Weather Dynamics Ch. 13, 14, 15

Basics of Heat Transfer Conduction – The mechanism of heat transfer in which highly energetic atoms or molecules collide with less energetic atoms or molecules, giving them some energy. Convection – The mechanism of heat transfer in which highly energetic molecules move from one place to another. Radiation – The mechanism of heat transfer in which atoms or molecules emit electromagnetic waves.

Solar Energy Space contains very little matter, so the energy that arrives at Earth’s outer atmosphere is about the same as when it left the sun. The amount of energy that reaches us is called the solar constant, 1367 J/m2s. The solar constant is the amount of radiant energy that hits 1 m2 of Earth’s outer atmosphere per second.

This constant applies only to rays that hit perpendicular to the surface. Some of this energy is reflected back into space or is absorbed. Draw a diagram on the board about how incidence angle of the radiation effects amount of radiation reflected. Different surfaces reflect different amounts of radiation. Heat is not evenly distributed on Earth's surface Tropics receive large amounts of heat. High latitudes receive small amounts of heat. Ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the sun's energy. Deserts and clouds, however, have high albedos; they reflect a large portion of the sun's energy. Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space. A cloud usually has a higher albedo than the surface beneath it, the cloud reflects more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere. We will go into more detail about what this means to weather and the climate of the earth in future classes.

Refer to diagram on Page 424.

Albedo Part of the solar energy that comes to Earth is reflected back out to space in the same, short wavelengths in which it came to Earth. The fraction of solar energy that is reflected back to space is called the albedo. Albedo is a measure of the reflectivity of a surface. 30% of incoming radiation is reflected back to space

Questions ???? So how does the Earth maintain a relatively constant temperature since some energy is absorbed, some is reflected? Why does the Earth not lose all its heat to the cold depths of space? Why do the oceans not boil due to the constant absorption of energy? The answer lies in our atmosphere that creates a “Greenhouse Effect”

Estimating the Magnitude of the Natural Greenhouse Effect   Estimating the Magnitude of the Natural Greenhouse Effect   If you consider the energy reaching the earth from the sun it only provides enough energy to heat the earth to a temperature of -19C. However, average global surface temperature is + 14C  Natural greenhouse effect warms the surface by 33C

The Greenhouse Effect As solar radiation reaches the Earth it is absorbed and heats up the surface. This heat is then radiated into the atmosphere as infrared radiation. There are gases in our atmosphere such as water vapour, CO2, methane and others that are very good at absorbing infrared radiation (heat). These gases then reradiate that heat back towards the surface thus further heating up the Earth.

The intensity of sunlight reaching the earth’s outer atmosphere has been very steady, ranging only slightly from its mean value of about 1370 watts of short-wave energy per square meter as it goes through its sunspot cycles. Although this value is called the solar constant, there is evidence that it can vary by larger amounts on century and millennial time scales. When averaged around the entire planet, this incoming energy is about 342 watts of energy for each square meter of the earth’s surface. About 31% of this radiation is reflected back to space by clouds, atmospheric dust and other aerosols in the atmosphere, and the earth’s surface. Changes in the amount of dust in the atmosphere, the extent and type of clouds and the reflective properties of the earth surface can cause the amount of reflected energy to vary considerably by season, from year to year and over longer time scales. The remaining 69% of the sun’s energy absorbed by the atmosphere and the earth’s surface heats the climate system and drives the dynamic behaviour of the atmosphere and the world’s oceans. To maintain a stable climate, just as much energy must be returned to space by some means as is absorbed by the climate system. The climate system accomplishes this through the emission of long wave (infrared) heat energy from the earth’s surface and atmosphere towards space. However, molecules of trace gases within the atmosphere, commonly known as greenhouse gases, as well as clouds and aerosols absorb most of this outgoing radiation, then re-radiate the absorbed energy in all directions, some back towards the surface. Much of the energy re-radiated upwards is also absorbed again by other greenhouse gas molecules and aerosols at higher elevations, and this process is repeated. While, for a balanced climate, the amount of energy that finally escapes into space is still, on average, equal to the absorbed solar energy, enough of the outgoing infrared is recycled to increase surface temperatures significantly.

Global Warming Global warming is the increase in the average measured temperature of the Earth's near-surface air and oceans since the mid-20th century, and its projected continuation. What evidence do we have that climate change occurs naturally? The Ice Ages (Last one was 15 000 years ago) El Nino Volcanic Eruptions Climate change occurs naturally but the concern now is that humans are accelerating the rate of global warming.

