The Diurnal Difference

Simply put, an area’s diurnal temperature variation is the difference between the high temperature and the low temperature of a single day.  That’s easy enough to understand…but as a perpetual wine student, I hear (and use) this term all the time – and wanted to know a bit more about this “diurnal swing” and the factors behind it. Thus began another wild internet search for information!

Here are some of the more interesting things I discovered:

1: Diurnal temperature variation throughout the world varies from a low of approximately 7°F/4°C (experienced in Hong Kong in July) to 100°F/56 °C (the Guinness World Record, set in Browning, Montana in January of 1916). On average, most locations experience a diurnal temperature swing of 20-30°F/12-17°C.

2: To understand the diurnal temperature variation, we need to understand how the portion of the earth that we inhabit warms up and cools down. Basically, there are two sources of heat: the sun (solar radiation) and the ground, which slowly absorbs the sun’s heat over the course of the day. After the sun sets, the ground begins to radiate its stored heat back out, heating the air until it is depleted, which on a hot, humid day usually occurs right around dawn.

3: Diurnal swing generally decreases with proximity to the sea and other large bodies of water, and increases with the impact and amount of solar radiation. Other factors include elevation, humidity, day length, and clouds; as described below.

• Elevation: High-elevation vineyard regions brag about their large diurnal swings for good reason: as mountain areas are located further from the large expanse of sun-heated ground surface, they receive less heat-radiation energy as the ground cools at night – meaning they cool off faster than do the surrounding valleys.
• Humidity: Water vapor in the air very efficiently absorbs radiation – both solar radiation and radiation that is released from the ground.  This reduces the amount of heat reaching the ground, keeping daytime temperatures low (although we humans may not always appreciate this), and also reduces the amount of heat released from the ground – keeping the ground warm and nighttime temperatures high. The lack of humidity is one of the main factors that desert areas are known for large diurnal temperature swings.
• Day length: Longer days understandably make for more heating, which can equate to larger diurnal variations. Day length in terms of daylight hours is determined by season as well as geographic location – high-latitude areas closest to the poles have longer days in summer, and shorter days in winter than those closest to the equator. In locations close to – or on – the equator, day length may vary by a mere 7 minutes over the course of a year!
• Clouds: The presence of clouds decreases the diurnal swing. During the day, clouds absorb and release sunlight, reflecting heat radiation back out into space – making for less heat overall. On cloudy nights, heat is trapped near the ground, making for warmer nights – and less day-to-night variation.
• Wind: Winds can cause the warm air radiating off the ground to mix with the cooler air a few feet above; thus mixing the warmer and cooler air and resulting in a decrease in diurnal swing.

We all know that diurnal temperature swings can be conducive to viticulture, by allowing the grapes to develop their sugary ripeness during the warm days, while still allowing the grapes to hold onto high levels of acidity when photosynthesis shuts down at night. Hopefully now – we also know why and how these fluctuations can occur!

References:

• http://www.sciencedirect.com/science/article/pii/S2212094714000346
• http://www.britannica.com/science/climate-meteorology#ref149502
• http://weather.about.com/od/temperatures/a/Diurnal-Cycle.htm
• https://en.wikipedia.org/wiki/Day_length