The temperature that constitutes “summer” is highly relative, influenced by geography, local climate, and humidity. A hot day in one location might be considered mild in another. The official definition of the season also varies depending on whether one uses an astronomical or a meteorological framework.
Defining Summer: Meteorological vs. Astronomical
The two primary ways summer is defined relate to Earth’s orbit and the calendar. Astronomical summer is based on the planet’s position relative to the sun, beginning at the summer solstice around June 21st in the Northern Hemisphere. This event marks the moment of maximum tilt toward the sun, resulting in the longest day of the year.
Scientists find this definition difficult to use when tracking climate trends. The meteorological definition simplifies this process by aligning with the calendar months that typically contain the warmest temperatures. In the Northern Hemisphere, meteorological summer runs from June 1st through August 31st.
Grouping three full calendar months allows meteorologists to consistently compare seasonal data and calculate long-term climate statistics. The astronomical definition varies slightly in start and end dates, which complicates the averaging of temperature records.
Typical Temperature Ranges and Regional Variation
For much of the population in temperate zones, a typical summer day features average high temperatures ranging from approximately 70°F to 90°F. This broad range is heavily influenced by geography, particularly latitude and proximity to large bodies of water. Coastal regions, which have a maritime climate, experience moderated temperatures because the ocean warms and cools more slowly than land.
Inland areas, which have a continental climate, exhibit much greater temperature extremes. For example, the Pacific Northwest coast might see July average high temperatures remain in the 65°F to 75°F range. By contrast, the desert Southwest experiences average daily summer highs that routinely exceed 100°F, sometimes reaching 117°F in locations like Death Valley.
Altitude also plays a significant role, as temperatures generally decrease by about 3.5°F for every 1,000 feet of elevation gain. This explains why a mountain town can experience a cool 75°F summer high while a nearby city in a valley swelters at 95°F.
Understanding Extreme Heat and the Heat Index
When temperatures reach dangerous levels, the air temperature alone is not the sole measure of risk. A heat wave is defined as a period of abnormally hot weather lasting multiple days, often exceeding a specific threshold relative to the local climate. The World Meteorological Organization defines a heat wave as five or more consecutive days where the daily maximum temperature surpasses the average maximum temperature by 9°F (5°C) or more.
To accurately reflect the danger to the human body, forecasters use the Heat Index, sometimes called the “feels like” temperature. This metric combines the air temperature with relative humidity. The body cools itself primarily through the evaporation of sweat from the skin.
When humidity is high, the air is already saturated with moisture, significantly slowing the rate at which sweat can evaporate. This lack of evaporative cooling prevents the body from regulating its internal temperature effectively. The heat index provides a more accurate public safety metric, indicating that a 90°F day with high humidity can feel like 105°F or higher.
How Climate Trends are Reshaping Summer Temperatures
Modern climate trends are causing a significant shift in the baseline and duration of summer heat. Global observations show a clear trend of rising average summer temperatures and an increased frequency and intensity of heat waves. This change is resulting in a phenomenon referred to as “summer creep,” where the period of high heat is expanding.
In the Northern Hemisphere, the length of the summer season has already extended by nearly three weeks over the last half-century. This expansion is due to summer temperatures starting earlier in the spring and lasting longer into the autumn. Projections indicate that if current warming trends continue, the season of summer heat could last nearly six months by the end of the century.
This lengthening of the hot season means that the number of days exceeding local temperature thresholds is increasing. For many regions in the United States, the annual number of days with a Heat Index above 100°F is projected to double or even triple. These trends underscore the rising health and environmental risks associated with prolonged extreme heat.