Climate and Average Weather Year Round in Iwamizawa Japan

In Iwamizawa, the summers are comfortable, wet, and partly cloudy and the winters are freezing, snowy, windy, and mostly cloudy. Over the course of the year, the temperature typically varies from 9°F to 79°F and is rarely below -3°F or above 86°F.

Based on the tourism score, the best time of year to visit Iwamizawa for warm-weather activities is from early July to late August.

The warm season lasts for 3.5 months, from June 8 to September 23, with an average daily high temperature above 68°F. The hottest month of the year in Iwamizawa is August, with an average high of 78°F and low of 63°F.

The cold season lasts for 3.5 months, from December 1 to March 18, with an average daily high temperature below 37°F. The coldest month of the year in Iwamizawa is January, with an average low of 9°F and high of 28°F.

In Iwamizawa, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year.

The clearer part of the year in Iwamizawa begins around May 13 and lasts for 6.4 months, ending around November 26.

The clearest month of the year in Iwamizawa is October, during which on average the sky is clear, mostly clear, or partly cloudy 63% of the time.

The cloudier part of the year begins around November 26 and lasts for 5.6 months, ending around May 13.

The cloudiest month of the year in Iwamizawa is January, during which on average the sky is overcast or mostly cloudy 75% of the time.

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days in Iwamizawa varies throughout the year.

The wetter season lasts 5.7 months, from July 20 to January 11, with a greater than 32% chance of a given day being a wet day. The month with the most wet days in Iwamizawa is November, with an average of 12.7 days with at least 0.04 inches of precipitation.

The drier season lasts 6.3 months, from January 11 to July 20. The month with the fewest wet days in Iwamizawa is March, with an average of 6.9 days with at least 0.04 inches of precipitation.

Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. Based on this categorization, the most common form of precipitation in Iwamizawa changes throughout the year.

Rain alone is the most common for 8.6 months, from March 15 to December 1. The month with the most days of rain alone in Iwamizawa is October, with an average of 11.7 days.

Snow alone is the most common for 3.4 months, from December 1 to March 15. The month with the most days of snow alone in Iwamizawa is January, with an average of 7.1 days.

Rainfall

To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day of the year. Iwamizawa experiences extreme seasonal variation in monthly rainfall.

The rainy period of the year lasts for 10 months, from February 23 to January 1, with a sliding 31-day rainfall of at least 0.5 inches. The month with the most rain in Iwamizawa is August, with an average rainfall of 5.3 inches.

The rainless period of the year lasts for 1.7 months, from January 1 to February 23. The month with the least rain in Iwamizawa is January, with an average rainfall of 0.4 inches.

Snowfall

As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day of the year. Iwamizawa experiences extreme seasonal variation in monthly snowfall.

The snowy period of the year lasts for 5.7 months, from October 20 to April 11, with a sliding 31-day snowfall of at least 1.0 inches. The month with the most snow in Iwamizawa is December, with an average snowfall of 15.1 inches.

The snowless period of the year lasts for 6.3 months, from April 11 to October 20. The least snow falls around July 20, with an average total accumulation of 0.0 inches.

The length of the day in Iwamizawa varies significantly over the course of the year. In 2024, the shortest day is December 21, with 8 hours, 59 minutes of daylight; the longest day is June 21, with 15 hours, 23 minutes of daylight.

The earliest sunrise is at 3:52 AM on June 15, and the latest sunrise is 3 hours, 13 minutes later at 7:05 AM on January 4. The earliest sunset is at 3:57 PM on December 8, and the latest sunset is 3 hours, 20 minutes later at 7:17 PM on June 26.

Daylight saving time (DST) is not observed in Iwamizawa during 2024.

The figure below presents a compact representation of key lunar data for 2024. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases.

We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.

Iwamizawa experiences extreme seasonal variation in the perceived humidity.

The muggier period of the year lasts for 2.2 months, from July 6 to September 13, during which time the comfort level is muggy, oppressive, or miserable at least 13% of the time. The month with the most muggy days in Iwamizawa is August, with 13.3 days that are muggy or worse.

The least muggy day of the year is March 11, when muggy conditions are essentially unheard of.

This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed in Iwamizawa experiences significant seasonal variation over the course of the year.

