Being a good dad I try and answer my kids science questions, but sometimes it’s really tough.

There is an awesome Python library called MetPy that can help with some of those challenging science and weather questions.

In this blog I’d like to introduce the MetPy library and show how to use it to solve questions like:

• Can they make snow when it’s above freezing ?
• How much thinner is the air in places like Denver?
• How can you figure out what the wind chill is?

## Getting Started with MetPy

To install MetPy:

`pip install metpy`

One of nice things about MetPy is that it manages the scientific units, so variables  are defined with their units. Below is a Python example where the units module is used for a simple temperature conversion. The temperature today is defined as 40 degF. The to() method can be used to convert the temperature to degC.

```>>>> from metpy.units import units
>>> tempToday =  * units.degF
>>> tempToday.to(units.degC)

Quantity([4.44444444], 'degree_Celsius')
```

You can also do some interesting mixing of units in math calculations, for example you can add 6 feet and 4 meters:

``` >>> print( *units.feet +  * units.m)
[19.123359580052494] foot```

MetPy has a very large selection of thermodynamic and weather related functions. In the next sections I will show how to use some of the these functions.

## How can they make snow when it’s above freezing ?

Ski resort can make snow by forcing water and pressurized air through a “snow gun”. Making snow can be an expensive operation but it allows ski resorts to extend their season. When you get a weather forecast the temperature is given as the ambient or dry bulb temperature. The wet bulb temperature takes the dry air temperature and relative humidity into account. The wet bulb temperature is always below the outside temperature. To start  making snow  a wet bulb temperature of -2.5°C or 27.5°F is required.

Metpy has a number of functions that can used to find humidity and wet bulb temperatures.  The wet_bulb_temperature function will find the wet bulb temperature using the pressure, dry temperature and dew point.

Below is an example where the temperature is below freezing, but it’s not possible to make snow because the wet bulb temperature is only -0.6°C  (not the required  -2.5°C).

```>>> import metpy.calc
>>>
>>> pressure =  * units.kPa
>>> temperature = [0.5] * units.degC
>>> dewpoint = [-2.5] * units.degC
>>>
>>> metpy.calc.wet_bulb_temperature(pressure, temperature, dewpoint)

Quantity(-0.6491265444587265, 'degree_Celsius')
```

Knowing that -2.5°C (27.5°F) is the wet bulb temperature upper limit for snow making, the relative_humidity_wet_psychrometric function can be used to create a curve of humidity and dry temperature points where it is possible to make snow.

The code below iterates between -10 and 10 deg °C getting humidity values at a wet bulb temperature of -2.5°C.

```
#
# Find when you can make snow
#
import matplotlib.pyplot as plt
import metpy.calc
from metpy.units import units

print("Get temps vs. humidity")
print("-------------------")

plt_temp = []
plt_hum = []
for temp in range (-10,11): # Check dry temperatures between -10 - 10 C
dry_temp = [temp] * units.degC
# Get the relative humidity
rel_humid = metpy.calc.relative_humidity_wet_psychrometric(dry_temp, wet_temp,pres)
# Strip the humidity units for charting, and make a percent (0-100)
the_humid = rel_humid.to_tuple() * 100
if (the_humid  0) : # Get valid points
plt_temp.append(temp) # append a valid temp
plt_hum.append(the_humid) # append a valid humidity
print (temp, the_humid)

fig, ax = plt.subplots()
ax.plot(plt_temp, plt_hum )
ax.set(xlabel='Temperature (C)', ylabel='Humidity (%)',
title='When you can make Snow')
ax.grid()
#fig.savefig("makesnow.png")
plt.show()

``` From the data we can see that it is possible to make snow when the temperature is above freezing and the humidity is low.

## How much thinner is the air … in Denver?

We all know that the air is thinner when we’re up in an airplane, but how much thinner is it in Denver or  Mexico City compared to New York City ?

Using the height_to_pressure_std function it is possible to get a pressure value based on altitude. The to() method can be used to convert the pressure to standard atmospheres.

