Position of Incheon

In Incheon, a Korean city, located in the northwest of the country, on the coast of the Yellow Sea and a short distance from Seoul, the climate is continental: winters are very cold but sunny, and summers are hot, sultry and rainy, though tempered by the breeze. Like the rest of Korea, the city is affected by the monsoon circulation: in winter, the northwest cold currents prevail, while in summer, they are replaced by hot and humid currents of tropical origin. Spring and autumn are transitional periods, mild and pleasant, especially the month of October.
The climate is very similar to that of Seoul, however, owing to the location on the coast, it is a bit milder (less cold in winter and less hot in summer) and also less rainy. For example, in January 2011, the temperature dropped to -18 °C (0 °F) in Seoul and to -15 °C (5 °F) in Incheon; in January 2016, it dropped to -18 °C (0 °F) in Seoul and to -16 °C (3 °F) in Incheon; in August 2016, it reached 36.6 °C (98 °F) in Seoul and 34 °C (93 °F) in Incheon.
Here are the average temperatures.
Incheon - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)-5-3281318222218114-3
Max (°C)241016212528292620125
Min (°F)232736465564727264523927
Max (°F)363950617077828479685441

In winter, the average temperature is around freezing (0 °C or 32 °F) or a few degrees below; sunshine is quite common, and a cold wind often blows from the Asian continent. On colder periods, the temperature remains below freezing during the day; in the worst moments, it can drop to -15 °C (5 °F) or below.
Although the winter is a dry season, sometimes it can snow.
In winter, and especially in spring (March to May, which is called yellow dust season), the city can be reached by yellow dust (HwangSa in Korean), which comes from the desert of Mongolia and from China (but in truth, there is also a contribution given by local pollution) and forms a characteristic haze; it can also cause breathing problems and eye irritation.
Spring is mild, but the rains gradually increase. However, because of the monsoon, the rainiest season is summer: from June to August, more than 700 millimeters (27.5 inches) of rain fall. The rains can sometimes be torrential. The worst month, the wettest and least sunny, is generally July. In summer, there can also be periods of good weather, which can be very hot, with highs about 35 °C (95 °F).
In Autumn, rainfall decreases, and by October, the rains are already quite rare. Here is the average precipitation.
Incheon - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)202040601001103202851555550201235
Prec.(in)0,80,81,62,43,94,312,611,26,12,220,848,6
Days757791015128687102

From July to early October (and occasionally, also in the second half of June), Incheon can be affected by typhoons, the tropical cyclones of South-East Asia, although this area of Korea is not as exposed as the southern coast.

Songdo

The amount of sunshine in Incheon is decent in winter and also in spring and autumn, while it reaches its minimum in July, when cloudy skies prevail (and the amount of sunshine drops to 35%) because of the summer monsoon. Here are the average sunshine hours per day.
Incheon - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours667777567766

In Incheon, the sea is very cold in winter, but it warms up a lot in summer, reaching 24 °C (75 °F) in August. Here are the average sea temperatures.
Incheon - Sea temperature
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Temp (°C)5558131722242319149
Temp (°F)414141465563727573665748