3.2: Daily Temperature Variations

Daily Highs and Lows

Daily changes are also called “diurnal.” The diurnal high and low temperatures of a location depend on many factors, but most importantly on the balance between incoming solar radiation and outgoing long-wave radiation. Remember that, in addition to the sun emitting solar radiation, Earth also emits infrared radiation.

Daytime Temperatures

Starting at sunrise, the sun rises and incoming solar radiation increases. To understand why this happens, you can shine a flashlight on a surface. When the flashlight is held at a low angle, the illuminated area of the surface is large but dim (Figure 3.2.1). The same is true when the sun is low on the horizon; solar intensity is low. On the other hand, as you increase the angle of the flashlight until it is pointing straight down onto the surface, the illuminated surface becomes smaller, but brighter (Figure 3.2.2). The same is true when the sun is high in the sky: solar intensity increases as it rises.

Figure \(\PageIndex{1}\): When a flashlight is shining on a surface from a low angle, the illuminated surface is large, but dim. (Public Domain, Alicia Mullens)

Figure \(\PageIndex{2}\): When the flashlight is shining directly overhead, the illuminated surface is smaller, but significantly brighter. (Public Domain, Alicia Mullens)

Once the sun rises, incoming solar radiation quickly exceeds outgoing long-wave radiation, and Earth begins to warm. As solar energy strikes the Earth's surface each morning, a shallow 1–3-centimetre (0.39–1.18 in) layer of air directly above the ground is heated by conduction. Heat exchange between this shallow layer of warm air and the cooler air above is very inefficient. On a warm summer's day, for example, air temperatures may vary by 16.5 °C (30 °F) from just above the ground to chest height. As the morning progresses, the amount of incoming solar radiation increases, further warming air temperatures. By noon, the sun peaks in the sky, but incoming solar radiation is still significantly more than the outgoing long-wave radiation Earth is emitting, and so Earth continues to warm. As the ground warms, the amount of long-wave radiation it emits increases. In the afternoon, the sun begins to lower in the sky, and the amount of incoming solar radiation declines. However, because the Earth is still warming, the amount of outgoing long-wave radiation continues to increase. Eventually, the two (incoming solar radiation and outgoing long-wave radiation) equal each other. At this time, warming stops, and the highest daytime temperature is achieved. While other factors (advection, cloud cover, etc.) can affect the timing of the highest daily temperature, the typical time of the highest temperature is between 3 and 5 pm. The relationship between incoming and outgoing radiation and how they affect temperature is shown in Figure 3.2.3.

Figure \(\PageIndex{3}\): The amount of radiative energy (y-axis, in W m-2) as a function of time (x-axis, in hours). The shaded area between the minimum and maximum temperature represents net warming, and the shaded areas before the minimum and after the maximum represent net cooling. The temperature is coolest at sunrise, warms during the day, and reaches its maximum temperature in the late afternoon. (CC BY-SA 4.0; Britt Seifert via Atmospheric Processes and Phenomena).

Temperature lag, also known as thermal inertia, is an important factor in diurnal temperature variation. The peak daily temperature generally occurs after noon, as the air continues to absorb net heat from morning through noon and for some time thereafter. After the daytime high temperature is reached, the amount of incoming solar radiation continues to decrease as the sun sets, and Earth begins to cool. This is why, even though the sun is still out in the evening, it may feel significantly cooler than earlier in the day.

Nighttime Temperatures

At night, no sunlight enters, yet Earth continues to emit long-wave radiation. As a result, Earth loses heat and begins to cool. This cooling process is called radiational cooling. This continues throughout the night as Earth loses radiation without any incoming sunlight. As a result, the coldest temperature of the night is typically at or just after sunrise. Because air is a poor conductor of heat, radiational cooling is most extreme on surfaces. As a result, temperatures near the ground (and on surfaces) can be significantly cooler than those in the air just a few feet above. You can experience this when walking outside barefoot at night: the ground feels considerably cooler than the air at eye level. Another consequence of this is the formation of dew on surfaces, even if the air above those surfaces is not cold enough to become saturated. This will be covered in a future chapter.

While radiational cooling occurs every night, certain conditions can enhance it, resulting in extremely cold temperatures on some nights. This happens when nights are:

• Long, which allows for more extended radiational cooling.
• Clear, which allows long-wave radiation to escape into space.
• Dry, because moisture is an excellent reservoir of heat, the absence of moisture allows for additional cooling.
• Calm, because winds mix in warmer air from above with cooler air near the ground, the lack of winds prevents mixing at the surface.

One final interesting feature of nighttime temperatures is that they can be significantly cooler on valley floors than on the adjacent hillsides. This occurs because cooler air sinks into the valley floor, leaving warmer air on the hillsides. Because many agricultural regions, such as central California, lie on valley floors, farmers use various techniques, including orchard heaters, to prevent their crops from freezing.