Atmosphere and Weather Notes

2 Atmosphere and Weather

2.1 Diurnal Energy Budgets

The Daytime Energy Budget

• An energy budget considers the amount of energy entering and leaving a system, as well as energy transfer within the system.
• Energy budgets are analyzed at both global (macro) and local (micro) scales.
• The daytime energy budget comprises six components:
  • Incoming solar radiation (insolation).
  • Reflected solar radiation.
  • Surface absorption.
  • Latent heat transfer (evaporation).
  • Sensible heat transfer.
  • Long-wave radiation.
• These components determine the energy gain or loss at a point on the Earth's surface.
• The daytime energy budget can be expressed as:
  energy \ available \ at \ the \ surface = incoming \ solar \ radiation - (reflected \ solar \ radiation + surface \ absorption + sensible \ heat \ transfer + long-wave \ radiation + latent \ heat \ transfers)
• Incoming solar radiation (insolation):
  • It is the primary energy input.
  • Affected by latitude, season, and cloud cover.
  • Less cloud cover and higher cloud altitude result in more radiation reaching the Earth's surface.
• Reflected solar radiation (albedo):
  • Varies with the color of the surface; lighter materials are more reflective.
  • Grass has an albedo of 20-30%, meaning it reflects 20-30% of the received radiation.

Table 2.1 Selected albedo values

• Surface
  • Water (Sun's angle over 40°) Albedo (%): 2-4
  • Water (Sun's angle less than 40°) Albedo (%): 6-80
  • Fresh snow Albedo (%): 75-90
  • Old snow Albedo (%): 40-70
  • Dry sand Albedo (%): 35-45
  • Dark, wet soil Albedo (%): 5-15
  • Dry concrete Albedo (%): 17-27
  • Black road surface Albedo (%): 5-10
  • Grass Albedo (%): 20-30
  • Deciduous forest Albedo (%): 10-20
  • Coniferous forest Albedo (%): 5-15
  • Crops Albedo (%): 15-25
  • Tundra Albedo (%): 15-20
    *Revised
• Surface absorption:
  • Energy reaching Earth's surface heats it up.
  • The amount of heating depends on the surface's nature.
  • Surfaces that conduct heat to lower layers remain cooler.
  • Energy concentrated at the surface leads to greater warming.
• Sensible heat transfer:
  • Movement of air parcels into and out of the studied area.
  • Convection occurs when surface-warmed air rises and is replaced by cooler air.
  • Common in warm areas during early afternoon.
• Long-wave radiation:
  • Radiation of energy from the Earth into the atmosphere and space.
  • Downward movement of long-wave radiation from atmospheric particles.
  • The difference between upward and downward flows is the net radiation balance.
• Latent heat transfer (evaporation):
  • Heat energy is used to convert liquid water into water vapor.
  • The heat is released when water vapor condenses.
  • Energy available for temperature increase is reduced when water evaporates from a surface.
• Dew refers to condensation on a surface.
• The air is saturated, generally because the surface temperature has dropped enough to cause condensation.
• Absorbed energy (insolation) received by the Earth will be reradiated as long-wave radiation.
• Some of this will be absorbed by water vapor and other greenhouse gases, thereby raising the temperature.

The Night-Time Energy Budget

• Consists of four components:
  • Long-wave radiation.
  • Latent heat transfer (condensation).
  • Absorbed energy returned to Earth (sub-surface supply).
  • Sensible heat transfer.
• Long-wave radiation:
  • Large loss of long-wave radiation from Earth during cloudless nights.
  • Clouds return some long-wave radiation to the surface on cloudy nights, reducing overall energy loss.
• Latent heat transfer (condensation):
  • Heat is released when water condenses.
  • Water vapor near the surface condenses as the air cools during the night.
• Sub-surface supply:
  • Heat transferred to the soil and bedrock during the day is released back to the surface at night.
  • Partly offsets night-time cooling.
• Sensible heat transfer:
  • Air movement.
  • Cold air reduces temperatures; warm air supplies energy.

