2.1 Diurnal Energy Budget Flashcards
What is the atmosphere and its structure?
Mixture of gases with different properties. All of our weather occurs in the troposphere.
Troposphere: 75% of atmospheric mass, pressure and temperature decrease with altitude. Contains all water vapour, clouds, pollution and all the weather. Temperature decreases at 6.5 degrees per 1000m
Tropopause is the ceiling between troposphere and stratosphere. Here the temperature inverts and starts to decrease. The height can be distorted by latitude or above jet streams. Falls to -60
Stratosphere: 10-50km ASL, temperature rises with height from -60 to freezing. Small amounts of ozone absorb UV and warm up - very dry
Mesosphere: 50-85km, temp falls with altitude again. No gas, matter or particles to absorb UV. Falls to -90.
Thermosphere: 85-500km ASL, temperature increase with height as oxygen molecules absorb UV radiation
What is the daytime/nighttime energy budget?
There are 6 components affecting the daytime energy budget. Insolation may be seen as an input. The outputs are reflected solar radiation, surface absorption, sensible heat transfer, LW radiation and latent heat transfer.
The nighttime energy budget only has 4 components as there is no insolation from the sun and components act differently during the night and so there is no surface absorption. As such the 4 components are LW earth radiation, latent heat transfer, absorbed energy returned to Earth and sensible heat transfer.
What are the percentages of each component of the daytime energy budget?
100% comes in from insolation Surface absorption - 48% Atmosphere absorption - 23% Reflected by clouds/ atmosphere - 23% Reflected by surface - 6%
LW radiation from Earth - 12%
Sensible heat transfer - 5%
Latent heat transfer - 25%
What is insolation?
Shortwave radiation which arrives at the top of the atmosphere - UV radiation from the sun. What happens to the radiation depends on the wavelength, composition of the atmosphere and its thickness.
5% is scattered back to space, 24% is reflected back to space and 23% is absorbed by atmospheric gases.
Cloud cover has a significant impact on the amount received, as does the angle of incidence of the sun as when it is more high in the sky the amount of insolation is much more intense.
There are 3 processes which affect this - absorption, scattering and reflection which compose the other parts of the budget.
What is reflection?
The proportion of insolation reflected back to space is known as albedo. Most of this comes from, the top of clouds, causing 23% of insolation to be reflected back to space. Light material are more reflective than dark materials.
Grass has an albedo of 15-30 and the ocean has a low albedo of 4. This insolation has no impact on the surface and so does not warm the surface.
Scattering may occur when it passes through the atmosphere due to small particles of dust, air and water vapour which cause it to scatter in different directions. This is why the sky changes colour as there is scattering of different wavelengths of light. 22% of light is scattered either out of the atmosphere or down to the surface.
What are some important albedos?
15% grass, 40% sand, 92% thick cloud, 32% thin cloud, 22% concrete, 80% snow
What is absorption?
In the upper atmosphere, 23% of insolation is absorbed in 3 places.
High up it is absorbed by oxygen molecules, at 50km oxygen molecules absorb UV and split into ozone and at 40km the insolation is absorbed by the ozone. In the lower layers most absorption comes from water vapour and clouds which account for 15 units.
Dark surfaces absorb much more radiation than surfaces with high albedo. Some of the energy is transferred into the soil and rocks by conduction, causing a heat transfer into the surface.
This depends on how conductive the rock is which is dependent on how light it is or moisture content, for instance sand has many air spaces which are not poor conductors so store heat well.
48% of insolation is absorbed into rocks and soil. This heat may then be rereleased at night back to the surface which can partly offset the night-time cooling.
What is sensible heat transfer?
Convection from the surface heats the air slightly above it, causing parcels of air to be heated and rise and get replaced by cooler air. The molecules expand and become less dense so start to rise.
Air is a poor conductor so the thin layer on the surface is warmed and warm air molecules expand and rise. The warm surface winds can be deflected upwards by obstacles and can go up high above the surface, so in the evening it gets cool in hotter areas as the heat is released into the atmosphere.
What is long wave radiation?
Radiation absorbed by the earth is reradiated as long wave infra-red radiation. This is much more easily absorbed by greenhouse gases and heats the Earth the most. The net long-wave radiation balance refers to the amount emitted back out and the downward movement of reflected LW radiation from particles in the atmosphere. Only 7% of heat escapes.
In the night, there is a large loss of LW radiation from the earth. If there are no clouds, then there is very little return to the surface so there is a net loss of energy. On a cloudy night the clouds may return LW radiation to the surface and so the loss of energy is reduced. Therefore, in hot deserts it gets cold at night.
What is latent heat transfer?
When water is evaporated by the heat, heat energy is used up. When this rises and condenses into a liquid, heat is released. When water is present at a surface, a proportion of energy is used to evaporate it, and less energy is available to raise local energy levels and temperature.
This latent heat is technically stored in the water vapour and may rise with it until it cools to condense into ice, causing the atmosphere to be warmed
In the night water vapour close to the surface can condense to form water as the air is cooled by the surface and so latent heat is released. This can in some cases cause evaporation at night.
What is humidity and dew
Humidity is a measure of the amount of water vapour in the atmosphere. If the relative humidity and temperatures are high then it is damp and murky, causing clouds, rain, snow, dew, frost and fog.
Dew is a type of precipitation which form when water droplets form on the ground or condensation is created on the ground, usually in the night and on grasses.
The dew point is the temperature at which the water vapour in the air is saturated and so condenses on available surfaces, forming droplets of dew (hence why grass is wet). This creates humid air around the dew.
What is the role of the atmosphere?
The atmosphere absorbs radiation at some wavelengths but allows radiation at other wavelengths. It is mostly transparent and so insolation can get through and re-emitted infrared radiation is absorbed by the long wave radiation which heats up the atmosphere, known as the greenhouse effect.
What is the role of clouds?
Increase the albedo but also re-emit long wave radiation. They reflect incoming insolation and help absorb outgoing long wave radiation - they are much more effective at absorbing long wave and often let a lot of short waves through.
By allowing the surface to heat more, more clouds form and more heating occurs. Low level cumulus clouds reflect a lot of insolation out to space, cooling the troposphere and acting against greenhouse gases.
In general, thin clouds allow more in but absorb more however thick overcast clouds can reflect up to 80% of incoming radiation and cool the Earth’s surface.
What is the role of albedo?
Urban areas have lower albedos and so experience higher temperatures whereas deserts and clouds have high albedos and so reflect lots of energy.
What is the role of latitude?
The amount of insolation received is dependent on the latitude as at higher latitudes, there is more atmosphere to travel through and so more energy is lost in absorption, reflection, and scattering.
There is a large area of atmosphere to heat up and so takes longer. At the equator there is a much more concentrated heat on a smaller part, meaning more intense heating and less atmosphere to heat up
