Climate and monthly weather forecast
Portsmouth, NH

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Bright neon lights and buildings next to the very calm waters of the iconic Piscataqua River in Portsmouth, New Hampshire
Bright neon lights and buildings next to the very calm waters of the iconic Piscataqua River in Portsmouth, New Hampshire

The climate of Portsmouth

Portsmouth, residing within the crux of the New Hampshire climate zone, falls primarily within the humid continental climate classification, dictated by the Köppen classification system. It is identified as Dfb which typifies it as having no dry season, but a warm summer. Throughout the year, Portsmouth undergoes notable variations in weather with each season introducing unique climatic attributes.

During a calendar year, temperature peaks in July at 76.3°F (24.6°C) and dips to a low of 21.2°F (-6°C) in January. Fluctuations in humidity remain low as percentages linger from 75% to 80% throughout the year. Rainfall varies lightly, reaching a high of 2.83" (72mm) in October and a low of 1.54" (39mm) in September. There is no significant dry season in Portsmouth as rainfall is consistent throughout the year. Nonetheless, May and June experience the highest number of rainy days, 14.8 and 15 respectively.

Portsmouth experiences its maximum snowfall in January at 4.02" (102mm) with zero snowfall observed from May to October. Daylight varies notably within a calendar year reaching a high of 15.3 hours in June and sinking to a low of 9.1 hours in December. Average UV Index ranges between 2 in the winter months to 6 during the summer season. Wind speed remains relatively stable, peaking at 11.7mph (18.8km/h) in January and dipping to 6.6mph (10.6km/h) in July.

The best time to visit Portsmouth

For those seeking a moderate weather experience, the months from May to September would be the most favourable time to visit Portsmouth. During this period, the temperature ranges between a comfortable 51.1°F (10.6°C) to 75.7°F (24.3°C). Minimal rainfall is observed in September, further enhancing the conditions for outdoor activities. Daylight hours are ample, peaking at 15.3 hours in June. Moreover, the vibrancy of Portsmouth's sun during this period is not to be understated with June through August offering over 10 daily sunshine hours.

The worst time to visit Portsmouth

For travellers averse to chillier conditions, the time from November to March might be less desirable. These months feature Portsmouth's lowest temperature averages, dropping to a chilling 21.2°F (-6°C) in January. Additionally, January and February bear the brunt of annual snowfall, reaching 4.02" (102mm) in January. Limited daylight and sunshine hours could also present constraints on outdoor activities.

Spring weather in Portsmouth

Spring in Portsmouth, enveloping the months from March to May, showcases an accelerated temperature rise from 40.5°F (4.7°C) to 61°F (16.1°C). Rainfall and humidity remain consistent, although March and April experience a significant number of rainfall days, reaching 13.8 days by April. Despite the rain, the city remains illuminated with daylight hours extending to 14.7 hours in May with almost 9 hours of sunshine.

Summer weather in Portsmouth

Summer in Portsmouth, NH, is marked by pleasant average temperatures, ranging from 69.3°F (20.7°C) to 76.3°F (24.6°C) from June to August. Despite slight increases in rainfall, the number of rainy days drops slightly in comparison to spring. The season also offers the maximum daylight hours, topping at 15.3 hours in June. It is also the ideal time for sun-seekers with 10.3 daylight hours on average throughout July.

Autumn weather in Portsmouth

Autumn, spanning from September to November, witnesses a steady reduction in temperatures from 69.1°F (20.6°C) to 47.1°F (8.4°C). Rainfall increases slightly during this period, peaking as high as 2.83" (72mm) in October. Days shorten reaching 9.7 daylight hours in November. Simultaneously, the city experiences a decline in sunshine duration, dipping to 5.5 hours daily in November.

Winter weather in Portsmouth

Winter in Portsmouth introduces a significant plunge in temperature reaching as low as 21.2°F (-6°C) in January. Rainfall, however, remains relatively consistent, ranging from 2.8" (71mm) in December to 2.09" (53mm) in February. However, snowfall become prevalent, reaching 4.02" (102mm) in January. The sunshine shrinks to its annual low in December, with only 4.8 hours of sun per day, and daylight decreases to a yearly low of 9.1 hours in December.

