Units Tutorial

Early in our scientific careers we all learn about the importance of paying attention to units in our calculations. Unit conversions can still get the best of us and have caused more than one major technical disaster, including the crash and complete loss of the $327 million Mars Climate Orbiter.

In MetPy, we use the pint library and a custom unit registry to help prevent unit mistakes in calculations. That means that every quantity you pass to MetPy should have units attached, just like if you were doing the calculation on paper!

In MetPy units are attached by multiplying them with the integer, float, array, etc. In this tutorial we’ll show some examples of working with units and get you on your way to utilizing the computation functions in MetPy.

import numpy as np

from metpy.units import units

Simple Calculation

Let’s say we want to calculate the area of a rectangle. It so happens that one of our colleagues measures their side of the rectangle in imperial units and the other in metric units. No problem! First we need to attach units to our measurements. For many units the easiest way is by find the unit as an attribute of the unit registry:

length = 10.4 * units.inches
width = 20 * units.meters
print(length, width)

Out:

10.4 inch 20 meter

Don’t forget that you can use tab completion to see what units are available! Just about every imaginable quantity is there, but if you find one that isn’t, we’re happy to talk about adding it.

While it may seem like a lot of trouble, let’s compute the area of a rectangle defined by our length and width variables above. Without units attached, you’d need to remember to perform a unit conversion before multiplying or you would end up with an area in inch-meters and likely forget about it. With units attached, the units are tracked for you.

area = length * width
print(area)

Out:

208.0 inch * meter

That’s great, now we have an area, but it is not in a very useful unit still. Units can be converted using the to() method. While you won’t see square meters in the units list, we can parse complex/compound units as strings:

print(area.to('m^2'))

Out:

5.2832 meter ** 2

Temperature

Temperature units are actually relatively tricky (more like absolutely tricky as you’ll see). Temperature is a non-multiplicative unit - they are in a system with a reference point. That means that not only is there a scaling factor, but also an offset. This makes the math and unit book-keeping a little more complex. Imagine adding 10 degrees Celsius to 100 degrees Celsius. Is the answer 110 degrees Celsius or 383.15 degrees Celsius (283.15 K + 373.15 K)? That’s why there are delta degrees units in the unit registry for offset units. For more examples and explanation you can watch MetPy Monday #13: https://www.youtube.com/watch?v=iveJCqxe3Z4.

Let’s take a look at how this works and fails:

We would expect this to fail because we cannot add two offset units (and it does fail as an “Ambiguous operation with offset unit”).

10 * units.degC + 5 * units.degC

On the other hand, we can subtract two offset quantities and get a delta. A delta unit is pint’s way of representing a relative change in two offset units, indicating that this is not an absolute value of 5 degrees Celsius, but a relative change of 5 degrees Celsius.

print(10 * units.degC - 5 * units.degC)

Out:

5 delta_degC

We can add a delta to an offset unit as well since it is a relative change.

print(25 * units.degC + 5 * units.delta_degF)

Out:

27.77777777777778 degC

Absolute temperature scales like Kelvin and Rankine do not have an offset and therefore can be used in addition/subtraction without the need for a delta version of the unit.

print(273 * units.kelvin + 10 * units.kelvin)

Out:

283 kelvin

print(273 * units.kelvin - 10 * units.kelvin)

Out:

263 kelvin

Compound Units

We can create compound units for things like speed by parsing a string of units. Abbreviations or full unit names are acceptable.

u = np.random.randint(0, 15, 10) * units('m/s')
v = np.random.randint(0, 15, 10) * units('meters/second')

print(u)
print(v)

Out:

[11.  9. 13.  6.  2. 11. 12.  4.  1. 13.] meter / second
[ 5.  3.  4.  4. 11. 11.  3.  7.  1.  1.] meter / second

Common Mistakes

There are a few common mistakes the new users often make. Be sure to check these when you’re having issues

• Pressure units are mbar or hPa for common atmospheric measurements. The unit mb is actually millibarns.
• When using masked arrays, units must be multiplied on the left side. This will be addressed in the future, but is a current limitation in the ecosystem. The expected error will be AttributeError: ‘MaskedArray’ object has no attribute ‘units’

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