"""
The zonal-mean seasonal cycle, animated month by month
===========================================================

The surface counterpart of :doc:`plot_05_zonal_mean_temperature`'s vertical
cross-section: zonal-mean surface temperature against latitude, stepped
through the twelve months of the year. A single annual-mean curve is the
right summary for climate; it is the wrong one for the seasonal cycle
itself, which is a genuinely different curve every month and whose defining
feature -- how much the two hemispheres disagree about which month is
warmest -- only shows up by watching the curve rock back and forth.

The seasonal swing is not uniform across latitude, and that unevenness is
the content. Near the equator the sun's elevation barely changes over the
year, so the curve there stays almost still; toward the poles the swing
grows to tens of degrees, so the animation's fastest-moving parts are its
edges. The two hemispheres are exactly out of phase -- July is northern
summer and southern winter at once -- which is why the curve does not
simply rise and fall in place but rotates, warming on one side while
cooling on the other.
"""
import os
import tempfile

import numpy as np
import plotpress

lat = np.linspace(-90.0, 90.0, 200)                 # degrees

T_ANNUAL = 300.0 - 45.0 * np.sin(np.radians(np.abs(lat))) ** 2

# Seasonal amplitude grows toward the poles and nearly vanishes at the
# equator, where the sun's elevation barely changes over the year.
AMPLITUDE = 25.0 * np.sin(np.radians(np.abs(lat))) ** 1.5
HEMISPHERE_SIGN = np.sign(lat)
HEMISPHERE_SIGN[HEMISPHERE_SIGN == 0.0] = 1.0        # the equator itself, arbitrarily

month = np.arange(1, 13)

# Phased so July peaks in the north (summer there) and troughs in the south
# (winter there) at the same instant -- the two hemispheres share a
# calendar but disagree about what it means.
phase = np.cos(2.0 * np.pi * (month[:, None] - 7.0) / 12.0)
temperature = T_ANNUAL[None, :] + AMPLITUDE[None, :] * HEMISPHERE_SIGN[None, :] * phase

fig, ax = plotpress.subplots(figsize=(8.6, 5.4))
ax.plot(lat, T_ANNUAL, color="#888888", linestyle="--", linewidth=1.3,
        label="annual mean")
ax.plot_frames(lat, temperature, slider_values=month, slider_label="month",
              color="#d62728", label="zonal-mean surface T")
ax.axvline(0.0, color="#333333", linewidth=0.7)
ax.set_ylim(220.0, 320.0)
ax.set_xlim(-90.0, 90.0)
ax.set_xlabel("latitude (deg)")
ax.set_ylabel("temperature (K)")
ax.set_title("Seasonal cycle: hemispheres out of phase, amplitude growing poleward")
ax.legend(loc="upper center")
ax.grid(True)
fig.tight_layout()

gif_path = os.path.join(tempfile.gettempdir(), "plotpress_seasonal_cycle.gif")
fig.save(gif_path, fps=4)