Figure 2.1 Daytime and night-time energy budgets for Washington DC

• Shows rural and urban energy budgets.
• Values are proportions of 100 units of incoming solar radiation.
• Rural surface:
  • Day: Short-wave radiation 100, Heat loss due to evaporation 24, Heat loss by air movement 8, Long-wave radiation from the ground 35, Long-wave radiation from clouds +4, Heating of the ground by conduction 24.
• Night: Heat given up by the ground to the surface 44, Long-wave radiation from the ground 54, Long-wave radiation from clouds 11, Heat loss by air movement 30
  • Urban surface:
  • Day: Incoming solar radiation 100, Short-wave radiation reflected back to space from clouds and ground 5, Heat loss due to evaporation 10, Heat loss by air movement 1, Long-wave radiation from ground 35 Long-wave radiation from clouds 2, Heating of the ground by conduction 53
• Night: Heat given up by the ground to the surface 22, Long-wave radiation from ground 69, Long-wave radiation from clouds 10, Heat loss by air movement 5.

2.2 The Global Energy Budget

Latitudinal Variations in Radiation

• The atmosphere is an open energy system that receives energy from the Sun and the Earth.
• Variations in solar radiation exist based on latitude and season.
• This creates an imbalance: excesses in the tropics and deficits in temperate and polar regions.
• Energy is transferred horizontally from the equator to the poles through winds and ocean currents to achieve balance.
• This transfer forms a secondary energy budget in the atmosphere.

Figure 2.2 Seasonal and latitudinal variations in insolation

• Shows variations in solar radiation with latitude and season, assuming no atmosphere.
• Explains high radiation amounts received at the poles in summer due to 24-hour daylight.

Atmospheric Transfers

• Excess net radiation in lower latitudes leads to poleward energy transfer via ocean currents and wind systems.
• This transfer occurs through sensible heat (warm air/ocean water) and latent heat (atmospheric water vapor).

Global Wind Systems

• Major wind systems are determined by temperature and pressure variations.
• Trade winds:
  • Blow from subtropical high-pressure belts (STHP) towards the equator.
  • Regular and predictable due to the strength of the STHP.
• Mid-latitude westerlies:
  • Blow from the STHP towards the poles.
  • Stronger in the southern hemisphere due to fewer land masses.
• Polar easterlies:
  • Blow from the polar high-pressure zone towards the mid-latitudes.

Sea Currents

• Ocean currents affect temperatures based on whether they are cold or warm.
• Warm currents from equatorial regions raise the temperatures of polar areas (aided by prevailing westerly winds).
• The Gulf Stream transports heat northwards and eastwards across the North Atlantic, resulting in mild winters and cool summers in northwest Europe.
• Cold currents, such as the Labrador Current, can reduce summer temperatures if the wind blows from the sea to the land.

Seasonal Variations

• Temperature:
  • Large-scale north-south temperature zones with seasonal variations.
  • In January, highest temperatures over land are in Australia and southern Africa (above 30°C), while lowest temperatures are in Siberia, Greenland, and the Canadian Arctic (less than -40°C).
  • In July, maximum temperatures are over the Sahara, Near East, northern India, and parts of the southern USA and Mexico, with cooler temperatures in the southern hemisphere.
• These patterns reflect the general decrease of insolation from the equator to the poles.
• Little seasonal variation occurs at the equator, but large seasonal differences occur in mid-to-high latitudes due to insolation changes and day length.

Pressure Variations

• Sea-level pressure conditions show differences between hemispheres.
• Greater seasonal contrasts in the northern hemisphere; more stable conditions in the southern hemisphere.
• Differences are related to unequal distribution of land and sea, as ocean areas are more stable in terms of temperature and pressure variations.

