The heat flow leaving the atmosphere changes during the day, a variation central to Earth’s energy budget. The planet’s temperature is governed by the balance between incoming shortwave radiation from the sun and outgoing thermal radiation. While incoming solar energy is completely absent at night, the heat leaving the Earth, known as Hout, is a continuous process. Hout varies daily, but not as dramatically as the incoming light. Understanding this daily cycle of heat loss is central to predicting weather and climate patterns.
Defining Hout: Earth’s Thermal Emission
Hout, or Outgoing Longwave Radiation (OLR), is the primary way the Earth system cools itself by shedding energy back to space. This energy is electromagnetic radiation in the infrared portion of the spectrum, typically ranging from 4 to 100 micrometers. The longwave designation distinguishes this thermal energy from the incoming shortwave radiation, such as visible light from the sun.
Any object above absolute zero, including the Earth’s surface, atmosphere, and clouds, continuously emits this thermal radiation. Hout represents the net longwave energy that ultimately exits the top of the atmosphere, acting as a counterbalance to the energy absorbed from the sun. The flux of this energy is measured in watts per square meter (\(\text{W}/\text{m}^2\)).
The Role of Solar Input and Surface Temperature
The difference between day and night in incoming solar energy drives the diurnal cycle. During the daytime, shortwave radiation from the sun is intense, while at night, this incoming energy drops to zero. This change drives surface temperature variations, which, in turn, influences the amount of heat the planet radiates.
The amount of thermal energy emitted is directly proportional to the fourth power of its temperature, as described by the Stefan-Boltzmann Law. As surface and near-surface air temperatures rise during the day due to solar heating, the local rate of Hout naturally increases. However, greenhouse gases and clouds absorb and re-emit a significant portion of this longwave radiation. This process acts like a thermal blanket, slowing the heating and cooling of the surface and preventing Hout from changing as quickly as the incoming sunlight.
Diurnal Variation in Outgoing Heat Flow
Hout changes over the course of a day, but the magnitude of this variation is significantly smaller than the swing in solar input. The variation is most noticeable over land, where surface temperature undergoes larger swings due to the lower heat capacity of soil and rock. Over land, the diurnal amplitude of Hout can be approximately \(50\ \text{W}/\text{m}^2\) in some regions.
The daily peak in Hout occurs several hours after local noon, typically in the mid-afternoon. This delay results from thermal inertia, as it takes time for absorbed solar energy to warm the surface and lower atmosphere to maximum temperatures. The minimum Hout generally occurs shortly after sunrise, reflecting the lowest temperatures reached after continuous night cooling. Over oceans, the immense heat capacity of water results in a much smaller diurnal variation in Hout, often peaking around \(25\ \text{W}/\text{m}^2\).
Atmospheric Factors Influencing Hout
The magnitude and timing of Hout’s daily change are complicated by clouds and water vapor in the atmosphere. Water vapor, a potent greenhouse gas, absorbs longwave radiation from the surface, reducing the amount of heat that immediately escapes to space. Changing humidity levels, which often fluctuate diurnally, modify the atmosphere’s opacity to longwave radiation, influencing the overall Hout.
Clouds can both warm and cool the planet depending on their type and altitude. Low, thick clouds reflect solar radiation during the day, causing cooling, but they also trap outgoing longwave radiation at night, reducing Hout. Conversely, high, thin clouds are less reflective of solar energy but are very effective at trapping outgoing heat, which reduces Hout and results in a net warming effect. Changes in cloud cover and cloud top temperature over a 24-hour period can significantly alter the local Hout measurement, sometimes dominating the signal from surface temperature changes.