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# Physical Constants
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MetPy provides a comprehensive library of over 60 physical constants essential for meteorological calculations. All constants include proper units through Pint integration, ensuring dimensional analysis and preventing unit errors in calculations.
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## Capabilities
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### Earth Constants
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Fundamental properties of Earth used in atmospheric and geophysical calculations.
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```python { .api }
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# Earth geometric properties
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earth_avg_radius = 6371008.7714 * units.m
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earth_gravity = 9.80665 * units('m/s^2')
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earth_avg_angular_vel = 7292115e-11 * units('rad/s')
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# Earth orbital properties  
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earth_sfc_avg_dist_sun = 149597870700.0 * units.m
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earth_ellipsoid_flattening = 1/298.257223563
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earth_eccentricity = 0.081819221456
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# Solar geometry
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earth_max_declination_angle = 23.45 * units.degrees
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```
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### Atmospheric Constants
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Properties of the atmosphere essential for meteorological calculations.
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```python { .api }
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# Dry air properties
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dry_air_molecular_weight = 28.9647 * units('g/mol')
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dry_air_gas_constant = 287.0 * units('J/(kg*K)')
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dry_air_spec_heat_press = 1004.0 * units('J/(kg*K)')  # Cp
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dry_air_spec_heat_vol = 717.0 * units('J/(kg*K)')    # Cv
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dry_air_density_stp = 1.275 * units('kg/m^3')
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# Atmospheric pressure
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atmos_pressure_sea_level = 101325.0 * units.Pa
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```
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### Water and Moisture Constants
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Water vapor and phase change properties for humidity and cloud physics calculations.
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```python { .api }
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# Water vapor properties
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water_molecular_weight = 18.0153 * units('g/mol')
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water_gas_constant = 461.5 * units('J/(kg*K)')
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# Latent heats
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water_heat_vaporization = 2.501e6 * units('J/kg')
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water_heat_fusion = 3.337e5 * units('J/kg')
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water_heat_sublimation = 2.834e6 * units('J/kg')
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# Specific heats
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water_specific_heat = 4186.0 * units('J/(kg*K)')
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ice_specific_heat = 2106.0 * units('J/(kg*K)')
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# Densities
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density_water = 1000.0 * units('kg/m^3')
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density_ice = 917.0 * units('kg/m^3')
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# Triple point properties
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water_triple_point_temperature = 273.16 * units.K
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water_triple_point_pressure = 611.657 * units.Pa
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```
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### Universal Physical Constants
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Fundamental constants from physics used in atmospheric radiation and thermodynamics.
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```python { .api }
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# Fundamental constants
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boltzmann = 1.380649e-23 * units('J/K')
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planck = 6.62607015e-34 * units('J*s')
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avogadro = 6.02214076e23 * units('1/mol')
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universal_gas_constant = 8.314462618 * units('J/(mol*K)')
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# Electromagnetic constants
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speed_of_light = 299792458.0 * units('m/s')
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stefan_boltzmann = 5.670374419e-8 * units('W/(m^2*K^4)')
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# Other constants
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gravitational_constant = 6.67430e-11 * units('m^3/(kg*s^2)')
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```
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### Derived Constants
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Commonly used ratios and derived quantities for atmospheric calculations.
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```python { .api }
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# Molecular weight ratios
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epsilon = water_molecular_weight / dry_air_molecular_weight  # ≈ 0.622
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poisson_exponent = dry_air_gas_constant / dry_air_spec_heat_press  # ≈ 0.286
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# Heat capacity ratio
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gamma = dry_air_spec_heat_press / dry_air_spec_heat_vol  # ≈ 1.4
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# Other derived quantities
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dry_adiabatic_lapse_rate = earth_gravity / dry_air_spec_heat_press  # ≈ 9.8 K/km
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```
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## Usage Examples
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### Basic Constant Usage
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```python
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import metpy.constants as constants
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from metpy.units import units
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# Use constants in calculations
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pressure = 1000 * units.hPa
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temperature = 20 * units.celsius
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# Calculate air density using ideal gas law
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density = pressure / (constants.dry_air_gas_constant * temperature.to('kelvin'))
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print(f"Air density: {density}")
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# Calculate scale height
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scale_height = (constants.dry_air_gas_constant * temperature.to('kelvin')) / constants.earth_gravity
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print(f"Scale height: {scale_height}")
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```
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### Thermodynamic Calculations
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```python
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# Calculate potential temperature manually using constants
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theta = temperature * (1000 * units.hPa / pressure) ** constants.poisson_exponent
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print(f"Potential temperature: {theta}")
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# Calculate saturation vapor pressure using Clausius-Clapeyron
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e_sat = 611 * units.Pa * np.exp(
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    (constants.water_heat_vaporization * (temperature.to('kelvin') - 273.15 * units.K)) /
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    (constants.water_gas_constant * temperature.to('kelvin') * 273.15 * units.K)
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)
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print(f"Saturation vapor pressure: {e_sat}")
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```
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### Unit Conversions with Constants
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```python
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# Convert between different temperature scales
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temp_celsius = 25 * units.celsius
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temp_kelvin = temp_celsius.to('kelvin')
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temp_fahrenheit = temp_celsius.to('fahrenheit')
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# Energy conversions
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kinetic_energy = 0.5 * 1000 * units.kg * (10 * units('m/s'))**2
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kinetic_energy_cal = kinetic_energy.to('calorie')
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# Pressure conversions using standard atmosphere
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pressure_mb = 850 * units.mbar
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pressure_pa = pressure_mb.to('pascal')
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pressure_inhg = pressure_mb.to('inch_Hg')
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```
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## Constants Access Patterns
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```python
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# Import all constants
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import metpy.constants as constants
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# Access individual constants
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g = constants.earth_gravity
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R_d = constants.dry_air_gas_constant
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L_v = constants.water_heat_vaporization
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# Use in meteorological calculations
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# Hydrostatic equation: dp/dz = -ρg
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# Ideal gas law: p = ρRT
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# Clausius-Clapeyron: de_s/dT = L_v*e_s/(R_v*T^2)
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```
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## Alternative Access Methods
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```python
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# Import specific constants directly
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from metpy.constants import earth_gravity, dry_air_gas_constant
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from metpy.constants import water_heat_vaporization as L_v
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# Access through metpy.constants namespace
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import metpy
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g = metpy.constants.earth_gravity
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```
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## Types
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```python { .api }
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# All constants are Pint Quantity objects with appropriate units
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from pint import Quantity
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# Example constant types
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earth_gravity: Quantity  # Length/Time^2 units
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dry_air_gas_constant: Quantity  # Energy/(Mass*Temperature) units  
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water_heat_vaporization: Quantity  # Energy/Mass units
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atmos_pressure_sea_level: Quantity  # Pressure units
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```