Figure 2.3 Seasonal variations in pressure

• Subtropical high-pressure belts (STHP) are a permanent feature, especially over ocean areas.
• In the southern hemisphere, this is almost continuous at about 30° latitude.
• In the northern hemisphere, the belt is more discontinuous due to land.
• Over the oceans, high pressure occurs as discrete cells, such as the Azores and Pacific Highs.
• Over continental areas (e.g., southwest USA, southern Asia, Sahara), major fluctuations occur: high pressure in winter and summer lows due to overheating.
• Pressure is low over the equatorial trough, around 1008-1010 mb, coinciding with the zone of maximum insolation.
• In the northern hemisphere in July, it is located north of the equator (25° over India), while in the southern hemisphere (January), it is just south of the equator.
• Pressure is generally lower in temperate latitudes than in subtropical areas.
• Large numbers of depressions (low pressure) and anticyclones (high pressure) do not show up on a map of mean pressure.
• In the northern hemisphere, there are strong winter low-pressure zones over Icelandic and oceanic areas, but high pressure dominates over Canada and Siberia due to the coldness of the land.
• In summer, high pressure is reduced, especially over continental areas.
• Pressure is relatively high in Polar areas throughout the year, especially over Antarctica, due to the coldness of the land mass.

Wind Belts

• Winds between the tropics converge on a line known as the intertropical convergence zone (ITCZ) or equatorial trough.
• Latitudinal variations in the ITCZ occur due to the movement of the overhead Sun.
• The ITCZ lies further north in June and in the southern hemisphere in December.
• Seasonal variation in the ITCZ is greatest over large land masses (e.g., Asia).
• Movement is far less over the Atlantic and Pacific Oceans.
• The word monsoon means reverse and refers to a seasonal reversal of wind direction.
• The monsoon is induced by Asia, causing winds to blow outwards from high pressure in winter and pulling the southern trades into low pressure in the summer.
• The monsoon is influenced by the reversal of land and sea temperatures between Asia and the Pacific during the summer and winter.
• In winter, surface temperatures in Asia can be as low as -20°C, while surrounding oceans have temperatures of 20°C.
• During the summer, the land heats up quickly and may reach 40°C, while the sea remains cooler at about 27°C.
• This initiates a land-sea breeze blowing from the cooler sea (high pressure) in summer to the warmer land (low pressure), whereas in winter air flows out of the cold land mass (high pressure) to the warm water (low pressure).
• The uneven pattern in Figure 2.4 is the result of seasonal variations in the overhead Sun.
• Summer in the southern hemisphere means that there is cooling in the northern hemisphere, thereby increasing the temperature differences between polar and equatorial air.
• Consequently, the high-level westerlies are stronger in the northern hemisphere in winter.

Figure 2.4 Surface winds

• Illustrates surface winds, including the ITCZ, SPCZ (South Pacific convergence zone), and ZAB (Zaïre air boundary).
• Shows the location of high-pressure centers and the equator.

Explaining Variations in Temperature, Pressure, and Winds

• Latitude:
  • Areas near the equator receive more heat than areas near the poles.
  • Incoming solar radiation (insolation) is concentrated near the equator but dispersed near the poles.
  • Insolation near the poles has to pass through a greater amount of atmosphere, increasing the chance of reflection back into space.

Figure 2.5 Latitudinal contrasts in insolation

• Illustrates how insolation is concentrated at the equator but dispersed over a wider area near the poles.

Distribution of Land and Sea

• There are differences between the specific heat capacities of land and water.
• Land heats and cools more quickly than water.
• It takes five times as much heat to raise the temperature of water by 2°C as it does to raise land temperatures.
• Water heats more slowly because:
  • It is clear, so the Sun's rays penetrate to a greater depth (distributing energy over a larger volume).
  • Tides and currents cause the heat to be further distributed.
• Distance from the sea influences temperature.
• In winter, sea air is much warmer than land air, so onshore winds bring heat to coastal lands.
• During the summer, coastal areas remain much cooler than inland sites.
• Areas with a coastal influence are termed maritime or oceanic, whereas inland areas are called continental.

Ocean Currents

• Surface ocean currents are caused by prevailing winds blowing steadily across the sea.
• The dominant pattern of surface ocean currents (known as gyres) is roughly a circular flow.
• The pattern of these currents is clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
• The main exception is the circumpolar current that flows around Antarctica from west to east.
• There is no equivalent current in the northern hemisphere because of the distribution of land and sea there.
• In the Pacific Ocean, there are two main atmospheric states.
  • Warm surface water in the west with cold surface water in the east (normal circulation).
  • Warm surface water in the east with cold in the west (El Niño events).
• In both cases, the warm surface causes low pressure.
• As air blows from high pressure to low pressure, there is a movement of water from the colder area to the warmer area.
• These winds push warm surface water into the warm region, exposing colder deep water behind them and maintaining the pattern.

