Atmospheric Relationships

Temperature Variations Weather is driven by uneven heating of the Earth’s surface. Heat energy is responsible for changes in all atmospheric variables. Temperature is greatly affected by intensity and duration of insolation (incoming solar radiation).

Daily and seasonal temperature variations are caused by changes in the balance between the amount of incoming insolation and the amount of outgoing insolation

The atmosphere only absorbs 16% of incoming insolation A higher percentage is by conduction via direct contact with the atmosphere

“Greenhouse” Gases Atmospheric water vapor, carbon dioxide, and methane are able to able to absorb reradiated energy.

Venus Mean temperature of Venus’ surface: 462°C

Factors Affecting Local Temperatures Latitude – higher means less insolation Altitude – average temperature decreases as you ascend Closeness to large bodies of water – water heats up and cools down slower than land Coastal regions have cooler summers and warmer winters than inland locations

Variations in Moisture The amount of moisture in the air changes constantly. Moisture can be found as Liquid (water droplets) Solid (ice and snow crystals) Gas (water vapor)

Saturation of air with water Warmer air can hold more moisture If the temperature of a sample of air decreases, the amount of water vapor it can hold – its capacity – also decreases. When air contains all of the moisture it can hold at a given temperature, we say it is saturated. Most of the time, air is not saturated

Dewpoint Temperature If air is unsaturated, more water must be added. Alternatively, air can be cooled to its dewpoint temperature – the temperature at which condensation occurs.

At the dewpoint, the air is holding all the moisture it can. The amount of water vapor present in the air is the absolute humidity.

There is a direct relationship between absolute humidity and dewpoint temperature The greater the absolute humidity, the higher the dewpoint temperature. When air is cooled below the dewpoint temperature, condensation occurs When moist air rises, it cools When rising air is cooled below its dewpoint temperature, clouds begin to form. As the air temperature approaches the dewpoint, precipitation becomes more likely.

Relative Humidity Relative humidity is a comparison between the amount of moisture in the air and the amount it can hold. The RH is 50% if the air holds half of the moisture it could… As dewpoint temperature and air temperature come closer together, RH increases As RH increases, the probability of rain increases

Daily Temperature Fluctuations and Dewpoint RH is naturally lowest in mid-afternoon When is RH the highest?

Determining Dewpoint Temperature Sling psychrometers are used to find the dewpoint temperature. The “wet bulb” sock loses heat to evaporation, lowering temperature The difference between the wet and dry bulb is used to calculate dewpoint temperature and relative humidity.

Dewpoint Temperature of Room 202 Student Dry bulb temperature Wet Bulb Temperature Duncan Morgana Spencer Hominy Alex Ethan Adam Lindsy

From Your ESRT

And….

Assume a dry-bulb temperature of 10°C and a wet-bulb temperature of 6°C What is the difference between the two bulbs? What is the dewpoint temperature?

Assume a dry-bulb temperature of 10°C and a wet-bulb temperature of 6°C What is the relative humidity?

Use the charts on page 12 of your Earth Science Reference Tables to calculate the answers. The dry-bulb temperature equals 18°C and the wet-bulb temperature equals 14°C. What is the dew point temperature?  What is the dew point temperature, if the difference between the dry-bulb temperature and the wet-bulb temperature is 2°C and the dry-bulb temperature equals -10°C?

The dry-bulb temperature equals 14°C and the wet-bulb temperature equals 11°C. What is the dew point temperature? What is the relative humidity? What is the dew point temperature, if the difference between the dry-bulb temperature and the wet-bulb temperature is 6°C and the wet-bulb temperature equals 18°C? 

What is the relative humidity, if the difference between the dry-bulb temperature and the wet-bulb temperature is 4°C and the wet-bulb temperature equals 10°C? What is the dew point temperature when the difference between the dry-bulb temperature and the wet-bulb temperature is 2°C and the dry-bulb temperature equals 19°C? Calculate relative humidity: wet-bulb temperature and dry-bulb temperature difference = 9°C wet-bulb temperature = 8°C   Calculate dew point temperature: wet-bulb temperature and dry-bulb temperature difference = 7°C wet-bulb temperature = 14°C wet-bulb temperature = 12°C dry-bulb temperature = 15°C

Pressure Variations Air has weight. The force (weight) of the air pushing on a unit of area is the air pressure (atmospheric pressure) The more dense a sample of air, the greater the air pressure Density and air pressure are closely related to temperature changes

Air pressure is… Inversely proportional to temperature changes Directly proportional to density changes As the temperature of air decreases, the air contracts (volume decreases) As the volume contracts, the density increases, pressure increases

