Climate and Average Weather Year Round in Hartley California, United States

In Hartley, the summers are arid, hot, and mostly clear and the winters are wet, cold, and partly cloudy. Over the course of the year, the temperature typically varies from 39°F to 95°F and is rarely below 31°F or above 104°F.

Based on the tourism score, the best time of year to visit Hartley for warm-weather activities is from early June to late September.

The hot season lasts for 3.6 months, from June 8 to September 25, with an average daily high temperature above 87°F. The hottest month of the year in Hartley is July, with an average high of 94°F and low of 61°F.

The cool season lasts for 3.0 months, from November 22 to February 22, with an average daily high temperature below 63°F. The coldest month of the year in Hartley is January, with an average low of 40°F and high of 57°F.

In Hartley, 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 Hartley begins around May 14 and lasts for 5.3 months, ending around October 24.

The clearest month of the year in Hartley is July, during which on average the sky is clear, mostly clear, or partly cloudy 90% of the time.

The cloudier part of the year begins around October 24 and lasts for 6.7 months, ending around May 14.

The cloudiest month of the year in Hartley is January, during which on average the sky is overcast or mostly cloudy 56% of the time.

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

The wetter season lasts 5.1 months, from November 4 to April 7, with a greater than 15% chance of a given day being a wet day. The month with the most wet days in Hartley is February, with an average of 8.1 days with at least 0.04 inches of precipitation.

The drier season lasts 6.9 months, from April 7 to November 4. The month with the fewest wet days in Hartley is July, with an average of 0.2 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. The month with the most days of rain alone in Hartley is February, with an average of 8.1 days. Based on this categorization, the most common form of precipitation throughout the year is rain alone, with a peak probability of 30% on February 20.

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. Hartley experiences significant seasonal variation in monthly rainfall.

The rainy period of the year lasts for 7.6 months, from October 2 to May 20, with a sliding 31-day rainfall of at least 0.5 inches. The month with the most rain in Hartley is February, with an average rainfall of 4.0 inches.

The rainless period of the year lasts for 4.4 months, from May 20 to October 2. The month with the least rain in Hartley is August, with an average rainfall of 0.0 inches.

The length of the day in Hartley varies significantly over the course of the year. In 2026, the shortest day is December 21, with 9 hours, 29 minutes of daylight; the longest day is June 21, with 14 hours, 51 minutes of daylight.

The earliest sunrise is at 5:43 AM on June 13, and the latest sunrise is 1 hour, 50 minutes later at 7:33 AM on October 31. The earliest sunset is at 4:46 PM on December 7, and the latest sunset is 3 hours, 49 minutes later at 8:35 PM on June 27.

Daylight saving time (DST) is observed in Hartley during 2026, starting in the spring on March 8, lasting 7.8 months, and ending in the fall on November 1.

The figure below presents a compact representation of key lunar data for 2026. 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.

The perceived humidity level in Hartley, as measured by the percentage of time in which the humidity comfort level is muggy, oppressive, or miserable, does not vary significantly over the course of the year, remaining a virtually constant 0% throughout.

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 Hartley experiences mild seasonal variation over the course of the year.

The windier part of the year lasts for 5.0 months, from April 4 to September 6, with average wind speeds of more than 7.5 miles per hour. The windiest month of the year in Hartley is July, with an average hourly wind speed of 8.5 miles per hour.

The calmer time of year lasts for 7.0 months, from September 6 to April 4. The calmest month of the year in Hartley is October, with an average hourly wind speed of 6.6 miles per hour.

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

The wind is most often from the west for 8.0 months, from February 27 to October 27, with a peak percentage of 66% on July 13. The wind is most often from the north for 4.0 months, from October 27 to February 27, with a peak percentage of 53% on January 1.

Hartley 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 some seasonal variation over the course of the year.

The time of year with warmer water lasts for 2.5 months, from August 12 to October 27, with an average temperature above 56°F. The month of the year in Hartley with the warmest water is September, with an average temperature of 57°F.

The time of year with cooler water lasts for 3.1 months, from March 22 to June 27, with an average temperature below 53°F. The month of the year in Hartley with the coolest water is May, with an average temperature of 52°F.

To characterize how pleasant the weather is in Hartley 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 Hartley for general outdoor tourist activities is from early June to late September, 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 Hartley for hot-weather activities is from late June to late August, with a peak score in the third week of July.

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 Hartley typically lasts for 10 months (318 days), from around January 27 to around December 11, rarely starting after February 27, or ending before November 18.

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 Hartley should appear around February 10, only rarely appearing before January 29 or after February 24.

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 extreme seasonal variation over the course of the year.

The brighter period of the year lasts for 3.5 months, from May 7 to August 24, with an average daily incident shortwave energy per square meter above 7.2 kWh. The brightest month of the year in Hartley is June, with an average of 8.3 kWh.

The darker period of the year lasts for 3.5 months, from November 3 to February 18, with an average daily incident shortwave energy per square meter below 3.4 kWh. The darkest month of the year in Hartley is December, with an average of 2.2 kWh.

For the purposes of this report, the geographical coordinates of Hartley are 38.417 deg latitude, -121.947 deg longitude, and 118 ft elevation.

The topography within 2 miles of Hartley contains only modest variations in elevation, with a maximum elevation change of 144 feet and an average elevation above sea level of 117 feet. Within 10 miles contains only modest variations in elevation (2,769 feet). Within 50 miles contains large variations in elevation (4,314 feet).

The area within 2 miles of Hartley is covered by grassland (48%) and cropland (47%), within 10 miles by cropland (48%) and grassland (35%), and within 50 miles by cropland (36%) and grassland (26%).