Global Warming Most experts agree that human activities are enhancing the natural greenhouse effect of the atmosphere and accelerating global warming. Climate change is more than a warming trend. Increasing temperatures will lead to changes in many aspects of weather, such as wind patterns, the amount and type of precipitation, and the types and frequency of severe weather events.

How will climate change affect us? Not all regions of the world will be affected equally by climate change. Low-lying and coastal areas face the risks associated with rising sea levels. Scientists have also determined that warming will be greater in polar regions than nearer to the equator, and that continental interiors will experience greater warming than coastal areas. Increasing temperatures will cause oceans to expand (water expands as it warms), and will melt glaciers and ice cover over land – ultimately increasing the volume of water in the world's oceans. Scientists estimate that sea levels could rise by an average of 5 cm per decade over the next 100 years. Some estimates suggest that sea levels could rise by almost a full meter by the year 2100. This has serious implications for sensitive polar ecosystems, their wild species and the human inhabitants. Interior regions may face more frequent and intense heat waves.

How will climate change affect us? Climate change is a global problem, affecting all countries. While greenhouse gases (GHGs) form naturally, many human activities add additional GHGs to the atmosphere. Heating and cooling buildings, using energy at home and work, driving vehicles to move people and goods, powering industrial processes – most things we do that consume energy contribute to the problem. In Canada, climate change will affect fishing, farming, forestry, lakes, rivers, coastal communities and the North

Projected Global Surface Temperature Change

Evidence that climate is changing……. Observations over recent decades also show… Evaporation & rainfall are increasing; More of the rainfall is occurring in downpours; Permafrost is melting; Corals are bleaching; Glaciers are retreating; Sea ice is shrinking; 2007 & 2008 have been the years with the most sea ice melt in history. Sea level is rising; Wildfires are increasing; Storm & flood damages are soaring.

Assignment Read pg. 422- 426 Do Questions 1-4 pg. 426.

Energy and Water Only about 30% of the Earth’s surface is land and most of the land is covered by cloud a large portion of the time. This means that most of the solar energy that reaches the Earth strikes water. As a result the interactions between solar energy and water have major influences on the Earth’s weather. Water absorbs 93% of the energy that reaches it, and yet the average temperature of most bodies of water on our planet does not vary greatly.

Energy and Water The temperatures of oceans and large lake remain relatively constant for many reasons. Some of the reasons relate to water’s unique properties such as it large specific heat capacity. This is defined as the amount of heat that is required to raise one gram of a substance 1oC.

Specific Heat Capacity Chemists have measured the specific heat capacity (c) of many substances. Using these measured values you can calculate the amount of heat (Q) required to raise the temperature of an amount of a substance (m) by a given temperature (∆T). Q = mc ∆T Q = heat (J); m = mass in grams; c = specific heat capacity (J/ g oC); ∆T = change in temperature (oC) Specific heat capacities of many substances are in Table 13.1 on pg. 427.

Example #1 Calculate the amount of heat needed to increase the temperature of 250g of water from 20oC to 56oC. Q = mc ∆T Q = ? m = 250g c = 4.18 J/g oC (from table 13.1) ∆T = 56 oC - 20oC = 36oC Q = 250 g x 4.18 J/g oC x 36oC Q = 37 620 J = 38 kJ

Example #2 2. Calculate the specific heat capacity of copper given that 204.75 J of energy raises the temperature of 15g of copper from 25oC to 60oC. Q = mc ∆T c = ? Q = 204.75 J m = 15g ∆T = 60oC - 25oC = 35 oC c = Q/m ∆T c = 204.75 J/15g x 35 oC c = 204.75 J/ 525 g oC c = 0.39 J/ g oC

The higher the specific heat capacity of a substance, the more heat it can absorb and the more time it will take for it to release that heat. Heat of Vaporization The second unique property of water that helps maintain constant temperatures is its heat of vaporization. This is the amount of energy required to convert 1.0 grams of a substance from a liquid state to a gaseous state.

Heat of Vaporization If the substance returns to a liquid state from the gaseous state, the same amount of energy that was used to convert it originally will be released. Scientists call this the latent heat of vaporization because the thermal energy that is used to evaporate the liquid does not become thermal energy again until the gas condenses back into a liquid.

Heat of Vaporization Not all of the energy that strikes water is used to heat it up. Some of it is used to vaporize the water. You can calculate the amount of heat that is required to evaporate a given mass (m) of liquid by using the values in Table 13.2 (pg. 428) and the following formula. Q = m ∆Hovap Q = heat m = mass in grams ∆Hovap = heat of vaporization of 1g of a substance. Table 13.2.

