Horizons Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horizons Observatory clear sky chart from 2005-11-06 to 2024-01-17.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	15% 8% 3 7% 6% 4 5% 5% 6% 7% 34%	74815
	Night	15% 8% 3 7% 6% 3 5% 5% 6% 7% 35%	75004
Transparency	Day	48% 8% 13% 19% 11% 1	74520
	Night	47% 8% 13% 20% 11% 1	74833
Seeing	Day	24% 17% 29% 19% 9% 3	13710
	Night	21% 14% 28% 28% 9% 1	22787

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	9% 11% 4 9% 7% 4 6% 6% 7% 7% 28%	1517
	11th to 20th	Day	15% 9% 4 8% 7% 4 6% 5% 6% 8% 28%	1488
	21st to end	Day	9% 10% 6% 8% 8% 4 6% 5% 7% 8% 30%	1584
February	1st to 10th	Day	12% 11% 4 9% 7% 5% 7% 5% 5% 7% 27%	1595
	11th to 20th	Day	9% 9% 5% 11% 8% 4 6% 5% 6% 7% 29%	1748
	21st to end	Day	15% 11% 5% 8% 7% 4 5% 4 4 6% 30%	1480
March	1st to 10th	Day	11% 8% 6% 8% 6% 4 6% 7% 5% 6% 32%	1797
	11th to 20th	Day	17% 9% 5% 7% 5% 3 5% 5% 5% 6% 32%	1844
	21st to end	Day	14% 10% 4 7% 6% 3 5% 4 5% 5% 35%	2371
April	1st to 10th	Day	17% 8% 4 8% 6% 3 4 3 5% 6% 35%	2160
	11th to 20th	Day	17% 6% 3 6% 7% 4 4 5% 6% 7% 36%	2134
	21st to end	Day	22% 9% 3 7% 6% 3 5% 4 5% 6% 30%	2322
May	1st to 10th	Day	21% 9% 4 7% 6% 3 4 4 4 6% 31%	2340
	11th to 20th	Day	17% 6% 3 6% 6% 3 5% 5% 5% 7% 38%	2476
	21st to end	Day	14% 7% 3 5% 5% 4 5% 5% 5% 8% 39%	2725
June	1st to 10th	Day	18% 7% 3 8% 6% 4 6% 6% 5% 7% 29%	2520
	11th to 20th	Day	17% 5% 2 5% 5% 4 6% 5% 5% 9% 36%	2520
	21st to end	Day	18% 7% 3 7% 6% 4 6% 6% 6% 8% 30%	2520
July	1st to 10th	Day	16% 6% 2 6% 5% 3 5% 5% 6% 8% 39%	2520
	11th to 20th	Day	11% 6% 3 6% 7% 4 7% 6% 6% 10% 35%	2520
	21st to end	Day	13% 5% 3 6% 6% 3 6% 6% 8% 8% 36%	2744
August	1st to 10th	Day	11% 6% 3 7% 6% 4 6% 7% 7% 9% 35%	2466
	11th to 20th	Day	12% 5% 3 7% 7% 4 5% 6% 7% 9% 35%	2235
	21st to end	Day	11% 5% 2 5% 6% 3 6% 5% 5% 9% 42%	2376
September	1st to 10th	Day	13% 5% 1 5% 5% 3 5% 5% 6% 9% 43%	2160
	11th to 20th	Day	13% 6% 2 4 5% 3 4 4 5% 6% 48%	2030
	21st to end	Day	15% 6% 2 4 4 3 5% 4 5% 8% 44%	1980
October	1st to 10th	Day	22% 6% 3 6% 5% 4 4 5% 6% 6% 35%	1948
	11th to 20th	Day	13% 8% 3 7% 6% 5% 5% 4 6% 7% 37%	1738
	21st to end	Day	23% 10% 3 7% 5% 4 4 4 5% 6% 29%	1782
November	1st to 10th	Day	13% 8% 3 6% 5% 2 5% 5% 7% 7% 39%	1671
	11th to 20th	Day	15% 9% 3 6% 5% 3 5% 5% 5% 7% 36%	1710
	21st to end	Day	17% 9% 4 8% 6% 3 5% 5% 6% 7% 31%	1596
December	1st to 10th	Day	16% 12% 5% 10% 7% 4 5% 5% 6% 6% 24%	1520
	11th to 20th	Day	20% 11% 4 7% 5% 4 5% 4 5% 6% 31%	1906
	21st to end	Day	20% 14% 3 9% 7% 4 6% 4 4 5% 24%	2772
January	1st to 10th	Night	12% 9% 4 7% 7% 4 6% 5% 6% 8% 32%	2649
	11th to 20th	Night	13% 10% 5% 7% 8% 4 6% 6% 6% 8% 28%	2616
	21st to end	Night	9% 7% 5% 8% 6% 4 6% 7% 7% 6% 34%	2772
February	1st to 10th	Night	11% 10% 4 8% 7% 4 7% 6% 7% 8% 27%	2360
	11th to 20th	Night	10% 8% 4 8% 8% 5% 7% 7% 6% 8% 30%	2098
