Northern Vietnam, including popular destinations like Hanoi, Ha Long Bay, Sapa, and Ninh Binh, experiences four distinct seasons, making the "best" time to visit somewhat dependent on your preferences. However, generally, the most favorable periods are spring (March to May) and autumn (September to November). Here's a breakdown by season:

1. Autumn (September to November): Generally considered the best time to visit
• Weather: Mild and comfortable temperatures (20-28°C / 68-82°F), lower humidity, and generally dry and sunny days, especially in October and November. Rainfall tends to taper off from early September.
• Highlights: Ideal for outdoor activities like trekking in Sapa, cruising Ha Long Bay, and exploring Hanoi and Ninh Binh. The rice fields in mountainous areas like Sapa and Mu Cang Chai turn golden for harvest, offering incredibly picturesque landscapes.
• Considerations: Can still have some rain in early September. Popular time, so expect more tourists.

2. Spring (March to May): Another excellent period
• Weather: Mild and comfortable temperatures (18-30°C / 64-86°F), with relatively low humidity. It can rain, especially in March, but it generally tapers off.
• Highlights: Landscapes are lush and green after the rainy season. Flowers are in bloom, especially in places like Moc Chau. Good for exploring cities, national parks, and enjoying outdoor activities. Many local festivals take place during this time.
• Considerations: March can still be misty and have some drizzle, potentially obscuring views in mountainous or bay areas.

3. Winter (December to February): Cooler and drier
• Weather: Cool to cold and generally dry. Temperatures in Hanoi and the delta areas average 17-22°C (63-72°F) but can drop significantly, especially in mountainous regions like Sapa (where it can even snow in January). Misty and cloudy conditions are common.
• Highlights: Good for those who prefer cooler temperatures and want to avoid intense heat and humidity. Great for experiencing local festivals, especially Lunar New Year (Tet) which usually falls in late January or February (check lunar calendar for exact dates).
• Considerations: Can be quite chilly, especially at night. Some outdoor activities might be less comfortable. High season due to holidays, so expect more crowds and potentially higher prices.

4. Summer (June to August): Hot, humid, and rainy
• Weather: Hot and humid, with high rainfall. Temperatures often exceed 30°C (86°F) and can reach 35-40°C (95-104°F). July and August are often the wettest months, with heavy, sometimes torrential, afternoon downpours and a higher chance of typhoons.
• Highlights: Lush green landscapes. Fewer crowds and potentially lower prices (low season). Some coastal areas like Cat Ba and Co To are popular for beaches.
• Considerations: The heat, humidity, and heavy rain can be uncomfortable for many travelers. Outdoor activities, especially trekking, can be challenging due to muddy trails and slippery conditions. Views in Ha Long Bay or mountainous areas might be obscured by mist or rain.

In summary:
• For comfortable weather and scenic beauty (especially rice fields): September to November
• For pleasant weather and cultural festivals: March to May
• For cooler temperatures and holiday festivities: December to February
• If you don't mind heat and rain and want to avoid crowds: June to August

Always check the specific forecast for the areas you plan to visit closer to your travel dates, as weather can vary within Northern Vietnam, especially between the delta, coastal, and mountainous regions.