CSV Weather Data
================

:Source: ``buem/weather/from_csv.py``

For single-building runs, BuEM loads pre-extracted weather data from a CSV
file rather than processing the full COSMO-REA6 grid.

CsvWeatherData
--------------

Reads CSV files with hourly columns:

- **T** — air temperature [°C]
- **GHI** — global horizontal irradiance [W/m²]
- **DNI** — direct normal irradiance [W/m²]
- **DHI** — diffuse horizontal irradiance [W/m²]

DNI Reconstruction
~~~~~~~~~~~~~~~~~~

NWP models store DNI as :math:`(GHI - DHI) / \cos(\theta_z)`.  Near the
horizon :math:`\cos(\theta_z) \to 0`, producing unphysical spikes
(> 4 000 W/m² observed).

``reconstruct_dni_from_ghi()`` applies **pvlib's DISC decomposition**
(Iqbal 1983) to re-derive DNI from GHI.  The result is capped at
extraterrestrial irradiance (1 316–1 413 W/m² seasonal).  DHI is then
back-computed as :math:`DHI = GHI - DNI \cos(\theta_z)`.

Caching
~~~~~~~

Processed data is stored as a ``.feather`` file next to the source CSV,
avoiding the 2–3 s pvlib computation on repeated imports.

Default Weather File
--------------------

BuEM ships with ``COSMO_Year__ix_390_650.csv`` — a representative weather
year extracted from the COSMO-REA6 reanalysis for Loenen, Netherlands.