Unit 4 Basic Weather Processes

1. Unit 4 Basic Weather Processes

2. Objectives Describe the structure and composition of the atmosphere. Define weather and list its elements. Describe the sun-earth radiation budget and the earth’s heat balance.

3. Objectives • Describe factors affecting the temperature of the earth’s surface and the lower atmosphere. • Describe the greenhouse effect and its influence on air temperature. • Describe temperature lag and the affect daily and seasonal temperature lags have on wildland fire behavior.

4. Our Atmosphere Encircling the earth is a blanket of gases bound to it by gravity. Similar to its oceans, the atmosphere is in constant motion.

5. Our “Thin” Atmosphere The atmosphere extends hundreds of miles above the earth’s surface. However, compared to the diameter of the earth of nearly 8,000 miles, our atmosphere is really quite thin.

6. Ninety-nine percent of its gases lie within 18 miles of the earth’s surface. Because the upper portion of the atmosphere gradually thins with increasing altitude, it is impossible to say exactly where it ends and interplanetary space begins. 18 miles

7. Our atmosphere can be divided into many layers based on its change in temperature with altitude. On average, temperature decreases with increasing altitude in the troposphere and mesosphere, and increases with altitude in the stratosphere and thermosphere.

8. Polar Region Troposphere The Tropics The Troposphere The lowest layer of the atmosphere varies in height from 9 to 12 miles above sea level over the tropics, to about 6 miles above sea level over the polar regions.

9. Stratosphere Tropopause Troposphere (weathersphere) The Tropopause This boundary separates the troposphere from the stratosphere, and marks the upper limit of nearly all weather in our atmosphere. Because nearly all weather occurs below the tropopause, the underlying troposphere is often referred to as the weathersphere.

10. Composition of the Atmosphere The earth’s atmosphere is principally composed of gasesandwater vapor. Nearly three-quarters of all these atmospheric gases are concentrated within the troposphere.

11. Dry Gases Nitrogen occupies 78 percent and oxygen about 21 percent of the total volume of dry gases in the troposphere. The remaining 1 percent of this volume includes argon, neon, helium, hydrogen, xenon and carbon dioxide.

12. Water Vapor • Is an extremely important element of the atmosphere. • Forms clouds that produce precipitation. • Stores and releases heat energy called latent heat that is used to power storms, such as thunderstorms and hurricanes.

13. Water Vapor Approximately half of all water vapor is found within the lowest 3 miles of the atmosphere; in other words within the troposphere. The concentration of this invisible gas varies greatly from place to place, and from time to time.

14. In tropical locations, water vapor may account for up to 4 percent of the atmospheric gases. In colder polar regions, its concentration may be a mere fraction of a percent.

15. Wildland Fire Environmental Factors • Wind, Stability, Temp, RH • Fuel Moisture • Fuel Temperature • Fuel Characteristics • Terrain • Aspect • Elevation

16. Of these three major components, weather is the most variable over space and time.

17. Because of its variability, weather can be difficult to predict, particularly at scales of less than 50 miles and greater than 24 hours. miles 50 0

18. What Is Weather? It is the short-term variations of the atmosphere. These variationsinclude: • Air pressure • Air temperature • Humidity • Wind • Clouds • Precipitation • Visibility 4-18-S290-EP

19. It cannot be over emphasized: A basic knowledge and awareness of weather is essential for making critical fire management decisions.

20. According to the Standard Firefighting Orders in the NWCG Fireline Handbook All firefighters should “keep informed on fire weather conditions and forecasts.” Watch Out Situations • Unfamiliar with weather and local factors influencing wildland fire behavior • Weather becoming hotter and drier • Wind increases and/or changes direction

21. To understand how weather can influence wildland fire behavior, we will begin with a discussion of the weather element atmospheric pressure.

22. Top of atmosphere Atmospheric Pressure Or simply air pressure, is defined as the amount of force exerted by the weight of air molecules on a surface area. This downward force or weight is the result of the pull of gravity. Pull Of Gravity

23. 99.9% of Total Weight 99% of Total Weight 90% of Total Weight 50% of Total Weight Atmospheric pressure always decreases with increasing altitude. In this figure, note that 50 percent of all gases are concentrated within the lowest 18,000 feet (3 miles) of the atmosphere. Millibar is the most common pressure unit used today.

