How to get the horizon line in the spatial position indicator?

Question

Test Driven Development

There is an image.

You need to get something like

, i.e., find the border between black (ground) and blue (sky),
There is shit code:

from PIL import ImageOps, Image, ImageGrab
import numpy as np
import cv2

def getSkyThresh(img):  # находит все голубые места
    hsv_min = np.array((19, 100, 100))
    hsv_max = np.array((35, 255, 255))
    hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
    return cv2.inRange(hsv, hsv_min, hsv_max)


def getGNDthresh(img):  # находит все серые места (соответствующие земле)
    hsv_min = np.array((0, 0, 0))
    hsv_max = np.array((255, 0, 130))
    hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
    return cv2.inRange(hsv, hsv_min, hsv_max)


def getRedThresh(img):  # находит все красные места (риски и символическое изображение самолета)
    hsv_min = np.array((100, 0, 0))
    hsv_max = np.array((130, 255, 255))
    hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
    return cv2.inRange(hsv, hsv_min, hsv_max)


def getFullThsesh(img):
    skyThresh = getSkyThresh(img)
    GNDtresh = getGNDthresh(img)
    redThresh = getRedThresh(img)
    print(skyThresh)
    x, y = np.shape(img)[0], np.shape(img)[1]
    img1 = np.array([[0 for i in range(y)] for j in range(x)])
    center = (93, 98)
    radius = 86
    for i in range(x):
        for j in range(y):
            if (i - center[0]) ** 2 + (j - center[1]) ** 2 > radius ** 2:
                img1[i, j] = 50
            elif redThresh[i, j] != 0:
                img1[i, j] = 100
            elif GNDtresh[i, j] != 0:
                img1[i, j] = 170
            elif skyThresh[i, j] != 0:
                img1[i, j] = 255
    return img1


def getLine(img):  # должен выделять линию
    skyThresh = getSkyThresh(img)
    GNDtresh = getGNDthresh(img)
    redThresh = getRedThresh(img)
    print(skyThresh)
    x, y = np.shape(img)[0], np.shape(img)[1]
    img1 = np.array([[0 for i in range(y)] for j in range(x)])
    center = (93, 98)
    radius = 86
    for i in range(x):
        for j in range(y):
            if (i - center[0]) ** 2 + (j - center[1]) ** 2 <= radius ** 2 and redThresh[i, j] == 0 and \
                    GNDtresh[i, j] == 0 and skyThresh[i, j] == 0:
                img1[i, j] = 255
    return img1


img1 = cv2.imread('filename')  # читаем изображение
Image.fromarray(img1).show()  # показываем его

Image.fromarray(getFullThsesh(img1)).show()  # здесь и далее магия
Image.fromarray(getLine(img1)).show()

getLine()highlights not only the line, but also various other things. I would like to highlight only the line. What if so:

Then the line will be interrupted by that red thing.
What then to make the algorithm for obtaining the horizon line?