Example Sam has a large pothole in his driveway where rainwater collects. If the hole contains 2.7 kg of water, how much energy is required to evaporate all the water? Q = ? m = 2.7 kg ∆Hovap = 2260 J/g Q = m ∆Hovap Q = 2700 g x 2260 J/g Q = 6 102 000 J or 6.1 x 106 J

Heat of Fusion A third unique property of water is its large heat of fusion. This is the amount of heat that is required to melt 1g of a solid into a liquid. In reverse, it is also the amount of energy that will be released when a liquid freezes. The formula is the same as for the heat of vaporization except that heat of fusion is used instead of heat of vaporization. Q = m ∆Hofus

Example A huge ice sculpture is delivered to a spring carnival. The sun delivered 3.0 x 108 J of energy before it completely melted away. How much did the ice sculpture weigh when it was delivered? Q = m ∆Hofus m = ? Q = 3.0 x 108 J ∆Hofus = 333 J/g m = Q/ ∆Hofus m = 3.0 x 108 J / 333 J/g m = 900 000 g or 900 kg

Summary Remember these points when deciding which of the 3 formulas to use: If there is a change in temperature of a substance use Q = mc ∆T. If a substance is changing state from a liquid to a gas or from a gas to a liquid use Q = m ∆Hovap If a substance is changing state from a liquid to a solid or from a solid to a liquid use Q = m ∆Hofus

Water in the Air Why doesn’t all of the water in the world evaporate into vapor? There is a finite amount of water that the air can hold. Air at different temperatures can hold different amounts of water. Warm air can hold more vapor than cold air. When there is as much water vapor in the air as possible at a given temperature, we say the air is saturated.

Water in the Air Humidity is a measure of the amount of water vapour in the air. Absolute humidity is the actual amount of water vapour in the air, expressed in units such as grams of water vapour per kilogram of air. Relative humidity is the percentage of water vapour in the air compared to the amount of water vapour that air would contain if it were saturated.

An example of Relative Humidity Why is the air so dry in homes in the winter? The temperature outside is say -12oC and the relative humidity is 75%. That air is pulled into your house and heated to 20oC. What is the RH now?? (using table 13.4) Remember that cold air can hold less moisture that warm air. So at 75% RH at -12oC the air would contain 0.75 x 1.53g water/kg air = 1.15 g. Air at 20oC can hold 15.0g water/kg air. So the RH would now be 1.15 g/15.0 g = 7.7 %

Water in the Air Dew Point: The dew point is the temperature to which a given parcel of air must be cooled for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point. It is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease.

(Pg. 433)

Assignment Read pg. 427- 433 Do Questions 1-8 pg. 434.

Atmospheric Pressure Every layer of air exerts pressure on the air below because every molecule in the air is pulled toward the Earth by gravity. Consequently, the lower layers are compressed by all the layers above. This is called atmospheric pressure. As you increase in altitude, the pressure decreases due to less atmosphere above you.

Interactions of Solar Energy with Land and Air As we learned earlier, different substances have different heat capacities. This determines how much heat a substance can hold, and how fast it will dissipate that heat. For example water has a higher heat capacity than asphalt. Thus water will take longer to warm up, but it will also cool off slower than asphalt. Convection produces many effects on air. As air heats up due to conduction and radiation from the surface, parcels of air react in a predictable way.

As air heats up at the surface it does several things: As air heats up at the surface it becomes less dense than the air around it, thus it will rise to air that is the same density. As the air parcel rises it begins to expand. This is due to the lower atmospheric pressure on the parcel as it gets higher in the atmosphere. This expansion also causes the air to cool at a rate of 10oC/1000m. These processes will be important when we discuss clouds.

The atmosphere is divided into 5 layers Troposphere: This is the layer from the surface up to 10 km above. This is the layer in which all weather occurs. Temperature drops as you increase in altitude in this layer.

Stratosphere: This layer contains most of the ozone which filters out much of the harmful ultraviolet (UV) radiation from the sun. Temperature increases as you rise in altitude.

Mesosphere: Temperature again drops as you increase in altitude in this layer. This is the layer that meteors burn up in. Thermosphere: This layer begins 100 km above the surface. It is extremely hot in this layer due to oxygen absorbing high energy UV light. Ionosphere: A layer of charged particles within the thermosphere and mesosphere. This layer is a collection of electrons that have been ejected from high energy molecules. This allows radio waves to be reflected off this layer and received large distances form their source.

Assignment Read pg. 435- 446 Do Questions 1-6 pg. 448.