Barometers

Atmospheric Pressure Standard pressure at sea level is 29.92 inches of Mercury (Hg) This is the length of the mercury column supported by air pressure Standard pressure in metric units: 1013.2 millibars Standard pressure also referred to as 1 atmosphere (atm)

Pressure Conversions: ESRT A pressure of 1036.0 mb when converted to inches would equal: A pressure reading of 29.50 inches converted to millibars would equal:

A barometric pressure of 1021 A barometric pressure of 1021.0 millibars is equal to how many inches of mercury? 29.88 30.15 30.25 30.50

The diagram represents an aneroid barometer that shows the air pressure, in inches of mercury. When converted to millibars, this air pressure is equal to 1009.0 mb 1012.5 mb 1015.5 mb 1029.9 mb

Pressure( mb) Pressure (inches of Hg) 973.0   29.45 1026.5 29.82 1005.5 29.26

Pressure on Weather Maps Pressure isolines (isobars) join places of equal air pressure The pressure interval is usually 4 millibars

Draw Isobars using 4 mb contour interval

Variables that affect air pressure Air temperature Moisture levels Altitude

Moisture and Air Pressure Moist air is lighter than dry air! H2O is 36% lighter than N2, and 44% lighter than O2 The more moisture, the lower the pressure The water molecules replace oxygen and nitrogen, lowering the pressure Barometers are the #1 tool for predicting weather changes

Altitude and Air Pressure As altitude increases, the density and pressure of the air decrease ½ of all air molecules are within the first 5.8 km (3.5 miles) of the Earth’s surface Fewer molecules at higher altitude means less pressure

Lab 12 Dew Point and Cloud Formation

A new graph to use! Shows the relationship between air temperature, dew point, and altitude

Practice Problem #1 At what altitude do clouds form if the surface temperature is 10°C and the dew point is - 10°C?

Practice Problem #2 Clouds covered the top half of Tremper Mountain on a morning when the air temperature was 20°C and the dew point temperature was 15°C. What was the altitude of the cloud base?

Outside we go!

Questions Why does the height of the cumulus cloud base change from day to day? What would happen to the height of the cloud base if the dew point temperature were lower? How would it be possible to have a day without any clouds? What relationship would you expect to find between the air temperature and dew point temperature at ground level if the area is covered by fog?

What happens to the air temperature of a descending mass of air? What happens to the dew point temperature of a descending mass of air? Explain why a descending mass of air would tend to become drier. Conclusion: describe, step by step, how you can determine the base altitude at which clouds form?

Moisture and Energy Input The amount of moisture entering the atmosphere depends on evaporation and transpiration.

Evapotranspiration Evapotranspiration is all of the water released into the atmosphere The oceans are the main source of moisture in the atmosphere Evapotranspiration requires large amounts of energy to change liquid water into water vapor.

Factors Affecting the Rate of Evaporation Amount of energy available – more heat, more evaporation Surface area – more surface area of water, more evaporation Amount of moisture in air – when air is saturated, there is no net evaporation (evaporation = condensation) Decreasing the amount of moisture in air increases the rate of evaporation

Warm air can hold more water vapor When the air temperature is a constant, and the amount of water vapor in the air increases, Humidity (absolute and relative) increase Dew point increases When the air temperature cools below dew point, water condenses by condensation

Other Energy Inputs Most atmospheric energy comes from conduction and radiation from Earth’s surface. The rate of energy input depends on the amounts of H2O and CO2 (main greenhouse gases) in the atmosphere. Frictional drag, caused by the Earth’s rotation, also adds heat to the atmosphere

Air Movement Large horizontal movements of air near the Earth’s surface – winds Smaller local horizontal movements – breezes Vertical air movements – currents Winds are named for the direction from which they come.

Pressure Gradients Primary causes of winds are differences in air temperature Air temperature differences produce differences in pressure Air always moves from areas of high pressure to areas of low pressure The rate of change in pressure between two locations is the pressure gradient When the isobars are closer, the pressure gradient is greater, wind speed is greater

Hurricane Sandy

Sandy Storm Track Data Date Lat Lon Wind (mph) Pressure Storm Type Oct/22/2012 13.5N 78.0W 30 1003 Tropical Depression Oct/23/2012 13.8N 77.8W 50 993 Tropical Storm Oct/24/2012 17.1N 76.7W 80 973 Category 1 Hurricane Oct/25/2012 22.4N 75.5W 105 964 Category 2 Hurricane Oct/26/2012 26.7N 76.9W 970 Oct/27/2012 29.0N 76.0W 75 958 Oct/28/2012 32.5N 72.6W 951 Oct/29/2012 37.5N 71.5W 90 943 38.8N 74.4W 940 Oct/30/2012 39.8N 75.4W 952 Post-Tropical Cyclone

Local Breezes Winds are caused by differences in pressure. Along shorelines, land heats up faster than water. The air over water is cooler, denser, higher pressure The air over land is hotter, less dense, lower pressure Beaches always have sea breezes during the day Air over land cools more quickly at night than the air over the water – land breezes result Local breezes are a form of convection.