The windier part of the year lasts for 7.5 months, from October 3 to May 20, with average wind speeds of more than 8.6 miles per hour. The windiest month of the year in Iwamizawa is February, with an average hourly wind speed of 10.4 miles per hour.

The calmer time of year lasts for 4.5 months, from May 20 to October 3. The calmest month of the year in Iwamizawa is July, with an average hourly wind speed of 6.6 miles per hour.

The predominant average hourly wind direction in Iwamizawa varies throughout the year.

The wind is most often from the south for 6.1 months, from April 5 to October 8, with a peak percentage of 58% on August 3. The wind is most often from the west for 5.9 months, from October 8 to April 5, with a peak percentage of 53% on January 1.

Iwamizawa is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.

The average water temperature experiences significant seasonal variation over the course of the year.

The time of year with warmer water lasts for 2.7 months, from July 15 to October 6, with an average temperature above 65°F. The month of the year in Iwamizawa with the warmest water is August, with an average temperature of 70°F.

The time of year with cooler water lasts for 4.6 months, from December 16 to May 2, with an average temperature below 47°F. The month of the year in Iwamizawa with the coolest water is March, with an average temperature of 41°F.

To characterize how pleasant the weather is in Iwamizawa throughout the year, we compute two travel scores.

The tourism score favors clear, rainless days with perceived temperatures between 65°F and 80°F. Based on this score, the best time of year to visit Iwamizawa for general outdoor tourist activities is from early July to late August, with a peak score in the first week of August.

The beach/pool score favors clear, rainless days with perceived temperatures between 75°F and 90°F. Based on this score, the best time of year to visit Iwamizawa for hot-weather activities is from late July to mid August, with a peak score in the first week of August.

Methodology

For each hour between 8:00 AM and 9:00 PM of each day in the analysis period (1980 to 2016), independent scores are computed for perceived temperature, cloud cover, and total precipitation. Those scores are combined into a single hourly composite score, which is then aggregated into days, averaged over all the years in the analysis period, and smoothed.

Our cloud cover score is 10 for fully clear skies, falling linearly to 9 for mostly clear skies, and to 1 for fully overcast skies.

Our precipitation score, which is based on the three-hour precipitation centered on the hour in question, is 10 for no precipitation, falling linearly to 9 for trace precipitation, and to 0 for 0.04 inches of precipitation or more.

Our tourism temperature score is 0 for perceived temperatures below 50°F, rising linearly to 9 for 65°F, to 10 for 75°F, falling linearly to 9 for 80°F, and to 1 for 90°F or hotter.

Our beach/pool temperature score is 0 for perceived temperatures below 65°F, rising linearly to 9 for 75°F, to 10 for 82°F, falling linearly to 9 for 90°F, and to 1 for 100°F or hotter.

Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).

The growing season in Iwamizawa typically lasts for 5.2 months (160 days), from around May 3 to around October 11, rarely starting before April 16 or after May 21, and rarely ending before September 24 or after October 27.

Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.

Based on growing degree days alone, the first spring blooms in Iwamizawa should appear around May 14, only rarely appearing before May 5 or after May 23.

This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.

The average daily incident shortwave solar energy experiences significant seasonal variation over the course of the year.

The brighter period of the year lasts for 3.9 months, from April 23 to August 20, with an average daily incident shortwave energy per square meter above 5.2 kWh. The brightest month of the year in Iwamizawa is June, with an average of 6.1 kWh.

The darker period of the year lasts for 3.3 months, from November 3 to February 13, with an average daily incident shortwave energy per square meter below 2.2 kWh. The darkest month of the year in Iwamizawa is December, with an average of 1.3 kWh.

For the purposes of this report, the geographical coordinates of Iwamizawa are 43.200 deg latitude, 141.760 deg longitude, and 56 ft elevation.

The topography within 2 miles of Iwamizawa contains only modest variations in elevation, with a maximum elevation change of 289 feet and an average elevation above sea level of 70 feet. Within 10 miles contains only modest variations in elevation (1,739 feet). Within 50 miles contains very significant variations in elevation (6,824 feet).

The area within 2 miles of Iwamizawa is covered by cropland (70%) and trees (19%), within 10 miles by cropland (51%) and trees (36%), and within 50 miles by trees (54%) and water (23%).