Weather Notes

Weather

Part 1: Weather Factors

• Factors Influencing Weather:
  • Temperature
  • Humidity
  • Air Pressure
  • Winds
• Station Models: Used to represent weather data.

What is Weather?

• Definition: Weather refers to the short-term atmospheric conditions in a specific area at a specific time.

Location of Weather Changes

• Troposphere: Most weather changes occur in the troposphere, the layer of the atmosphere just above Earth’s surface.
• Water Vapor: The troposphere contains most of the water vapor, which is crucial for weather phenomena.

Factors Affecting Weather

• Key Factors: Temperature, humidity, air pressure, and winds.
• Interrelation: These factors are interrelated; a change in one factor influences the others.

WEATHER FACTOR #1: TEMPERATURE

Measuring Temperature

• Temperature: Measurement of the average kinetic energy of a substance.

Thermometers

• Thermometer: A sealed glass tube with graduations, containing mercury or alcohol.
  • The liquid expands with heating and contracts with cooling.

Temperature Scales

• Three Major Scales:
  • Celsius: Used by most of the world and in scientific fields.
  • Fahrenheit: Commonly used in the U.S.
  • Kelvin: Used in scientific fields.
• ESRT Page 13: Provides temperature scale conversions.

Temperature Scale Conversions

• Fahrenheit, Celsius, Kelvin:
  • Water Boils: 212°F, 100°C, 373K
  • Room Temperature: 68°F, 20°C, 290K
  • Water Freezes: 32°F, 0°C, 273K

Temperature Scales & Conversions Checkpoint

• Room Temperature: 68°F, 20°C, 293K
• Water Freezes: 32°F, 0°C, 273K
• Water Boils: 212°F, 100°C, 373K

Modeling Temperature on Maps

• Isotherms: Isolines connecting points of equal temperature.

Temperature and Density

• Relationship: As temperature increases, density decreases.
• Air Movement: Warm air rises because it is less dense, while cold air sinks because it is denser.

Altitude and Temperature

• Altitude: Height above sea level.
• Atmospheric Pressure: Decreases with altitude (e.g., Mount Everest: 8,850 m, 0 kPa).

Altitude and Temperature in the Troposphere

• Decrease with Altitude: As altitude increases in the troposphere, temperature decreases.
• Reason: The air becomes thinner at higher altitudes.
• Reference: See ESRT page 14.

WEATHER FACTOR #2: HUMIDITY

What is Humidity?

• Humidity: The amount of water in the air.

Moisture Capacity

• Moisture Capacity: The amount of water air can hold.
  • Warm air can hold more moisture than cold air.
  • Warm air has more space between particles.

Relative Humidity

• Relative Humidity: The amount of water in the air compared to how much it could hold, expressed as a percentage.
  • 100% relative humidity means the air is saturated and precipitation is likely.

Modeling Relative Humidity

• Beakers as Models: Different-sized beakers represent different air temperatures, with water representing the amount of moisture.

Relative Humidity Checkpoint

• Temperature Representation: The size of the beaker represents temperature (larger = warmer).
• Modeling: Pouring water into smaller beakers shows that as temperature decreases, relative humidity increases.
• Relationship: As air temperature increases, relative humidity decreases.
• Precipitation Model: When there is more water than the beaker can hold, the overflow represents precipitation.

Cloud Formation

• Process:
  1. Warm air rises.
  2. It expands and cools.
  3. Reaches dew point temperature to form a cloud.
• Dew Point Temperature: The temperature to which air must be cooled to be saturated.

Conditions at Dew Point Temperature

• When air temperature equals dew point temperature:
  1. Relative humidity equals 100%.
  2. Water vapor will condense (form clouds).
  3. Precipitation will occur.

Condensation Nuclei

• Requirement: A solid surface is needed for condensation.
• Condensation Nuclei: Dust particles in the atmosphere provide these surfaces.

Cloud Formation Checkpoint

• Cloud Formation: Clouds usually form when air temperature reaches the dew point.

