Mr. Prizzi's
Class Notes

The Atmosphere

    How is the atmosphere structured?

The atmosphere consists of four distinct layers, which are seperated by a boundaries that end in the suffix "pause". There is a lot of information in the Earth Science Reference Tables page 14.
  • Troposphere-This layer of the atmosphere is closest to the earth and 99% of all weather takes place here because most of the water vapor is located here.
  • Stratosphere-This layer is above the troposphere and contains ozone, which is an important gas, because it absorbs cancer-causing ultra violet rays from the Sun.
  • Mesosphere-This layer is above the stratosphere.
  • Thermosphere-This is the uppermost layer of the atmosphere. Earth Science Reference Table page 14:

      How do we describe the present condition of the atmosphere?

  • The present condition of the atmosphere is called WEATHER and it is described using observations of temperature, air pressure, humidity, wind, cloudiness and precipitation.

      What is temperature?

    1. Temperature is a measure of the average kinetic energy of the particles of a substance. In other words, the more molecules vibrate within a substance, the higher the temperature.
    2. Measurement - There are three scales that measure temperature: CELSIUS, FAHRENHEIT AND KELVIN scales. There is a conversion scale on page 13 of your ESRT.
    3. Maps - Meteorologists find it useful to map temperatures and they use lines to connect equal temperatures called Isotherms.

      What is air pressure and how is it used to predict the weather?

    1. Air Pressure - Air pressure is also called barometric or atmospheric pressure. By definition, it is the weight of the atmosphere pushing down on the earth.
    2. Barometer - A barometer is the instrument that measures air pressure and it uses units of millibars (mb) or inches of mercury (inches).

        Factors that affect air pressure:

      1. Temperature - As the temperature of the air increases, the air pressure decreases, which is an indirect relationship.
      2. Humidity - As the humidity of the air increases, the air pressure decreases, which is an indirect relationship.

        You might ask "WHY?": Humid air is lighter than dry air. When water vapor enters dry air, it does not squeeze in between the molecules of air. Instead, the water vapor pushes out many of the air's heaviest molecules like Nitrogen and Oxygen. In other words, lighter molecules of water vapor replace heavier molecules and the result is the air weighs less and therefore has less air pressure.

    3. High air pressure - means that the atmosphere is heavy at that location and from our notes above, you can conclude that the air is probably cool and dry (if you are not sure, see notes above). So, if the barometer indicates high pressure or rising air pressure, you can conclude that the weather will be cool and dry.
    4. Low air pressure - means that the atmosphere is not heavy at that location and from our notes above, you can conclude that the air is probably warm and humid (if you are not sure, see notes above). So, if the barometer indicates low pressure or falling air pressure, you can conclude that the weather will be warmer and humid.
    5. Maps - Meteorologists find it useful to map air pressure and they use lines to connect equal equal air pressures called Isobars

    This is a map that show isobars (air pressure lines)

    The white lines represent equal air pressures. Notice that the moisture (green)is located near the "L" which means low pressure. This makes sense with what we learned above.

      What is humidity and why is the most important gas in the atmosphere?

    Humidity is water in the gas phase. Although water exists in the atmosphere in all three phases, humidity (also called water vapor) greatly affects our weather. Air that is said to be "saturated" is holding all the humidity it can, which means that the relative humidity equals 100%.
  • Dew Point Temperature-This is the temperature at which air becomes saturated and if the temperature falls below this temp., CONDENSATION occurs.
  • Relative Humidity-This show how much water vapor air is holding at a certain temperature compared to how much it could hold. It is expressed as a %. Calculating Dew Pt. Temp. and Relative Humidity:
    • 1st ) Subtract the wet-bulb temperature from the dry-bulb temperature. This is called the wet-bulb depression.
    • 2nd) Locate the wet-bulb depression along the top of the chart and locate the dry-bulb temperature along the side of the chart.
    • 3rd) Where these two numbers intersect on the chart is the value of either the dew point temperature or relative humidity. The two charts work exactly the same way.
    Calculate dew point temperature and relative humidity with the following values: dry-bulb temperature = 8°C and wet-bulb temperature = 3°C

    Click here for the answer.

      What is wind and how does air pressure cause wind?

  • Wind – Wind is defined as the horizontal movement of air and they are named for the direction from which they come. An anemometer is the instrument that measures wind speed and a wind vane measures wind direction.
    For example, the name of winds that blow from Albany to NYC are north winds because the blow from the north.

  • Air pressure differences cause air to move from one location to another (wind). Winds always blow from areas of high pressure to areas of low pressure. Differences in air pressure are caused by uneven heating of the Earth’s surface.
    For example, imagine a bicycle tube. When you pop your tire, the air comes out because there is higher air pressure within the tube compared to outside. So, the air is flowing from a high pressure area (inside tube) to a low pressure area (outside of tube).