```>>> import metpy.calc
>>>
>>> New_York_alt = *units.ft
>>> metpy.calc.height_to_pressure_std(New_York_alt).to(units.atm)
Quantity([0.99880745], 'standard_atmosphere')

>>> Denver_alt =  * units.ft
>>> metpy.calc.height_to_pressure_std(Denver_alt).to(units.atm)
Quantity([0.82328412], 'standard_atmosphere')

>>> Mexico_city_alt = *units.ft
>>> metpy.calc.height_to_pressure_std(Mexico_city_alt).to(units.atm)
Quantity([0.76132418], 'standard_atmosphere')
```

Relative to New York City the air is about 18% thinner in Denver and 24% thinner in Mexico City.

Using the height_to_pressure_std function it is possible to create a chart of atmospheric pressure between sea level and the top of Mt. Everest (29,000 ft). At the top of Mt. Everest the air is 70% thinner than at sea level !!

```
#
# How much does the air thin as you climb ?
#
import matplotlib.pyplot as plt
import metpy.calc
from metpy.units import units
print("Get height vs. Atm Pressure")
print("---------------------------")
# create some plot variables
plt_ht = []
plt_press = []

# Check Atmospheric Pressure from sea level to Mt. Everest (29,000 ft) heights
for temp in range (0,30000,1000): # Check dry temperatures between -10 - 10 C
height = [temp] * units.feet
pressure = metpy.calc.height_to_pressure_std(height)
atm = pressure.to(units.atm)
print (height, atm)
plt_ht.append (height.to_tuple()) # put the value into plt list
plt_press.append (atm.to_tuple()) # put the value into plt list

fig, ax = plt.subplots()
ax.plot(plt_ht, plt_press )
ax.set(xlabel='Mountain Height (ft)', ylabel='Pressure (atm)',
title='How does the Pressure change Mountain Climbing?')
ax.grid()
fig.savefig("height_vs_press.png")
plt.show()

``` ## How can I figure out the Wind Chill ?

The MetPy windchill function will return a “feels like” temperature based on a wind speed and ambient temperature. For example an outside ambient temperature of 40 deg °F with a wind of 20 mph feel like 28 deg °F.

```>>> import metpy.calc
>>>
>>> temp =  * units.degF
>>> wind =  * units.mph
>>> metpy.calc.windchill(temp, wind, face_level_winds=True)
Quantity([28.42928573], 'degree_Fahrenheit')

```

When the temperature starts going below -20 °C parents should be keep a close eye on their kids for frost bite. Below is a code example that shows a curve of -20 °C based on ambient temperature and wind.

```
#
# Wind Chill
#
import matplotlib.pyplot as plt
import metpy.calc
from metpy.units import units

# Create some plotting variables
plt_temp = []
plt_speed = []

for temp in range (0,-46,-1): # Check dry temperatures between -10 - 10 C
the_temp = [temp] * units.degC
for wind in range(1,61,1):
the_wind = [wind] * units.kph
windchill =
metpy.calc.windchill(the_temp,the_wind,face_level_winds=True)
# Select points with a wind chill around -20
if (windchill.to_tuple()) = -20.1) :
plt_temp.append(temp)
plt_speed.append(wind)

fig, ax = plt.subplots()
ax.fill_between(plt_temp, plt_speed, label="-20 C - Wind Chill", color="red" )

ax.set(xlabel='Temperature (C)', ylabel='Wind Speed (kph)',
title='When is the Wind Chill -20C ?')
ax.grid()
fig.savefig("windchill.png")
plt.show()

``` ## Summary

MetPy didn’t solve all my kids questions but the Metpy library is an excellent for science questions around water and weather.

If you have a budding chemist or chemical engineer in your house try taking a look at the Python Thermo library.
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