THE ATMOSPHERE AND WEATHER

Air surrounds the earth and extends out for about 1,000 miles, gradually fading into space at that point. It is called the atmosphere. The air gets thinner the farther it is from the earth. It has no color, no smell and no taste but it is really several gases mixed together. You cannot see it, yet it is as real as land and water. The atmosphere is made up of 78 per cent nitrogen and 21 per cent oxygen. The remaining 1% consists of small amounts of argon and other gases.

The atmosphere is divided into several layers with the Troposphere being the lower level. It extends up to about eight miles or 42,000 feet. It is the region of most weather. The Stratosphere extends to about 25 miles. It is a stable area with minimum temperatures and little change. The Mesosphere extends to about 50 miles and the Thermosphere extends to the threshold of space.

The Troposphere is the area of interest because it is the area where most “weather” occurs. The most rapid changes in temperature also take place. This changing temperature allows both horizontal and vertical motion and the resultant mixing. Consequently, this is a generally mixed, sometimes turbulent layer. Practically all clouds, storms and other changes that affect fire occur here. Horizontal winds usually increase with height in this layer.

In addition to nitrogen and oxygen: water vapor, dust, salt particles, smoke, pollen, and various industrial pollutants are frequently found in the lower level. The solid particles affect visibility and act as nuclei for water vapor to form water droplets.

Atmospheric Moisture

Water is always present in the lower atmosphere in one or more of its three states. It can exist as a gas (water vapor), as a liquid (rain, dew or clouds), or as a solid (snow, hail, frost or ice crystals). The amount is variable – from near zero to 4 or 5 percent by weight. Water tends to act as an independent gas. The molecules of moisture float freely among the molecules of nitrogen and oxygen. It stores vast quantities of energy gained in evaporation and it has a profound effect on the weather processes. Without it, there would be no clouds or rain.

The amount of moisture in the atmosphere varies widely. When the atmosphere is warm, it can hold more water vapor then when it is cold. Relative humidity is a measure of the amount of water in the air compared to the amount it could hold at that temperature. It affects the amount of water in the fuel as well as in the air. When relative humidity is high, fuel will absorb moisture; when low, fuel will lose moisture. When relative humidity reaches 100%, moisture in the air begins to condense. The temperature at this point is called the dew point. Further cooling causes some of the vapor to condense into liquid droplets that form clouds, fog or dew. If the droplets continue to develop, they will eventually form rain. The closer the dew point is to the temperature, the higher the relative humidity. Relative humidity usually increases with height over normal surfaces. It can change rapidly from one hour to the next.

Relative humidity is a better indicator of fire behavior then dew point. Most news media now include relative humidity as part of the weather.

ABSOLUTE HUMIDITY: Amount by weight of moisture in the atmosphere.

RELATIVE HUMIDITY: Ratio of the amount of moisture in the air at a specific temperature to the total amount that the atmosphere could hold at that temperature.

DEW POINT: The temperature at which the present air becomes saturated.

Weight

The atmosphere also has weight. At the outer limits, it has virtually no weight. At the surface, the air above it compresses the atmosphere. At sea level, the average pressure on one square-inch is 14.7 lbs. The body is built to withstand this weight (pressure) and it is not noticeable. This is considered normal pressure and is referred to as “Standard Atmospheric Pressure”. A column of mercury is the standard method of measuring the pressure. At sea level it is 29.92 inches.

However, actual pressure will vary. In low-pressure systems, it will be lower and in high-pressure systems, it will be higher. The pressure also decreases with height until at the threshold of space - it is zero. A more common unit of pressure measurement used in meteorology is the millibar (mb). A barometer reading of 29.92 inches of mercury is equivalent to 1013.25 mb.  

Heat Energy and Temperature

What is energy? Energy is simply the capacity to do work. Almost all energy comes either directly or indirectly from the sun. It cannot be created or destroyed. It can however, be transformed from one form to another. The more common forms of energy are: thermal, radiant, mechanical, chemical, and electrical. For example, when lightning starts a forest fire, electrical energy has been converted to thermal energy.

Tremendous quantities of energy are continuously fed into the atmosphere, setting it into motion and creating weather. Their common source is the radiant energy from the sun. Absorption of this energy warms the surface of the earth and heat is exchanged between the earth’s surface and the lower atmosphere.

Radiation is the process by which the earth receives heat energy from the sun, about 93 million miles away. The intensity of solar radiation received at the outer limits of the earth’s atmosphere is relatively constant. However, the amount reaching the earth’s surface is highly variable depending on cloud cover and the angle it strikes the earth. Some solar energy is reflected from the clouds back into space. Small amounts are scattered by solid particles. The solar radiation that reaches the earth warms the surface. A large portion is absorbed and then reflected back as long-wave radiation, which is absorbed by the water vapor in the atmosphere. A large amount is also used in the evaporation of surface moisture and is transmitted into the atmosphere as latent heat.

