***********************
Modeling the Atmosphere
***********************
The ``pwv_atm`` module provides models for the atmospheric transmission due to
precipitable water vapor at Kitt Peak National Observatory. These models can be
retrieved by providing either a datetime and airmass value or a PWV column
density along line of sight.

Transmission for Datetime
=========================

Modeling the atmospheric transmission function for a given datetime requires
SuomiNet data to be available on the local machine. See the
:ref:`accessing_data:Updating Data` section for more details.

To find the atmospheric transmission for a given datetime, use the
``trans_for_date`` function.

.. autofunction:: pwv_kpno.pwv_atm.trans_for_date

Examples:
---------

For an airmass of 1.2, the transmission function at 2013-12-15 05:35:00 is
given by:

.. code-block:: python
    :linenos:

    >>> from datetime import datetime
    >>> from pwv_kpno import pwv_atm
    >>> import pytz
    >>>
    >>> obsv_date = datetime(year=2013,
    >>>                      month=12,
    >>>                      day=15,
    >>>                      hour=5,
    >>>                      minute=35,
    >>>                      tzinfo=pytz.utc)
    >>>
    >>> pwv_atm.trans_for_date(date=obsv_date, airmass=1.2)

      wavelength  transmission
       Angstrom
      ---------- --------------
          3000.0 0.999999991637
          3001.0 0.999999991629
          3002.0 0.999999991621
             ...            ...

Transmission for PWV
====================

Instead of relying on SuomiNet measurements, users can also retrieve the
modeled transmission function by directly specifying a PWV column density. This
can be done using the ``trans_for_pwv`` method.

.. autofunction:: pwv_kpno.pwv_atm.trans_for_pwv

Examples:
---------

For a 13.5 mm PWV column density along line of sight, the transmission function
is given by:

.. code-block:: python
    :linenos:

    >>> from pwv_kpno import pwv_atm
    >>> pwv_atm.trans_for_pwv(13.5)

      wavelength  transmission
       Angstrom
      ---------- --------------
          3000.0 0.999999922781
          3001.0 0.999999922702
          3002.0 0.999999922625