359544843

Finally, there are the northern islands.

The island of Jan Mayen, situated between the Atlantic Ocean and the Arctic Ocean, has a subarctic climate.

Further to the north, we find the Svalbard archipelago, which is the northernmost part of Norway, and is colder in winter.

When to go

The best time to go to Norway is the summer, from June to August. Temperatures are very cool along the coast and in mountainous areas, and even cold along the northern coast. Sometimes it can even be a bit hot during the day, while nights remain cool. In Lapland, temperatures are a bit more variable, so it can get hot during the day, but also cold at night.
In Norway there's no shortage of rain even in summer, especially along the western coast. June is a bit cooler than July and August, but it is (relatively) drier and sunnier, and has longer days, with the white nights in the south and the midnight sun in the north.
For skiing, the month of March is preferable than the winter months, because the temperature is less cold and the days are longer.
In the northern islands, temperatures are definitely low even in summer, but at least they are above freezing, albeit slightly.
As we said, the ocean, which has a surface temperature of 5/6 °C (41/43 °F) in winter, remains cool during the summer, and reaches 15 °C (59 °F) in August at Bergen, and only 11 °C (52 °F) at Tromsø: with these temperatures, sea bathing takes a lot of courage! It goes a bit better in the Skagerrak strait and near Oslo, where the water temperature reaches 17 °C (63 °F) in July and August.

What to pack

In winter: for Oslo, as well as inland and northern areas: very warm clothes, synthetic thermal long underwear, fleece, down jacket, hat, gloves, scarf. For the west coast: sweater, down jacket, hat, raincoat or umbrella.
In summer: spring/autumn clothes, being ready to add or remove the outer layer, t-shirt, but also long pants, sweatshirt or sweater; jacket, raincoat or umbrella.
It can be a useful to bring windbreaker and raincoat for the wind and the rain, especially along the coast and for a ferry trip in the fjords. In inland areas, in Oslo and along the southern coast, temperatures are generally mild, but a sweater for the evening is still advisable.
For the northern islands (Jan Mayen, Svalbard): warm clothing, down jacket, hat, gloves, windbreaker, raincoat.