Pennsylvania’s climate is characterized by four distinct seasons, placing the state largely within the humid continental zone. The summer months bring a predictable peak in heat that affects residents across the commonwealth, though not uniformly. Understanding the hottest month and how local geography modifies the warmth is useful for preparing for the mid-year weather.
Identifying Pennsylvania’s Peak Heat
The month of July is historically the warmest period across Pennsylvania. This peak heat is a consistent feature of the state’s annual weather cycle, surpassing the average temperatures seen in both June and August. The statewide average high temperature during July settles around 84°F, with average low temperatures dropping to 62°F.
This average confirms July’s status as the height of summer heat before the gradual cooling trend begins in late August. The higher average low temperatures in July are notable, as they prevent nighttime cooling and contribute to the overall accumulation of heat.
Geographic Differences in Summer Temperatures
Pennsylvania’s summer warmth is significantly altered by its varied topography, preventing a single temperature from defining the entire state. The Appalachian Mountain system, the Great Lakes, and major river valleys each play a role in creating distinct microclimates. This geographical diversity means the experience of July heat in one city can differ substantially from another.
The state’s southeastern region, including the Philadelphia area, records the highest July temperatures. Average high temperatures here can reach 88°F, with warm nighttime lows near 70°F. The concentration of concrete and pavement in this dense urban environment exacerbates the heat through the urban heat island effect, causing temperatures to remain elevated long after sunset.
In contrast, Western Pennsylvania cities like Pittsburgh experience milder conditions, with July average highs closer to 83°F and lows around 63°F. The influence of the Ohio River valley and varied terrain can lead to greater temperature fluctuations than the flat coastal plain areas to the east. The northern and mountainous regions, such as the areas around Scranton, see the coolest July averages, though they still experience warm days.
Understanding Summer Heat Metrics
The feeling of heat in Pennsylvania during July is often more intense than the air temperature alone suggests, a phenomenon explained by the role of atmospheric moisture. While the air temperature, or dry bulb temperature, measures the air directly, it does not account for the human body’s difficulty in cooling itself through sweat evaporation. The actual discomfort is driven by humidity.
The dew point is a precise measure of the moisture content in the air, representing the temperature at which the air must be cooled to become saturated. When the dew point rises above 60°F, the air begins to feel noticeably humid and sticky. In parts of Pennsylvania, particularly the southeast, the average July dew point is near 65°F, which defines uncomfortably “muggy” conditions.
This combination of air temperature and dew point is quantified by the Heat Index, which is the “feels like” temperature. A measured air temperature of 83°F with high humidity can result in a Heat Index exceeding 90°F, making physical activity outdoors hazardous. For this reason, the Heat Index is the figure officials reference when issuing heat advisories during the peak July weeks.