The earth’s surface is heated directly by radiation from the sun. Dark surfaces (Ex. a burned area) generally absorb more of the radiation than light surfaces and will become hotter. Light surfaces will reflect more of the heat. The atmosphere in turn, is heated indirectly by conduction and convection of heat and long-wave radiation from the earth’s surface. Thus, the temperature at the surface is higher than it is in the atmosphere and the temperature in the atmosphere generally decreases the higher you go.

Using a yard thermometer on a sunny day. Place it upright on the ground in the sun. After about five minutes, check the temperature – then again at ten minutes. (If the ambient (shade) temperature before you start is in the eighties or higher, keep an eye on the temperature as it may rise rapidly and "peg" out.)

Water is fairly transparent to incoming radiation and it will penetrate deeply into the water spreading the heat throughout a larger volume. In solid materials, the heat is concentrated in a shallow layer. This is especially true in wood and in soil as they are poor conductors of heat. This is the reason that land surfaces will be warmer then water is during the day.

The amount of radiation (heat) received in any given area depends on the angle with which sun’s rays strike the earth. Heating begins as soon as the sun’s rays strike the earth in the morning. The amount of heating will increase until the sun is overhead and then decreases again to near zero at sunset. At night, no appreciable radiation is received and the surface cools rapidly since the heat is concentrated at the surface. The seasons are also the result of the sun’s rays striking the earth at different angles due to the tilt of the earth.  This temperature cycle is the source of the diurnal weather cycle.

When solids and liquids are heated, their temperature increases, their molecular activity increases and they expand. They contract as the temperature falls. The reaction of a gas tends to be more complex. A change in temperature may change either the volume or pressure of a gas - or both. If the gas is confined, the pressure increases, (Ex. vehicle tire or pressure cooker). If pressure remains the same, volume increases, (Ex. hot-air balloon). The change is also much greater for gases. Therefore, a change in temperature will cause significant changes in density. When gas expands and density decreases, it must perform work in the process and therefore expend some of its internal energy. This in turn, lowers its temperature. Thus, expansion is essentially a cooling process and compression is a heating process.

Changes of State

Large amounts of energy are required to change solids (ice) to liquid (water) and to change a liquid to a gas (water vapor).  The heat required to convert a pound of ice to liquid at 32 degrees is 144 b.t.u. This is called “heart of fusion”. To change this pound of water into gas (water vapor) requires 972 b.t.u.! This is known as “heat of vaporization”.

When this process is reversed, (water vapor to water and water to ice), tremendous amounts of energy is released into the atmosphere. This occurs when clouds and ice are formed in the atmosphere.

Sources of Atmospheric Moisture

The water vapor in the atmosphere comes from three sources: Evaporation from a body of water (or moist surface), evaporation from soil and transpiration from plants.

The oceans are a major source of atmospheric moisture as they cover more than three-fourths of the earth’s surface but other sources can be important locally. Plants have a large leaf surface for transpiration. An area of dense vegetation can contribute up to eight times as much as an equal area of bare ground. The amount of evaporation or transpiration will vary according to the amount of air mass, rate of growth, temperature, etc. Wind increases evaporation by blowing away the moisture-laden air and replacing it with dryer air.

Atmospheric Stability

We have already learned the two basic concepts that are necessary to understand atmospheric stability:

*Pressure in the atmosphere decreases with height.

*The temperature of a parcel of air decreases as it expands.

Land and water surfaces warm and cool at different rates because of their different heat transfer properties. This differential heating produces differences in pressure, which in turn, cause movement of air in an attempt to regain balance. Cumulative differences in temperature and pressure develop broad areas of high and low pressure. Air tends to sink in high-pressure areas, flows to low-pressure areas at the surface, and then rises in the low-pressure areas thus resulting in vertical motion. The rising air expands as it rises and heat is lost.

Understanding atmospheric stability is essential to predicting where how smoke from a prescribed burn will disburse and where it might go.

Air Masses and Fronts

An “Air Mass” is a large body of air that has assumed fairly uniform characteristics. They are classified according to their source region such as “Continental Polar” or Maritime Tropical”. They tend to lose or gain various properties depending on the type of surface over which they are traveling. A front is where two air masses meet. The cooler front, being denser, will push under a warmer front lifting it over the cooler air. A warm front, being lighter, will ride over the cooler air. The ascending air cools and forms clouds. A large difference in temperature, results in a more intense front. Turbulence, strong, gusty winds and possibly rain ahead of the front is the result.

Cold fronts move faster then warm fronts. The band of changing weather is narrow, winds shift in a clockwise direction and winds become stronger and gusty. A squall line could develop. Warm fronts are less distinct and the band of changing weather will last longer. Adding moisture or lifting air until the dew point is reached forms clouds. Clouds are also formed by thermal lifting (heated surface).