Cloud Formation & Precipitation Checkpoint

• Sleet Formation: Rain freezes as it falls through colder air before hitting the ground.
• Precipitation Sequence: As a storm moves eastward, sleet will immediately follow freezing rain.
• Cloud Formation Factors: Air rises and cools.

Forms of Precipitation

• Rain: Falling liquid drops larger than 0.2 mm in diameter; may be melted snow.
• Drizzle: Falling liquid drops from 0.2 mm to 0.5 mm in diameter.
• Snow: Falling ice crystals formed by combining cloud ice crystals (below 32°F).
• Sleet: Solid pellets of ice that form by the freezing of rain drops as they fall.
• Freezing Rain: Rain or drizzle that freezes on contact with surfaces.
• Hail: Layers of ice, snow, and water formed by up-and-down movements in thunderstorm clouds.

Measuring Humidity

• Sling Psychrometer: Instrument used to measure humidity.
  • Dry Bulb Thermometer: Measures normal air temperature.
  • Wet Bulb Thermometer: Has a wet cloth, causing temperature to lower due to evaporation.

Using ESRT Page 12

• Dew Point & Relative Humidity Charts:
  • Top row: Difference between wet-bulb and dry-bulb temperature.
  • Left column: Dry-bulb temperature (°C).
  • Find where the correct row and column meet.

Air Temperature, Dew Point, and Relative Humidity

• Numerical Example:
  • Location A: Dry-Bulb 14°C, Wet-Bulb 9°C, Difference 5°C
  • Location B: Dry-Bulb 18°C, Wet-Bulb 15°C, Difference 3°C

Dewpoint & Relative Humidity Charts

• To find the dew point, use the Dewpoint Temperature chart depending on the Dry – Wet difference.
• To find the relative humidity, use the Relative Humidity (%) chart depending on the Dry – Wet difference.

Numerical Example Continued

• Location A:
  • Difference: $14-9 = 5$ so the difference is 5˚C
  • Dew Point Temperature: $4˚C$, Relative Humidity: $50$
• Location B:
  • Difference: $18-15 = 3$, so the difference is 3˚C
  • Dew Point Temperature: $13˚C$, Relative Humidity: $72$
• Location C:
• Dry-Bulb: $23˚C$, Wet-Bulb: $21˚C$, Dew Point Temperature: $20˚C$, Relative Humidity: $83.5$
• Location D:
• Dry-Bulb: $28˚C$, Wet-Bulb: $27˚C$, Dew Point Temperature: $27˚C$, Relative Humidity: $93$
• Location E:
• Dry-Bulb: $30˚C$, Wet-Bulb: $30˚C$, Dew Point Temperature: $30˚C$, Relative Humidity: $100$

Conclusions

1. The closer the air temperature and dew point, the higher the relative humidity.
2. The further the air temperature and dew point, the lower the relative humidity.

Sling Psychrometer Example

• Based on the readings, the dewpoint of the air is $3˚C$.
• Based on the readings, the relative humidity of the air is $27$.

Graphing Example

• Highest Relative Humidity: 6 a.m.
• Lowest Relative Humidity: 4 p.m.
• Equal Air Temperature and Dew Point: 100% relative humidity.
• Farthest Apart: 25% relative humidity.
• Most Likely Precipitation: 6 a.m. because the Air temperature and dewpoint temperature are equal, where the Relative humidity is 100%.
• Most Evaporation: 4 p.m. because air temperature is greatest, and relative humidity is lowest.

Final Conclusion

• When the relative humidity is highest, air temperature and dew point are closest together. Precipitation is most likely to occur at this time.

WEATHER FACTOR #3: AIR PRESSURE

What is Air Pressure?

• Air Pressure: Measurement of the weight of gases pushing on Earth’s surface.
  • High Pressure: A lot of gases pushing on Earth.
  • Low Pressure: Less gases pushing on Earth.

