Part 3 Temperature

Temperature Recall that temperature is the average kinetic energy of the molecules in that substance A change in temperature requires: The net energy budget The specific heat Whether or not phase changes occur

Surface Temperature Thermometers are placed in the shade Thermometer is at a height of 1.5 meters Therefore, surface temperature is the temperature of the air near the ground, not the temperature of the ground surface The average global surface temperature is 59˚ F (15˚ C) The extremes in surface temperature: Highest: 136.4˚ F (58˚ C) – El Azizia, Libya Lowest: -129˚ F (-89˚ C) – Vostok, Antarctica

Surface Energy Budget An energy balance exists if energy gains equal energy losses The Earth-atmosphere system, averaged over a year, is in energy balance As a result, the average global surface temperature does not change Over short periods of time, or in localized regions, there usually is an energy imbalance Energy gains > energy losses: temperature rises Energy losses > energy gains: temperature falls

Global Energy Budget Figure 3.2, Page 59

Surface Temperatures The surface air temperature is determined by energy exchanges with the surface Turbulence is irregular air motions that result as heat and moisture from the surface mix upward Conduction, convection, radiation, sensible and latent heat transfers, and turbulence all act at the same time to transfer heat There is no simple equation to express the relationship between temperature and surface conditions

Temperature Cycles Diurnal means “daily” The typical diurnal temperature cycle is warmest in the afternoon and coolest near dawn The typical seasonal (or annual) temperature cycle is warmest in the summer and colder in the winter

Temperature Variables The diurnal temperature range is the difference between the maximum and minimum temperatures of any given day The daily mean temperature is simply the average between the high and low temperature for that day The monthly mean temperature is calculated by averaging all the daily mean temperatures for each day of the month The annual temperature range is the difference between the warmest and coldest monthly mean temperatures The annual average temperature is the average of the monthly mean temperatures

Temperature Variables Figure 3.3, Page 60

Influences of Temperature Latitude Surface Type Elevation and Aspect Effects of Large Bodies of Water Cloud Cover

Latitude The latitude of a location dictates the angle of insolation during an entire year Insolation = INcoming SOLar radiATION The lower the latitude, the higher the Sun is in the sky year round The intensity of the Sun’s rays and number of daylight hours depend on latitude Because the Sun’s location changes dramatically in the subtropics during the day, greater diurnal temperature variations occur The maximum temperature of a location lags the time of maximum solar input (summer solstice) After the summer solstice, energy gains still exceed energy losses which results in temperatures still increasing

Effect of Latitude Figure 3.4, Page 61

Insolation of New York vs. Miami Figure 3.5, Page 61

Surface Type The surface of the Earth absorbs approximately 50% of the insolation received at the top of the atmosphere The atmosphere is heated by the surface Surface type plays an important role in determining the surface air temperature Dry sand – poor conductor of heat and low specific heat, so the top heats up rapidly Desert locations get very hot and have a large diurnal and annual temperature range Vegetation modifies the diurnal annual temperature range in two ways: Plants consume some solar energy for photosynthesis Transpiration by plants also consumes solar energy Evaporation uses energy that would otherwise heat the surface

Effect of Surface Type Figure 3.6, Page 62

Elevation and Aspect Higher elevations are colder than lower elevations for a few reasons: The air is less dense as you go up (fewer molecules to absorb energy) Terrestrial radiation can more easily escape to outer space with fewer molecules Higher winds aloft mix energy throughout Aspect is the direction that a mountain slope faces North-facing slopes receive less solar radiation than south-facing slopes South-facing slopes are therefore warmer More solar radiation results in increased evaporation, reduced soil moisture, and sparse vegetation on the south-facing slope

Effect of Elevation Figure 3.7, Page 62

Effect of Aspect Figure 3.8, Page 63

Effects of Large Bodies of Water Diurnal and annual temperature ranges are less for locations near large bodies of water Factors that contribute to the difference between continental and maritime locations: The specific heat of water is about 3 times greater than land (heats up and cools down more slowly) Evaporation of water consumes energy Mixing and transparency of water allows the solar radiation to be distributed throughout a large depth Proximity of warm and cold ocean currents can also affect the temperature

Effect of Nearby Bodies of Water Figure 3.10, Page 64

Effect of Ocean Current Temperature Figure 3.11, Page 65

Cloud Cover Clouds reflect and absorb solar energy They reduce the amount of solar radiation reaching the surface, and cause daytime cooling The thicker the cloud, the more pronounced the cooling Clouds also cause nighttime warming due to the emission of longwave radiation to the ground Cloudy regions are warmer than clear regions at night