2.3 Weather Processes and Phenomena

Atmospheric Moisture Processes

• Atmospheric moisture exists in all three states: vapor, liquid, and solid.
• Energy is used in the change from one phase to another, for example, between a liquid and a gas.
• In evaporation, water changes from a liquid to a gas, and heat is absorbed.
• Evaporation depends on three main factors:
  • Initial humidity of the air: strong evaporation occurs if the air is very dry; very little occurs if it is saturated.
  • Supply of heat: the hotter the air, the more evaporation that takes place.
  • Wind strength: under calm conditions, the air becomes saturated rapidly.
• When condensation occurs, latent heat locked in the water vapor is released, causing a rise in temperature.
• Condensation occurs when either (a) enough water vapor is evaporated into an air mass for it to become saturated or (b) when the temperature drops so that the dew point (the temperature at which air is saturated) is reached.
• The first is relatively rare, the second common.
• Such cooling occurs in three main ways:
  • Radiation cooling of the air.
  • Contact cooling of the air when it rests over a cold surface.
  • Adiabatic (expansive) cooling of air when it rises.
• Condensation requires particles or nuclei onto which the vapor can condense.
• In the lower atmosphere, these are quite common, for example, as sea salt, dust, and pollution particles.
• Some of these particles are hygroscopic – they attract water.
• When water vapor freezes, heat is released.
• In contrast, heat is absorbed in the process of sublimation (when a solid, such as ice, transforms directly to a gas).
• In contrast, deposition occurs when a gas is changed directly to a solid.
• When liquid freezes, heat is released and temperatures drop.
• In contrast, when solids melt, heat is absorbed and temperatures rise.

Humidity

• Absolute humidity refers to the amount of water in the atmosphere. For example, there may be 8 grams of water in a cubic meter of air.
• Relative humidity refers to the water vapor present, expressed as a percentage of the maximum amount air at that temperature can hold.

Precipitation

• The term 'precipitation' refers to all forms of deposition of moisture from the atmosphere in either solid or liquid states.
• It includes rain, hail, snow, and dew.
• Because rain is the most common form of precipitation in many areas, the term is sometimes applied to rainfall alone.
• For any type of precipitation to form, clouds must first be produced.

Causes of precipitation

• Atmospheric stability and instability are closely linked to weather phenomena.
• Stability means that air does not rise. Stability can lead to the formation of fog, mist, and frost.
• Under clear skies, temperatures may drop enough to form frost.
• Where there is moisture present, the cooling of air at night may be sufficient to produce mist and fog.
• Instability, however, produces unstable or rising air, forming clouds and possibly rain.
• Convectional rainfall:
  • When the land becomes very hot it heats the air above it. This air expands and rises. As it rises, cooling and condensation take place. If it continues to rise rain will fall. This is very common in tropical areas. In temperate areas, convectional rain is more common in summer.
• Frontal or cyclonic rainfall:
  • Frontal rain occurs when warm air meets cold air. The warm air, being lighter and less dense, is forced to rise over the cold, denser air. As it rises it cools, condenses and forms rain. It is most common in middle and high latitudes, where warm tropical air and cold polar air converge.
• Orographic uplift of air (relief rainfall):
  • Air may be forced to rise over a barrier such as a mountain. As it rises it cools, condenses and forms rain. There is often a rain shadow effect whereby the leeward slope receives a relatively small amount of rain. Altitude is important, especially on a local scale. In general, there are increases of precipitation up to about 2 km. Above this level rainfall decreases because of the air temperature being so low.
• Radiation cooling:
  • Radiation cooling occurs in low-lying areas during calm weather, especially during spring and autumn. The surface of the ground, cooled rapidly at night by radiation, cools the air immediately above it. This air then flows into hollows by gravity and is cooled to dew point, causing condensation. Ideal conditions include a surface layer of moist air and clear skies to allow maximum radiation cooling to occur quickly. As the Sun rises, it warms the surface, warm air rises and radiation fog clears away.