  • Wind velocity (speed) is greatest when there is a greater pressure gradient. This means winds are faster when a large difference in air pressure exists over a short distance.
    For example, the air flows out of the bicycle tire faster when the tire is inflated completely (much higher air pressure within the tube compared to outside the tube). Compare this to when the tube is only slightly inflated and the air flows out of the tube slowly (smaller difference in air pressure between the tube and outside).

  • There is an easy way to find an area on a map that has the greatest pressure gradient and therefore is likely to have the greatest wind speeds. Where the isobars are closest together is where the pressure gradient is greatest and where the wind speeds will be greatest.

    On the map below, compare the spacing of the isobars at New York and Texas. New York is likely to have high winds compared to Texas' more calm winds.

      How does Earth's rotation affect wind?

  • The rotation (spinning on it's axis) of the Earth causes free flowing objects near the Earth’s surface to curve to the right of their path in the Northern Hemisphere. This greatly effects the direction of winds & oceans currents. This is called the Coriolis Effect.

      How can differences in the Earth's surface cause small-scale winds called breezes?

    • Earth's surface is made of different materials, which heat up at different speeds (and cool down at different speeds). The specific heat of a material is a measure of how fast or slow it heats up (and cools down). Water has a specific heat equal to one (1), which is the highest of any natural substance. This means water takes the longest to heat up and the longest to cool down. This characteristic makes water a useful substance for heating our homes.

    • Local Breezes are small-scale movements of air and there are basically two types:
      1. Sea Breezes – During the day along coastal areas, the land will heat up faster than the water. The air over the land will become warmer causing it to be less dense, which results in a Low-pressure area. At the same time, the air over the body of water remains cool and more dense, which results in an area of HIGH pressure. The difference in air pressure creates a small-scale wind called a sea breeze that blows from high to low pressure. A sea breeze is also known as an on-shore breeze.
      2. Land Breezes – At night, the land cools down faster than the water (due to land’s lower specific heat). This causes the air above the land to become cooler than the air above the water. This causes the air to be more dense, which creates an area of high pressure over the land. The air above the water is warmer and less dense, which creates an area of low pressure above the water. This difference in air pressure creates a breeze from the land to the sea.

      How are ocean currents caused?

  • Wind is the major factor in moving oceans. As the wind blows over the ocean, it transfers energy to the water and sort of "drags" the water in the same direction. Both prevailing (constant) winds and ocean currents are affected by the Coriolis effect. See ESRT page 4 for more information on ocean currents.
    • Convection cells – Convection is the movement of a gas or liquid caused by differences in density. Large-scale convection cells occur in the atmosphere, which cause the prevailing winds.
    • Zones of diverging and converging air result from the convection cells in the atmosphere, which cause dry or wet areas.

      How do clouds form?

  • Condensation is the process by which a gas turns into a liquid. For condensation to occur, two things must be present or happen:
    1. The temperature of the air mass must reach it’s dew point temperature, AND
    2. A surface must be present for the water vapor to condense upon. This surface can be dust, pollution, or volcanic ash in the atmosphere. On the surface of Earth it can be a car, lawn or a flower. All of these are called condensation nuclei.
  • Example:
  • Cold glass of soda: As air near the glass cools and the air temperature approaches the dew point temperature. When the air temperature equals or drops below the dew point temperature, the water vapor in the air turns to liquid (the process of condensation) on the glass (glass acts as condensation nuclei).

  • Clean air is free of any condensation nuclei, so condensation cannot take place.

  • When condensation occurs, energy is released into the atmosphere. Exactly 540 calories per gram of H2O is released into the atmosphere and is the “fuel” for many severe weather events, like Hurricanes (See ESRT front page). Energy is released during condensation because gas is the highest energy phase and as the molecules become liquid, the "excess" is released into the atmosphere.
  • Cloud Formation - A cloud is a mass of air that has suspended droplets of water or ice in it. In order for a cloud to form, a few events must occur.
    1. Air must rise.
    2. The air will expand due to less air pressure aloft, which causes the air temperature to decrease.
    3. Condensation will occur if the temperature decreases to the dew point temperature and there is condensation nuclei available.
    4. Finally, a cloud is born!
  • Precipitation – Any falling liquid or solid water from clouds is considered precipitation. Once a cloud forms, precipitation can take place.
    • Coalescing – This is the process by which tiny water droplets in clouds “knock” into one another and grow into larger water droplets. The water droplets will fall to ground when they become too large to be suspended.

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