Energy & surface heating

What happens to solar energy before it reaches the ground?

Separate transmission, reflection, scattering and absorption.

Radiation transfers energy without moving a parcel

Radiation carries energy as electromagnetic waves and does not require contact or a material medium. All bodies above absolute zero emit radiation; their temperature affects the total emission and its spectrum. The much hotter Sun emits mainly at short wavelengths compared with the Earth's thermal emission. Solar energy includes visible light, ultraviolet and near-infrared radiation.

Not all radiation incident at the top of the atmosphere reaches the surface. Clouds and the surface reflect part of it back to space; gases, aerosols and droplets scatter some into different directions; atmospheric constituents absorb selected wavelengths. Ozone absorbs much ultraviolet radiation, while water vapour and other gases absorb in particular bands. Filtering is selective, not a single fixed percentage applicable to every weather situation.

The surface receives direct sunlight and diffuse radiation scattered by the atmosphere. A high solar angle concentrates energy over a smaller surface area and gives a shorter atmospheric path than a low angle. Day length, latitude, season, cloud and surface reflectivity all alter the received or absorbed energy. Snow reflects much more sunlight than many dark land surfaces; strong sunshine therefore does not imply identical surface heating everywhere.

Why can the ground cool under a clear night sky?

Compare incoming and outgoing energy rather than thinking that radiation stops at sunset.

A continuing infrared exchange

The Earth emits thermal radiation mainly at longer infrared wavelengths than solar radiation. Emission continues by day and night. Whether the surface warms or cools depends on the net energy balance, including incoming solar radiation, downwelling atmospheric radiation, emitted infrared, sensible heat, latent heat and heat stored in the ground or water. A negative net radiation balance often cools the surface after sunset.

Water vapour, carbon dioxide and other absorbing gases intercept part of the outgoing terrestrial infrared and emit radiation in all directions, including downwards. Some wavelengths pass more readily through atmospheric windows. The atmosphere is consequently not transparent to all terrestrial radiation, even though much incoming visible sunlight can pass through it. The surface and atmosphere continually exchange energy; no single ray needs to be imagined as permanently trapped.

Cloud droplets and ice also absorb and emit infrared effectively. Cloud bases supply more downwelling infrared than a clear, dry sky typically does, reducing net surface cooling at night. Cloud-top infrared cooling can destabilise and mix a cloud layer from above. The effect on surface temperature must still be assessed alongside cloud height, thickness, humidity, wind and surface properties.

How does a warm or cold surface change the air above it?

Start with molecular contact, then explain how a shallow influence spreads through a deeper layer.

Conduction has a short reach in air

Conduction transfers internal energy through molecular interactions from warmer to colder material without bulk transport of the material. Air is a poor conductor. A sun-warmed runway can therefore directly warm the air touching it, but molecular conduction alone cannot quickly heat a kilometre-deep atmospheric layer. At night the direction can reverse: the cooler ground takes heat from the immediately adjacent air.

Turbulent eddies mix parcels and transfer heat, moisture and momentum through a much deeper layer. Mechanical turbulence generated by wind over rough ground and thermal turbulence driven by buoyancy both contribute. If the surface is warmer than the air, the turbulent sensible heat flux is generally upwards; if the surface is colder, it can be downwards. The strength of that exchange depends on wind, roughness and stability.

Over land, a small effective heat capacity and shallow heating depth often allow a rapid daytime temperature rise and night-time fall. Water distributes energy through mixing and transmits some radiation below the surface; evaporation consumes energy as well. The sea usually has a smaller daily temperature range than nearby dry land. Air flowing over either surface is modified most effectively when there is a substantial temperature difference and sufficient contact and mixing.

Is the heat travelling vertically or horizontally?

Use convection and advection precisely, including cases where both occur together.

Convection is vertical transport

In meteorological usage, convection commonly describes vertical transport associated with buoyant air motion. A surface heated by sunlight warms the air immediately above it; if the parcel is less dense than its surroundings, it rises and cooler air replaces it. Thermals over sun-heated land are one example. Cold air passing over relatively warm sea can also become unstable and convective, sometimes producing showers even at night.

A rising parcel expands and cools. If it reaches saturation, cloud develops; if stability aloft stops the ascent, the cloud may remain shallow. Forced ascent over terrain or along a front is not necessarily freely buoyant convection at the start, although it can release conditional instability and initiate deep convection. Identify the lifting mechanism as well as the stability of the air.

Advection is horizontal transport

Advection describes the transport of a property by horizontal air movement: warm advection brings warmer air into a region; cold advection brings colder air. It can warm or cool a station even when the local radiative cycle suggests the opposite. Moisture advection changes dew point and cloud or fog potential. It does not itself mean the air is being heated while it moves: the air may simply carry a temperature acquired elsewhere.

Processes can combine. Warm moist air advected over a cold sea loses sensible heat to the surface and may form advection fog. Cold air advected over warmer water can gain heat, become unstable and generate convective cloud. In both cases the horizontal transport is advection; the local vertical exchange and buoyant motions are separate parts of the explanation.

Why can condensing cloud sustain an updraught?

Track where energy is absorbed, transported and released.

A phase change can transfer energy without a temperature rise

Evaporation changes liquid water to vapour and absorbs latent energy from the water and its surroundings. The resulting vapour can be transported by wind and vertical motion. When vapour condenses into cloud droplets, latent energy is released into the parcel. This offsets part of the cooling caused by expansion, so a saturated rising parcel normally cools more slowly than an otherwise comparable unsaturated one.

Melting and sublimation also absorb energy; freezing and deposition release it. These transfers are examined in the phase-change chapter. The central idea is that energy can be stored in the water's phase and transported before being released elsewhere. The atmosphere and surface exchange both sensible heat, associated with temperature differences, and latent heat, associated with water phase changes.

Latent heat release does not mean a rising cloud parcel must become warmer with altitude. It generally still cools, but at a reduced rate. If it remains warmer than the environment it can stay buoyant and continue to rise. Conversely, evaporation of falling rain into dry air absorbs energy, cools the air and can strengthen a descending current. The effect on buoyancy depends on the parcel's final temperature relative to its surroundings.

Why is the warmest time usually after solar noon?

Follow the net balance and the redistribution of heat through one day and night.

Energy balance controls the timing

Solar input is strongest near solar noon, but the surface can continue receiving more energy than it loses afterwards. Stored energy and ongoing net heating delay the usual temperature maximum into the afternoon. At night the surface can continue cooling until shortly after sunrise, when increasing incoming solar energy finally offsets the outgoing and other losses. Advection, fronts, cloud changes and terrain can override this typical timing.

Cloud generally reduces daytime solar heating and reduces night-time radiative cooling, narrowing the daily temperature range under otherwise similar conditions. Clouds do not always cool the surface at every time: their shortwave shading and longwave emission have competing effects. A sudden clearing near dawn can intensify surface cooling, while a cloud deck arriving at night can limit further temperature fall.

Wind redistributes the surface influence

Light wind on a clear night allows a strong shallow cold layer to develop. Moderate mixing brings warmer air down from above an inversion and carries cooled air upwards, often reducing the minimum-temperature fall at screen level. Over heated land by day, mixing carries surface heat through a deeper layer and can reduce the near-surface temperature contrast. Stronger wind also increases evaporation when moisture is available.

Do not infer the temperature tendency from wind speed alone. A strong flow can advect a much colder air mass and lower the temperature despite vigorous mixing. Diagnose three things in order: the surface energy balance, exchange between the surface and the air, and horizontal transport from upstream. This sequence explains why two airports with similar sunshine can have very different temperature and fog evolution.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.