Environmental Sciences- Notes-01

Environmental Sciences- Notes-01

Atmosphere is the gaseous envelope that surrounds earth. It approximately exten Is up to 500 km above. Our atmosphere is composed of different gases, the two most abundant of which are N2 and O2. N2 accounts for 79% of the atmosphere and O2 for 20%. Oxygen can also exist in a triatomic form (O3) called ozone, which has the significant property of absorbing ultraviolet solar radiation. 

Water vapor, whose abundance can vary from almost nothing in the driest of locales to about 4% of the atmosphere, is responsible for the formation of clouds (when water vapor condenses into liquid droplets or freezes into ice crystals) and precipitation as well. Additionally, water vapor is a greenhouse gas, and plays a key role in the heat budget of the Earth. Another significant greenhouse gas is carbon dioxide (CO2) that emitted as a byproduct of many industrial processes, and its increasing abundance in the atmosphere is the source of much concern regarding future climate change via global warming.

3. Atmospheric properties

Temperature: Temperature is the measure of thermal or internal energy of the molecules within an object or gas. We can measure temperature of an object using either direct contact or remote sensing. Temperature of air is closely related to other atmospheric properties, such as pressure, volume and density.

Density: Density measures the heaviness of an object or how closely packed the substance is. Density is related to both the type of material that an object is made of and how closely packed the material is,

Pressure: Pressure is the force exerted over a given area or object, either because of gravity pulling on it or other motion the object has. Molecules in the air produce pressure through both their weight and movement, and this pressure is connected to other properties of the atmosphere.

Humidity: 

Humidity is a measure of the amount of moisture in the air. It tells you how comfortable it is to be outside, and if there is enough moisture to create clouds and rain.

4. Variation of temperature

In the atmosphere, temperature is related to volume, pressure, and density. Temperature is inversely related to density but directly related to pressure and volume. This means, for example, when temperature increases, density decreases, and volume and pressure of the gas also increase. So, air that is warm and dry will tend to rise when surrounded by cooler air because warm air is less dense than the cooler air around it.

Temperature controls planting dates and the growth of plants as well as insect pests and crop diseases. As an integral part of weather, temperature also determines the type of precipitation (rain/snow/sleet) that might occur if you are in location that is experiencing near freezing conditions.

Temperature is a measure of how much internal energy an object or gas has. For example, 

Heat energy transfer is the cause of temperature change and like any other fluid system, in atmosphere also the main mode of energy transfer is a process called convection. It works because in a fluid, "chunks" of matter (or parcels) can move up or down with respect to the rest of the fluid as they are being heated or cooled, respectively. The processes of convection are, however, governed by the laws of thermodynamics. Understanding these laws helps us quantify these processes, make predictions on the formation of clouds and fog, and explain how the vertical profile of temperature in the atmosphere is determined.

4.1. Thermodynamic properties of dry air - adiabatic temperature change

4. The equation of state - ideal gas law

If air contains no water it is called dry air. The state of a parcel of dry air is described by three properties: temperature (T, expressed in °K, where 273 deg * K = 0 deg * C ) pressure (p, force per unit area, expressed in Newtons m²) and density (p, the mass of a unit volume, in Kg m³). In a gas these properties are related by a relatively simple physical law called the ideal gas law (ideal because it is not exact, albeit quite accurate for most applications in meteorology). This law states that: p=pR T

R is a coefficient, called the gas constant. It does not depend on either p. p. or T. The gas constant depends only on the composition of gases that make up the air (every gas has its own gas constant). Since this composition (for dry air) is roughly constant throughout most of the atmosphere R of air is constant and equal to 287 Joules kg¹ °K-¹).

To understand the equation of state, it is assumed that we have a fixed mass of air enclosed in a container with rigid walls (hence with fixed volume). If we warmed the container, say by putting it over a flame, the temperature of the air (ie. kinetic energy of the air molecules) will rise and the pressure (i.e., the force exerted by these molecules on the container walls) will increase. The density of the air will not change since we are not increasing the amount of gas in the container northe volume of the container. The ideal gas equation states that the increase in pressure is directly proportional to the increase in temperature.

Now if we replace the rigid wall of the container with flexible ones, that are allowed to stretch freely if the pressure inside rises above that on the outside. In that case, when we raise the temperature, the pressure inside will remain constant (and equal to the outside pressure), but the container's volume will increase. This means that the density will decrease (because the mass inside does not change). The ideal gas law states that the density decrease will be inversely proportional to the increase in temperature.