Effect of Clouds on Energy Budget Figure 3.12, Page 66

Effect of Cloud Cover Figure 3.13, Page 66

Interannual Temperature Variations Normal Temperatures = 30-year average Interannual temperature variations are temperature changes from one year to the next Anomalies = Departures from the normal value The interannual temperature pattern has been a persistent upward trend since the 1990s Volcanic eruptions, oceanic temperature phenomena (El Niño & La Niña) can cause anomalies in this pattern

Global Annual Temperatures over 120 Years Figure 3.14, Page 67

Diurnal Temperature Cycle Averaged over many years, a regular pattern of temperature change can be seen over the course of a day Temperature changes are driven by the difference in insolation vs. outgoing energy losses Sunrise – ground warms the atmosphere Air temperature increases because insolation offsets outgoing emission Noon – insolation values peak After noon – insolation still offsets outgoing emission, so temperature continues to increase About 4 p.m. – energy losses begin to offset insolation, highest temperature of the day Sunset – loss of insolation Energy losses exceed gains all night long, so temperatures fall until sunrise Variations from this pattern arise with changes in latitude, surface type, elevation and aspect, relationship to large bodies of water, and cloud cover

Diurnal Variation in Temperature Figure 3.16, Page 71

Temperature Variations with Height Lapse rate – the change in temperature with increasing altitude The average lapse rate in the troposphere is 6.5˚ C per kilometer Lapse rates are assumed to be negative (cooling with height) Environmental lapse rate – the specific change in temperature with altitude at any particular time and location Environmental lapse rates can change hour-to-hour and day-to-day Environmental lapse rates are measured by weather balloon

Stability The temperature difference between the environment and an air parcel determines the stability of the atmosphere If a parcel is lifted and is warmer than the environment at that level, it will be buoyant and continue to rise on its own If a parcel is lifted and is colder than the environment at that level, it will be negatively buoyant and will sink back down to its original level If the two temperatures are identical, the parcel will remain at the new elevation

Types of Stability Absolutely Unstable: The environment has a lapse rate greater than dry adiabatic In an absolutely unstable environment, a parcel will always be warmer (no matter if it is lifted dry or moist adiabatically) Absolutely unstable environments only exist very near the ground (mirages form because of this) Absolutely Stable: The environment has a lapse rate less than moist adiabatic In an absolutely stable environment, the parcel will always be colder than the environment (no matter if it is lifted dry or moist adiabatically)

Inversions Lapse rates are always assumed to be negative (cooling with height) Inversion: temperature increases with height Inversions are an extreme case of a stable atmosphere Inversions act as a “lid”, suppressing vertical air motions High air pollution incidents are common with inversions The stratosphere and thermosphere are two layers of the atmosphere with inversions Inversions can happen in the troposphere whenever warm air is above cold air Topography (valleys) commonly develop inversions

Nocturnal Inversions At the surface, 3 p.m. usually has the highest temperature By 8 p.m., the Earth’s surface has cooled because energy losses > gains Air in contact with the cool ground loses heat and cools as well An inversion develops until around mid-morning

Typical Lapse Rate: 3 p.m. Figure 3.18A, Page 77

Typical Lapse Rate: 8 p.m. Figure 3.18B, Page 77

Typical Lapse Rate: 5 a.m. Figure 3.18C, Page 77

Typical Lapse Rate: 10 a.m. Figure 3.18D, Page 77

Nocturnal Inversion Factors To develop a nocturnal inversion, the following is helpful: Lack of clouds (allow easy escape of terrestrial radiation) Lack of winds (winds provide mixing to disrupt an inversion) Winter nights (longer period of darkness), but inversions can happen with any season Condition of ground (snow allows surface to cool off rapidly)

Cold Air Draining in a Valley Figure 3.20, Page 79

Wind-Chill Temperature The cooling power of the wind is measured by the wind-chill factor Calm winds: thin layer of air insulates us High winds: insulating layer is blown away making it feel colder to us The wind-chill describes the increased loss of heat by the movement of air The wind chill is relevant to humans and other animals (not cars, buildings, etc.)

Wind-Chill Equivalent Temperature Expressed in degrees Translates the body’s heat losses under the current temperature and wind conditions into air temperature that would produce equivalent heat losses Has been recently updated to be more accurate Cold temperatures plus wind cause danger to exposed flesh

Temperature and Agriculture Cold air outbreaks can damage crops and be costly to farmers Nocturnal inversions can result in crop damage Surface temperatures are measured at 1.5 meters, not next to the ground where small crops are growing Ways to prevent vegetation damage from nocturnal inversions: Covering with plastic sheets (prevents heat escape) Large heaters (supply heat and mixes air in inversion) Mechanical mixing (large fans) Freezing water on the plants Plant damage occurs at -2˚ C, not 0˚ C Latent heat release can prevent the temperature from dropping down to -2˚ C