24. Another common air pressure unit used in aviation and on television and radio broadcasts is inches of mercury.

25. Standard Atmospheric Pressure At mean sea level, or the average height of the ocean surface, the average, or standard, value for atmospheric pressure is 29.92 inches of mercury. This value is equivalent to 1013.25 millibars.

26. If we weigh a column of air with a cross section of 1 square inch, extending from sea level to the top of the atmosphere, it would weigh nearly 14.7 pounds per square inch at its base. This value also represents the standard atmospheric pressure. Surface area

27. Measuring Air Pressure Barometer – It is the instrument used to measure air pressure. More precisely, it is a calibrated weather instrument used to measure the weight of the atmosphere on a surface area, normally one square inch in size. 1 square inch

28. Two Common Types of Barometers Calibrated Scale Vacuum The mercury barometer – forprecision. Partial Vacuum The aneroid barometer – forconvenience. Aneroid Barometer Mercury Barometer

29. On average, air pressure decreases approximately one inch of mercury every 1000 foot increase in elevation.

30. EXERCISE 1 Our Atmosphere

31. What Drives Our Weather? The Sun – The principal source of light and heat energy 4-31-S290-EP

32. On a much smaller scale, heat also originates from large fires, and other natural and human related heat-release processes.

33. Solar and Terrestrial Radiation Shortwave solar radiation travels through the atmosphere and heats the earth’s surface. This heat is then transferred to the atmosphere as longwave terrestrial radiation through conduction and convection. Longwave terrestrial radiation Shortwave solar radiation

34. Solar-Earth Radiation Budget

35. Incoming Solar Radiation The Earth’s Heat Balance Any change in this equation will cause the earth to either warm or cool. Outgoing Terrestrial Radiation = Incoming Solar Radiation Outgoing Earth Radiation 4-35-S290-EP

36. Latitudinal Distribution of Heat Polar regions lose more heat than they gain. Tropics gain more heat than they lose.

37. Factors Affecting the Temperature of the Earth’s Surface and Lower Atmosphere • Solar angle and duration • Atmospheric moisture and air pollutants • Surface properties of terrain and vegetation

38. The Seasons in the Northern Hemisphere

39. Solar Angle and Duration Solar angle, length of daylight, slope, aspect, and shape of the countryside all affect the amount of solar radiation striking a point on the earth’s surface. Basically, the higher the solar angle and the longer the daylight, the greater the solar heating.

40. Seasonal Change In Solar Angle At 40ºN Latitude

41. Atmospheric Moisture and Air Pollutants Clouds, water vapor and air pollutants absorb, reflect and scatter both solar and terrestrial radiation. Their presence and amount significantly influence the temperature of the earth’s surface and its atmosphere.

42. Heat Loss At Night Cloudy nights tend to be warmer than clear nights because of the insulating effect of cloud cover. Clear nights tend to be cooler than cloudy nights because terrestrial heat is allowed to escape freely to space.

43. Surface Properties of Terrain and Vegetation • Influence the amount of heat absorbed and reflected by the terrain and vegetation. • Effect on surface air temperature can be quite dramatic.

44. Surface Properties of Terrain and Vegetation • Color and texture • Transparency • Conductivity • Specific heat • Evaporation • Condensation

45. Earth’s Power Plant The difference in surface air temperature can be quite large because of these properties, such as the 30 degree difference between a shoreline and a rocky cliff just 20 miles apart.

46. Color and Texture Rough textured, irregular and dark colored materials are good absorbers of solar radiation. Whereas smooth, uniform and light colored materials such as snow, water and sandy soils are good reflectors of solar radiation.

47. Albedo Refers to the ability of a substance to reflect light and heat energy. Rough textured and dark colored materials, such as tree bark, a rocky cliff, granite, a newly plowed field, a forest canopy, and the surface of a lake at high sun angle, all have a low albedo. Smooth and light colored materials, such as a field of snow, sandy soil, and the surface of a lake at low sun angle, all have a high albedo.

48. Transparency Affects the distribution of light and heat through a substance. Water allows solar radiation to travel to a much greater depth than soil and rock. Soil will concentrate heat within the top layer.

49. Conductivity The transfer of heat between molecules in contact with one another.

50. metal granite sandstone ice at 0oC wet snow mud water soil wood dry air Conductors and Insulators Materials that allow for the efficient transfer of heat energy, such as metal and granite, are referred to as conductors. Materials that are poor conductors of heat energy, such as dry air, wood and water, are referred to as insulators.