Defining Weather and Atmospheric Variables
Weather is the present condition of the atmosphere at a specific location. It is described by atmospheric variables, which include:
- Air Temperature is measured with thermometers.
- Air Pressure is measured with barometers.
- Humidity is measured with hygrometers or psychrometers.
- Wind Speed and Direction are measured with anemometers and wind vanes.
- Precipitation is measured with rain gauges.
- Cloud Cover is observed visually or with ceilometers.
- Weather is not the same as climate!
- Weather refers to short-term atmospheric conditions, while climate describes long-term patterns.
The Role of Solar Radiation
Solar radiation is the primary source of heat energy in the atmosphere. It influences weather by:
- Heating Earth's Surface: Causing air temperature changes.
- Driving Air Circulation: Creating wind and pressure differences.
- Affecting Humidity: By evaporating water.
Remember, weather is a result of unequal heating of Earth's surface, leading to air movement and changes in atmospheric variables.
Air Temperature: Measuring Heat Energy
Factors Influencing Air Temperature
- Angle of Solar Radiation: More direct angles = higher temperatures.
- Duration of Sunlight: Longer exposure = more heating.
- Atmospheric Interactions: Clouds can reflect or absorb solar energy.
- Surface Characteristics: Different materials absorb heat differently.
Temperature is measured in Celsius, Fahrenheit, or Kelvin. Meteorologists often use Celsius.
Measuring Temperature
- Thermometers use expansion of liquids like mercury or alcohol.
- Maximum-Minimum Thermometers record the highest and lowest temperatures.
- Thermographs provide continuous temperature readings.
When measuring air temperature, place thermometers in shaded, vented boxes to avoid direct sunlight.
Air Pressure: The Force of the Atmosphere
Understanding Air Pressure
- Air pressure is the force exerted by air molecules on a surface.
- It is measured in millibars (mb) or inches of mercury.
Normal atmospheric pressure at sea level is 1,013.2 millibars.
Measuring Air Pressure
- Mercury Barometers use liquid mercury in a vacuum tube.
- Aneroid Barometers use a spring inside a sealed can.
Rising pressure often indicates fair weather, while falling pressure signals an approaching storm.
Humidity: The Moisture in the Air
Types of Humidity
- Absolute Humidity is the Grams of water vapor per cubic meter of air.
- Relative Humidity is the ratio of current water vapor to the maximum possible at a given temperature, expressed as a percentage.
Relative humidity of 100% means the air is saturated, not that it is all water vapor.
Measuring Humidity
- Hygrometers use human hair to measure changes in length.
- Psychrometers consists of a dry-bulb and wet-bulb thermometer.
- Winds blow from high-pressure to low-pressure areas.
- This movement is influenced by the Coriolis effect, which causes winds to curve.
Land and Sea Breezes
- Sea Breeze is when during the day, cool air from the sea moves toward the land.
- Land Breeze is cool air moving from the land toward the body of water.
Think of wind as nature's way of balancing pressure differences, like water flowing from a full to an empty container.
Clouds and Precipitation: Formation and Types
How Clouds Form
- Air Rises due to heating, obstacles (like mountains), or frontal boundaries.
- As air rises, it expands and cools.
- Water vapor condenses on condensation nuclei (e.g., dust, salt).
The dew point is the temperature at which air becomes saturated and condensation begins.
Types of Clouds
- Cirrus clouds are high, wispy clouds made of ice crystals.
- Cumulus clouds are fluffy, white clouds with a flat base.
- Stratus clouds are low, gray clouds covering the sky.
- Nimbus clouds are rain-bearing clouds.
- Clouds are classified by altitude and shape.
- Understanding these types helps predict weather conditions.
Types of Precipitation
- Rain is liquid water droplets.
- Snow is ice crystals.
- Sleet is frozen raindrops.
- Hail is ice pellets formed in thunderstorms.
Precipitation removes dust and pollutants from the atmosphere but can also form acid rain if pollutants react with water.
Weather Maps and Forecasting
Synoptic Weather Maps
- Station Models summarize weather conditions at specific locations.
- Field Maps use isolines to connect points of equal value (e.g., isotherms for temperature, isobars for pressure).
Closely spaced isobars indicate strong winds due to a steep pressure gradient.
Air Masses and Fronts
- Air Masses are large regions of air with uniform characteristics.
- Fronts are boundaries between air masses.
Most weather systems in the U.S. are mid-latitude cyclones, moving from west to east.
Hazardous Weather: Thunderstorms, Tornadoes, and Hurricanes
Thunderstorms
- Thunderstorms form from strong convection.
- They produce lightning, thunder, and heavy rain.
Thunderstorms are common in warm, moist conditions with intense surface heating.
Tornadoes
- Tornadoes are small, intense rotating columns of air.
- They form from severe thunderstorms.
- Their wind speeds can exceed 500 km/h.
Tornadoes are most common in the central United States, known as "Tornado Alley."
Hurricanes
- Hurricanes are large cyclones forming over warm oceans.
- Their sustained winds exceed 119 km/h.
- They weaken over land due to lack of moisture and heat.
Hurricanes cause storm surges, flooding, and high winds, making them extremely dangerous.
Emergency Preparedness and Forecasting
Mitigation and Preparedness
- You must develop an emergency plan by knowing evacuation routes and shelter locations.
- It is important to secure property by protecting windows and removing loose objects.
- Stay Informed by monitoring weather alerts and warnings.
The National Weather Service issues watches (conditions are favorable) and warnings (hazard is imminent) for severe weather.
Forecasting Methods
- Synoptic Forecasting is the analyzing weather maps and patterns.
- Statistical Forecasting is the usage of historical data to predict probabilities.
- Numerical Forecasting uses computer models based on physical laws.
Short-term forecasts (1-3 days) are more accurate than long-term forecasts due to the complexity of atmospheric systems.
Reflecting on Weather and Its Impact
- Weather is a dynamic and complex system influenced by a variety of factors.
- Understanding the interrelationships between atmospheric variables allows us to predict weather patterns and prepare for hazardous conditions.
- As you continue to study weather, consider the following:
- How do changes in one atmospheric variable affect others?
- What role does technology play in improving weather forecasts?
- How can individuals and communities better prepare for severe weather events?