Factors Affecting Air Pressure

1. Temperature:
   • As temperature increases, air pressure decreases.
   • Cold air is denser and sinks, resulting in high air pressure.
   • Warm air is less dense and rises, resulting in low air pressure.
2. Humidity:
   • As humidity increases, air pressure decreases.
   • Dry air is denser and sinks, creating high air pressure.
   • Humid air is less dense and rises, creating low air pressure.
3. Altitude:
   • As altitude increases, air pressure decreases due to fewer air molecules.
   • Low Pressure Air Movement: Warm, moist air rises, expands, and cools to the dewpoint, forming clouds.
   • High Pressure Air Movement: Cool, dry air sinks, contracts, and warms, preventing cloud formation.

Measuring Air Pressure

• Barometer: An instrument used to measure air pressure.
• Mercury Barometer: Tube with a closed top placed in mercury.
  • Air pressure pushes down on the mercury, causing it to rise in the tube.
  • Height is measured in inches of mercury.
• Aneroid Barometer: Metal can with a partial vacuum that expands and contracts with air pressure changes.

Conversion using ESRT

• ESRT Page 13: Used to convert air pressures.
• Each line = 1 millibar.
• Each line = 0.01 inches.

Converting Air Pressures Checkpoint

• Millibars to Inches of Hg:
  • 968.0 mb = 28.58 in
  • 1000.0 mb = 29.53 in
  • 1021.0 mb = 30.15 in
• Inches of Hg to Millibars:
  • 30.65 in = 1038.0 mb
  • 29.50 in = 999.0 mb
  • 29.41 in = 996.0 mb

Modeling Air Pressure on Maps

• Isobars: Isolines that connect points of equal air pressure at a 4 millibar interval.

Changes in Air Pressure

• Barometric Trend: How air pressure increases or decreases over time.
  • Increasing Pressure: Clear weather is coming.
  • Decreasing Pressure: Stormy weather is coming.

Air Pressure Practice Checkpoint

1. Average Air Pressure at Sea Level: 1013.25 mb and 29.92 in of Hg.
2. Temperature and Air Pressure: As temperature increases, air pressure will most likely decrease.
3. Warm, Moist Air: As warm, moist air moves into a region, barometric pressure readings will generally decrease.
4. Air Pressure Prediction: Based on air temperature and pressure, 1017 millibars would most likely occur at noon.

Pressure Center

• Pressure Center: Highest or lowest pressures on a weather map.
• Isobars: Form circles around the pressure centers.

Weather Conditions and Pressure Centers

• High Pressure System (Happy High):
  • Cooler and dry weather.
  • Surface winds circulate outwards and clockwise.
  • Sinking, diverging air.
• Low Pressure System (Lousy Low):
  • Warmer and wet weather.
  • Surface winds circulate inward and counterclockwise.
  • Converging, rising air.

Isobars & Pressure Centers Checkpoint

1. The correct isobar pattern for the given weather system is shown in the diagram.
2. The correct isobar and wind flow pattern for a Northern Hemisphere low-pressure center is shown in the diagram.
3. The surface wind pattern around a Northern Hemisphere high-pressure center is shown in the diagram.
4. The most likely location of clouds associated with pressure centers is shown in the diagram.

Drawing Isobars & Pressure Centers

• Isobars for 988 mb, 992 mb, 996 mb, 1000 mb, and 1004 mb were correctly drawn around the high and low pressure centers.
• Label the high pressure center with a capital H.
• Label the low pressure center with a capital L.
• Draw 3 large arrows around the high pressure center indicating the wind direction (outward and clockwise).
• Draw 3 large arrows around the low pressure center indicating the wind direction (inward and counterclockwise).

WEATHER FACTOR #4: WIND

Why Does Air Move?

• Wind: Horizontal movement of air caused by differences in air density due to the uneven heating of Earth’s surface.

Wind Direction

• Flow: Winds always flow from high pressure to low pressure.
  • Warm areas have low pressure.
  • Cool areas have high pressure.