Let us remove the flame that heated our flexible walled container, and put it in a chamber where the pressure can be controlled from the outside, lowered or raised at will. What will happen to the density of our air parcel when we lower the pressure surrounding our container? What will happen to its temperature?

Here too the pressure on both sides of the flexible container walls will equalize as the outside pressure drops, the container will expand and the pressure inside will drop by the same amount. The density of the air parcel in the container will decrease as well, in agreement with the ideal gas law. But what the ideal gas law cannot tell us is what will happen to the temperature. To find that out we need to consider the first law of thermodynamics a physical law that extends the principle of conservation of energy to include the concepts of heat and work.

In thermodynamics the simplest form of energy conservation is the balance between internal energy (the kinetic energy of the body's internal molecular motion directly proportional to its temperature), and the amount of heat added to the body minus the work done by the body on its surroundings.

As our air parcel expands in response to the lowering of the outside pressure, the force of its internal pressure is moving the walls of the container outwards. When a force is moving an object

over a given distance it does work. Thus the expanding air parcel does work on its surroundings. This work must come at the expense of internal energy (remember, heat is neither added nor taken away from the parcel in this experiment). Thus the molecular motion within the parcel will slow down, and the parcel's temperature will drop.

The expanding parcel will experience not only lowering of its pressure and density, but also of its temperature. All three state variables: pressure, density, and temperature will remain in balance as described by the ideal gas law. The process described above is called adiabatic expansion, implying the change in parcel density without the exchange of heat with its surroundings, and its consequential cooling. The opposite will occur when the parcel is compressed. Adiabatic compression leads to warming.

Using the equation of state, the first law of thermodynamics, and the hydrostatic equation we can find that the rate of adiabatic temperature change in an ascending air parcel (also termed the adiabatic lapse rate and denoted Fa) is constant:

ΓΑ - - ΑΤ/ΔZ-9.8 °C km²¹

Note that Fa is defined as the negative of the actual temperature change, so that Fa is the amount of cooling that the rising parcel experiences. Sinking air will warm at the same rate as it is being compressed by the increasing pressure.

Variation of density:

The technical definition of density is mass per unit volume. Generally, density describes how tightly packed something is. An object with a lot of material in a small space is denser than an object that has lots of air space included. In the atmosphere, gas that is less dense has a lower concentration of molecules per volume than a denser gas and will tend to rise compared to the air around it.

Why do I care? When planting crops or plants, soil density is very important. If the soil is packed too tightly, the plant or crop won't be able to absorb any water or nutrients from the soil and will not be able to grow properly. Density in the atmosphere is also important in the formation of clouds and precipitation.

Warm air is less dense than cooler air. Air density varies with the relative humidity (amount of water vapor molecules in the air) along with temperature. Water vapor molecules (H₂O in the gaseous phase) are composed of Hydrogen (H) and Oxygen (O) molecules. Hydrogen has a molecular weight of 1.01 g/mol. Dry air is composed mostly of Nitrogen (N) molecules since Earth's atmosphere is 78% Nitrogen and 21% Oxygen. Nitrogen has a molecular weight of 14.0 g/mol. In the atmosphere

Pressure is force exerted over a given area. In the atmosphere, the molecules in the air apply pressure to everything on earth, including us. For instance, individual molecules in the air push against tiny areas on the top of our head. The force that air exerts is called air pressure. The more air molecules there are above you, the greater the force they exert, so the greater the pressure.

Pressure is important because it is related to volume, density, and temperature. In the atmosphere, warm surfaces can heat the air above them, causing the air to become less dense and to rise. This can eventually result in clouds and precipitation in the areas of rising motion, such as in the center of low pressure systems. High pressure in the atmosphere causes the air to compress and sink, leading to clear skies and calm conditions.

We all live near the bottom of an ocean of air. At sea level, the weight of the air overhead. presses on us with a pressure of -10° N m ^ - 2 = 14.7 lbs in³, We are not aware of this great weight because the air presses on us from all sides, even from our insides (due to the air in our lungs). due to th

weight overhead At higher altitudes, there is less air and less weight overhead, and the pressure is less. Also, because air is readily compressible, the lower layers of air are compressed by the weight of the air above. Thus, the pressure and density of air decrease at higher altitudes. That's why a helium balloon rises: the pressure on the underside of the balloon is greater than the pressure on the top.