Lab 13 Absorption & Radiation by Land and Water

Planetary Convection Cells Convection cells are cyclical movements of fluids due to differences in density and the effects of gravity. Gravity pulls cool, dense air towards Earth’s surface, forcing warm, less dense air to rise

Variations in insolation result in unequal heating of Earth’s surface and the atmosphere above it. Unequal heating leads to density differences in the atmosphere, producing convection cells. In portions of these cells where air movement is vertical, pressure belts are produced.

Zones of Divergence In high pressure regions, air is descending. When this air reaches the surface, it spreads out (diverges) High pressure belts are known as zones of divergence Zones of divergence are dry, often arid deserts.

Zones of Convergence Air from divergent zones meet in regions of low pressure, where they rise. Low-pressure belts are called zones of convergence. Convergent zones are moist.

Winds are deflected towards the right in the Northern Hemisphere Winds are deflected towards the left in the Southern Hemisphere

Planetary Winds Air moves from regions of high pressure to regions of low pressure. If the earth did not rotate (and if its surface were evenly heated) winds would be part of one large cell.

Consequences of Earth’s Rotation and Uneven Heating of its Surface Winds flow from regions of high pressure to low pressure Wind direction is modified by the Coriolis Effect.

The result is the planetary wind system. The pressure belts and wind belts at the Earth’s surface follow the Sun’s vertical rays The belts shift north and south as the sun follows its migratory path between the Tropic of Cancer and the Tropic of Capricorn.

Planetary winds are a major influence on weather and climate. We live in a part of the Earth where the prevailing winds are from the southwest.

The Jet Stream Winds at high altitudes have a major influence on air masses closer to the Earth’s surface. Jet streams travel 7-8 miles above the surface, and have an eastern, wavelike motion.

Atmospheric Transparency On clear days, the atmosphere is transparent, and scattering/reflecting is minimal. Aerisols (dust and water vapor) scatter and reflect more radiation. Natural processes produce aerisols: volcanic eruptions Pollution from cars, factories, and homes produce aerisols

Aerisols

Condensation and Sublimation Condensation – when water vapor changes into liquid water For condensation to occur, the air must be saturated with vapor and contain dust particles (condensation nuclei) or ocean salt. At temperatures below 0°C, water vapor sublimates directly into water-ice or snow.

Condensation is a heating process For every gram of water that condenses, 2260 J of heat energy is released. This energy is the fuel for many storms. Hurricanes are fueled by the latent heat released by condensation

Dew and Frost When water vapor condenses directly onto a cold surface, dew is formed. If water vapor comes in contact with a freezing surface, the vapor will sublimate, forming frost

Clouds Clouds are collections of tiny water droplets or ice crystals suspended in the atmosphere. Clouds form when moist air expands and cools as it rises vertically. When air cools to the dewpoint temperature, it becomes saturated and condensation occurs.

Reaching the Dewpoint When moist air rises and cools to the dewpoint, condensation takes place and clouds form. Rising air results from heating, convection currents, air moving over a mountain, or a frontal boundary.

When air near Earth comes into contact with surfaces that have been heated, the air is heated by conduction, and forced upwards. Fluids (liquids and gases) transfer heat by convection by differences in density. Convection differences include orthographic lifting and frontal wedging

Types of Clouds Clouds are divided into four groups: High, middle, low, and vertical

Moisture and Energy Transfer Moisture and energy are transferred in the atmosphere by three methods: Convection – driven by density differences Winds Adiabatic process

Adiabatic Changes in Temperature An adiabatic temperature change is a change in the temperature of a system without heat being added or removed from the system. When gases expand, temperature decreases When gases compress, temperature increases When air descends, it is compressed by surrounding air and its temperature increases When air rises, it expands and its temperature decreases

Dry Adiabatic Lapse Rate When a dry parcel of air rises or descends, its temperature changes adiabatically at the rate of 10°C/km

Moist Adiabatic Lapse Rate When moist air rises and cools to the dew point, condensation takes place and clouds form. With condensation, the temperature drops at a lower rate, 5-6°C/km, because of the latent heat released.