Wind Direction & Speed Checkpoint

Wind moves from Forest High Pressure Cool --to-- Low Pressure Warm areas.
Wind speed was greater because difference in millibars is increased.

Wind Vane

• Wind Vane: Indicates the compass direction the wind is coming from.
• Naming: Winds are named for the direction they come from.

Determining Wind Direction Checkpoint

• South Wind: The arrow shows the wind coming from the South.
• West Wind: The arrow shows the wind coming from the West.
• Southwest Wind: The arrow shows the wind coming from the Southwest.

Wind Speed

• Pressure Gradient:
  • The greater the pressure gradient, the faster the wind blows.
  • The lower the pressure gradient, the slower the wind blows.

Wind Direction & Speed Checkpoint

Wind moves from High Pressure Cool --to-- Low Pressure Warm areas.
Wind speed was faster depending on pressure gradient.

Measuring Wind Speed

• Anemometer: Instrument that measures the speed of the wind.

Modeling Wind Speed on a Map

• Isobar Distance: Wind speed can be determined by the distance between isobars.
  • Closer isobars indicate faster wind.
  • Farther isobars indicate slower wind.

Winds Checkpoint

1. Greatest Wind Speed: Point B.
2. Wind Direction: Winds blow from regions of high air pressure to regions of low air pressure.
3. Wind Velocity: Is most dependent upon the gradient of the air pressure field.
4. Albany, NY: Was most probably experiencing a high wind velocity.
5. Isobar Distance: As wind velocity increases, the distance between isobars on a weather map will decrease.
6. Weather Instruments: Indicate that it is used to measure weather variable.
7. Low pressure System Diagram. Draw two additional isobars around the outside of the $1000-mb$ isobar using a $4-millibar$ interval and indicate the direction the air is spinning.
8. Pressure Gradient Calculations:

Pressure Gradient Calculations Checkpoint

• Based on the maps and the map scale, indicate the pressure gradient and area of speed.
• 1) Predict which area will have the greater wind speed. Explain your prediction: Pressure gradient A will have a greater wind speed because the isobars are closer together.
• 2) Calculate the pressure gradient in area A: Area $= (1020 mb – 1004 mb) / 100 miles = 16 mb / 100 miles = 0.16 mb/mile$
• 3) Calculate the pressure gradient in area B: Gradient $= (1028 mb – 1020 mb) / 100 miles = 8 mb / 100 miles = 0.08 mb/mile$
• 4) Which area had the faster wind speed? Did the calculations support your prediction? Area A had a faster wind speed than Area B, and the calculations supported my predictions.

Weather Instruments Checkpoint

• Diagram a) Name of Weather Instrument: Barometer, and Variable is: Air pressure
• Diagram b) Name of Weather Instrument: Sling psychrometer, and Variable is: Relative humidity & Dewpoint temperature
• Diagram c) Name of Weather Instrument: Anemometer, and Variable is: Wind speed
• Diagram d) Name of Weather Instrument: Wind vane, and Variable is: Wind direction
• Diagram e) Name of Weather Instrument: Thermometer, and Variable is: Temperature
• Diagram f) Name of Weather Instrument: Barometer, and Variable is: Air pressure

Specific Heat of Land vs. Water

• Coastal Breezes: Due to the difference in specific heats, land and water have different temperatures at different times of the day.

Coastal Breezes

1. Sea (Ocean) Breeze:
   • During the day, surface wind blows from the water towards the land.
   • Warmer air over land = low pressure.
   • Cooler air over ocean = high pressure.
2. Land Breeze:
   • At night, surface wind blows from the land towards the water.
   • Cooler air over land = high pressure.
   • Warmer air over ocean = low pressure.

Monsoons

• Definition: Seasonal reversing of winds and precipitation patterns caused by large differences in temperature of the land and the ocean water in certain seasons.

Monsoons: Summer vs. Winter

• Summer: Heavy rains occur when moist wind flows from the ocean onto land.
• Winter: Dry conditions occur when dry wind flows from the land to the ocean.