Weather in January

Despite chilly conditions, Portsmouth experiences adequate daylight with nine and a half hours in January. The characteristic freezing temperatures underscore the traditional winter conditions of this New Hampshire city. Compared to February, lower snowfall occurs on fewer days. Weather in January »

Weather in February

February's charm in Portsmouth lies within the slight temperature rise as it ushers in incrementally longer days, signaling the impending spring. However, on the similarity front, February mirrors January in the feature of relative humidity. Weather in February »

Weather in March

As winter melancholy fades, the promise of spring sunshine beckons in March. Portsmouth ventures towards warmer days with increased sunshine hours, hinting at the approaching spring respite. Relative humidity remains consistent, juxtaposing the partly cloudy skies with brief bursts of watery sunshine. Weather in March »

Weather in April

April sets the stage for Portsmouth's transition from winter to spring, as a noticeable warm-up in temperatures takes over. Increased daylight amps up the warmth levels, providing a cushion against the crisp ocean breeze. April sees a further drop in snowfall, accounting for just 1.22" (31mm) spread over around one and a half days. Weather in April »

Weather in May

The charm of Spring is fully founded in May and Portsmouth transforms into a sun-kissed realm gradually. A dive into the sea becomes appealing as the seawater temperature creeps up. The scenery flourishes under the generous sunshine, and May boasts its absence of snowfall. Weather in May »

Weather in June

June straddles the edge of spring and summons the heat of summer as temperatures climb up further. Portsmouth experiences a balance of sun and rains, with humidity peaking for the first time in the year. The seaside town dons its bright sunny look, thanks to the increased daylight hours. Weather in June »

Weather in July

July hotfoot it as the warmest month in Portsmouth, tapping into the rhythm of high summer. The bright sunny days are indulgent, befitting for a range of outdoor activities. A soothing sea breeze making the hot days manageable, the need for a summer hat goes without saying. Weather in July »

Weather in August

August mirrors July in terms of temperature trends, but with fewer daylight hours and reduced sunlight. Portsmouth experiences an overall dip in rainfall days, though the amount of rainfall remains consistent. On the brighter side, the sea water retains its warmth even as the peak summer starts to wind down. Weather in August »

Weather in September

September brings a subtle switch from summer and slowly introduces the cool autumnal breeze in Portsmouth. The transition is impeccable with a considerable reduction in humidity, fostering comfort, and ease. The rainy days decrease, and the city gets its first taste of fall. Weather in September »

Weather in October

With October, the full swing of the fall season arrives, and Portsmouth blossoms into autumnal hues. A perceptible edge to the air arises as temperatures drop, marking the onset of mild winter. While water temperature continues to plunge, land temperatures follow suit, causing a delightful crisp in the air. Weather in October »

Weather in November

November witnesses the last vestiges of fall and begins marching towards the upcoming winter with lower temperatures and short daylight hours. Portsmouth sees a dive in sea temperatures down to 49.8°F (9.9°C) making it less inviting for swimming. Weather in November »

Weather in December

December arrives on a freezing note with temperatures falling further as Portsmouth heralds winter's commencement. Daylight stays relatively limited, with the sun peaking out for barely nine hours. It's a splendid sight to witness the snowfall, which rises to 3.9" (99mm) usually falling across seven and a half days. Weather in December »

Frequently asked questions

What time of the year is the coldest in Portsmouth?

January is the coldest month, exhibiting average temperatures of 31.8°F (-0.1°C) at the peak and 21.2°F (-6°C) at the base.

How many days does it rain?

In Portsmouth, during the entire year, the rain falls for 144.2 days and collects up to 27.48" (698mm) of precipitation.

When does it snow?

January through April, October through December are months with snowfall.

What is the snowiest month?

The month with the most snowfall is January, when snow falls for 8.8 days and typically aggregates up to 4.02" (102mm) of snow.

How much does it snow?

Throughout the year, there are 35.2 snowfall days, and 16.97" (431mm) of snow is accumulated.

What are the months with the lowest UV index?

The months with the lowest UV index are January through March, November and December, with an average maximum UV index of 2.

What are the months with the least sunshine?

The months with the least sunshine are February and December, with an average of 4.8h of sunshine.

What month is the ocean coldest?

March is the month with the coldest seawater, with an average ocean temperature of 38.5°F (3.6°C).

When is Daylight Saving Time (DST)?