Coastal Breezes & Monsoons Checkpoint

1. Earth’s surface winds generally blow from regions of higher air pressure toward regions of lower air pressure.
2. The circulation of air above Earth’s surface at a coastal location during the day and at night is best described as an example of convection resulting from temperature and pressure differences above land and water.
3. The wind will occur at 2 p.m., when the air over land is 80˚F and the air over the lake is 70˚F.
4. The locations of high air pressure and low air pressure near a beach on a cool summer night.
5. Moist air flows from the Indian Ocean to India.

Weather Factors on a Map

• Station Model: Simple diagram that summarizes 10 pieces of weather information.
• A station model has no units!

Station Model Components

• Cloud Cover: Percent of circle shaded in.
• Barometric Pressure: Air pressure abbreviated (Actual = 1019.6 mb).
• Barometric Trend: Change in air pressure over past 3 hours (Actual +1.9mb). + or / = rising, - or \ = falling.
• Precipitation: Amount over the past 6 hours.
• Wind Direction: Stick points in direction wind is coming from.
• Wind Speed (knots): Total number of feathers. Whole feather = 10 knots, half feather = 5 knots.
• Present Weather: Type of precipitation occurring.
• Air Temperature: In degrees Fahrenheit.
• Visibility: Distance you can see in miles.
• Dewpoint Temperature: In degrees Fahrenheit.

Station Model Air Pressure Abbreviations

• To convert actual air pressure to the abbreviation, take off the 9 or 10 in the front, remove the decimal, and remove units.
• Example A: 987.0 mb = 870
• Example B: 1010.4 mb = 104
• If the abbreviation is greater than 500, add a 9 to the front, a decimal between the last two numbers, and units (millibars).
• Example: 870 = 987.0 mb
• If the abbreviation is less than 500, add 10 to the front, a decimal between the last two numbers, and units (millibars).
• Example: 104 = 1010.4 mb

Station Model Air Pressure Abbreviations Checkpoint

• Actual Air Pressure to Station Model Abbreviation:
  • 988.8 mb = 888
  • 1016.5 mb = 165
  • 976.8 mb = 768
  • 1000.0 mb = 000
  • 998.7 mb = 987
• Station Model Abbreviation to Actual Air Pressure:
  • 001 = 1000.1 mb
  • 082 = 1008.2 mb
  • 871 = 987.1 mb
  • 888 = 988.8 mb
  • 222 = 1022.2 mb

Station Model Practice

• 1) For Station 1, the chart with Weather Factor with:
  • Temperature: $52˚F$,
  • Dew Point: $46˚F$,
  • Barometric Pressure: $1006.4 mb$,
  • Wind Direction: West,
  • Wind Speed:: $20 knots$,
  • Precipitation: $0.30 inch$,
  • Cloud Cover: $0$,
  • Barometric Trend: $1.4 mb$ increase,
  • Present Weather: None,
  • Visibility: $10+ miles$
• 2) For Station 2, the chart with Weather Factor with:
  • Temperature: $33˚F$,
  • Dew Point: $30˚F$,
  • Barometric Pressure: $1024.7 mb$,
  • Wind Direction: Northeast,
  • Wind Speed:: $10 knots$,
  • Precipitation: $0.50 inch$,
  • Cloud Cover: $100$,
  • Barometric Trend: $2.4 mb$ increase,
  • Present Weather: Sleet,
  • Visibility: $1 mile$
• Station 2 shows the highest relative humidity based on the closer temperature and dewpoint, 100% cloud cover, and precipitation.

Impossible Station Models

• a) Problem: Units for the temperature and dewpoint temperature are in place. $876 +30/$.
• b) Problem: The barometric pressure is not abbreviated and is written outside the station model. $993.6 +70/$.
• c) Problem: The barometric pressure has units $mb$. $162 +81/$.
• d) Problem: The barometric trend is not abbreviated and instead has $1.5$. $960 -1.5/$.
• e) Problem: The air temperature is lower than the dewpoint temperature. $310 +10/$.