On Sunday, March 10. 2024, at 2:00 am, Daylight Saving Time starts, and the time zone changes from EST to EDT. Daylight Saving Time ends Sunday, November 3. 2024, at 2:00 am, and the time zone reverts from EDT to EST.

What are the least humid months?

April and November, with an average relative humidity of 75%, are the least humid months.

When it does not snow?

In Portsmouth, snow does not fall in May through September.

What is the most humid month?

The most humid month is June, with an average relative humidity of 80%.

When are the longest days?

June has the longest days of the year, with an average of 15h and 18min of daylight.

What time of the year is the hottest?

Portsmouth's warmest weather arrives in July, when temperatures average a high of 76.3°F (24.6°C) and a low of 62.6°F (17°C).

What is the month with the highest UV index?

The months with the highest UV index are July and August, with an average maximum UV index of 6.

What month is the ocean warmest?

August is the month with the warmest seawater, with an average ocean temperature of 65.7°F (18.7°C).

What month has the most sunshine?

With an average of 10.6h of sunshine, August has the most sunshine of the year.

What is the driest month?

The month with the least rainfall is September, when the rain falls for 11.3 days and typically collects 1.54" (39mm) of precipitation.

What is the wettest month?

The month with the most rainfall is October, when the rain falls for 13.3 days and typically aggregates up to 2.83" (72mm) of precipitation.

What is the month with the shortest days?

With an average of 9h and 6min of daylight, December has the shortest days of the year.

Average temperature
Portsmouth, NH

Average temperature - Portsmouth, NH

The warmest month (with the highest average high temperature) is July (76.3°F).
The month with the lowest average high temperature is January (31.8°F).

The month with the highest average low temperature is July (62.6°F).
The coldest month (with the lowest average low temperature) is January (21.2°F).

Average pressure
Portsmouth, NH

Average pressure - Portsmouth, NH

The month with the highest atmospheric pressure is November (30.06"Hg).
The month with the lowest atmospheric pressure is June (29.92"Hg).

Average wind speed
Portsmouth, NH

Average wind speed - Portsmouth, NH

The windiest month (with the highest average wind speed) is January (11.7mph).
The calmest month (with the lowest average wind speed) is July (6.6mph).

Average humidity
Portsmouth, NH

Average humidity - Portsmouth, NH

The month with the highest relative humidity is June (80%).
The months with the lowest relative humidity are April and November (75%).

Average rainfall
Portsmouth, NH

Average rainfall - Portsmouth, NH

The wettest month (with the highest rainfall) is October (2.83").
The driest month (with the least rainfall) is September (1.54").

Average rainfall days
Portsmouth, NH

Average rainfall days - Portsmouth, NH

The month with the highest number of rainy days is June (15 days).
The month with the least rainy days is January (7.5 days).

Average snowfall
Portsmouth, NH

Average snowfall - Portsmouth, NH

The month with the highest snowfall is January (4.02").
The months with the least snowfall are May, June, July, August and September (0").

Average snowfall days
Portsmouth, NH

Average snowfall days - Portsmouth, NH

The month with the highest number of snowfall days is January (8.8 days).
The months with the least snowfall days are May, June, July, August and September (0 days).

Average sea temperature
Portsmouth, NH

Average sea temperature - Portsmouth, NH

The best month for swimming (with the highest average sea temperature) is August (65.6°F).
The coldest month (with the lowest average sea temperature) is March (38.5°F).

Average daylight / Average sunshine
Portsmouth, NH

Average daylight / Average sunshine - Portsmouth, NH

The month with the longest days is June (Average daylight: 15h and 18min).
The month with the shortest days is December (Average daylight: 9h and 6min).

The month with the most sunshine is August (Average sunshine: 10h and 36min).
The months with the least sunshine are February and December (Average sunshine: 4h and 48min).

Average sunshine days
Portsmouth, NH

Average sunshine days - Portsmouth, NH

The month with the most sunshine days is September (16.8 days).
The month with the least sunshine days is April (13.2 days).

Average UV index
Portsmouth, NH

Average UV index - Portsmouth, NH

The months with the highest UV index are July and August (UV index 6).
The months with the lowest UV index are January, February, March, November and December (UV index 2).

Average visibility
Portsmouth, NH

Average visibility - Portsmouth, NH

The months with the highest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).
The months with the lowest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).

Portsmouth, New Hampshire, USA

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