	21st to end	Night	13% 9% 3 7% 6% 4 6% 5% 5% 7% 34%	1776
March	1st to 10th	Night	12% 9% 3 8% 7% 4 5% 5% 7% 6% 33%	2150
	11th to 20th	Night	15% 8% 4 6% 7% 4 5% 6% 5% 6% 33%	2115
	21st to end	Night	13% 10% 3 6% 5% 3 4 5% 5% 5% 40%	1980
April	1st to 10th	Night	16% 7% 3 6% 6% 4 5% 5% 5% 7% 34%	1800
	11th to 20th	Night	15% 7% 4 6% 6% 4 4 5% 5% 7% 37%	1775
	21st to end	Night	20% 7% 3 6% 5% 3 3 5% 6% 7% 33%	1638
May	1st to 10th	Night	21% 10% 3 6% 6% 3 4 4 5% 7% 32%	1620
	11th to 20th	Night	17% 7% 3 6% 6% 4 4 3 6% 6% 38%	1433
	21st to end	Night	14% 5% 3 7% 5% 3 5% 6% 7% 8% 39%	1550
June	1st to 10th	Night	17% 6% 3 6% 5% 3 5% 6% 6% 8% 35%	1440
	11th to 20th	Night	19% 5% 2 5% 4 3 5% 4 5% 7% 40%	1440
	21st to end	Night	15% 5% 3 5% 6% 4 6% 4 7% 8% 38%	1440
July	1st to 10th	Night	15% 4 2 4 5% 3 4 5% 5% 8% 42%	1440
	11th to 20th	Night	13% 4 2 4 5% 3 4 4 7% 10% 44%	1440
	21st to end	Night	12% 4 3 4 5% 3 5% 5% 6% 8% 46%	1574
August	1st to 10th	Night	11% 4 2 5% 5% 3 5% 4 6% 8% 46%	1454
	11th to 20th	Night	12% 5% 2 6% 6% 4 4 4 7% 8% 42%	1662
	21st to end	Night	14% 6% 2 4 4 3 3 4 6% 8% 46%	1980
September	1st to 10th	Night	12% 5% 2 4 4 2 4 4 6% 8% 49%	1800
	11th to 20th	Night	12% 5% 2 4 4 2 4 4 4 7% 51%	1928
	21st to end	Night	14% 6% 2 4 4 2 4 4 6% 7% 47%	1980
October	1st to 10th	Night	20% 6% 3 6% 6% 3 4 5% 6% 7% 36%	2010
	11th to 20th	Night	16% 7% 3 5% 6% 4 6% 6% 5% 8% 36%	2222
	21st to end	Night	21% 9% 3 6% 5% 3 5% 4 7% 6% 31%	2574
November	1st to 10th	Night	13% 8% 2 6% 5% 4 6% 5% 6% 5% 39%	2407
	11th to 20th	Night	15% 7% 3 6% 5% 4 6% 5% 6% 7% 38%	2470
	21st to end	Night	14% 8% 4 6% 6% 4 5% 4 7% 8% 35%	2579
December	1st to 10th	Night	16% 11% 5% 9% 8% 4 5% 5% 5% 7% 24%	2660
	11th to 20th	Night	18% 9% 4 8% 5% 3 5% 5% 5% 7% 31%	3288
	21st to end	Night	20% 12% 5% 11% 7% 3 5% 4 5% 5% 23%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	51% 9% 13% 19% 8%	1518
	11th to 20th	Day	54% 10% 13% 14% 8% 2	1488
	21st to end	Day	50% 7% 13% 17% 10% 2	1584
February	1st to 10th	Day	55% 8% 13% 15% 8% 1	1595
	11th to 20th	Day	53% 8% 12% 17% 9% 2	1757
	21st to end	Day	55% 6% 11% 14% 12% 1	1480
March	1st to 10th	Day	51% 7% 13% 15% 10% 3	1800
	11th to 20th	Day	52% 6% 13% 13% 14% 3	1844
	21st to end	Day	49% 7% 10% 15% 15% 4	2371
April	1st to 10th	Day	50% 5% 10% 21% 11% 3	2160
	11th to 20th	Day	46% 8% 11% 18% 15% 1	2134
	21st to end	Day	55% 5% 10% 16% 11% 3	2322
May	1st to 10th	Day	55% 6% 7% 19% 11% 2	2340
	11th to 20th	Day	45% 6% 10% 20% 16% 3	2480
	21st to end	Day	43% 9% 14% 22% 11% 2	2725
June	1st to 10th	Day	52% 8% 14% 16% 9%	2520
	11th to 20th	Day	45% 8% 12% 19% 14% 2	2520
	21st to end	Day	52% 9% 12% 17% 10% 1	2520
July	1st to 10th	Day	42% 9% 12% 22% 15% 1	2450
	11th to 20th	Day	43% 10% 15% 23% 9% 1	2445
	21st to end	Day	40% 10% 15% 24% 11% 1	2591
August	1st to 10th	Day	41% 11% 16% 22% 10%	2452
	11th to 20th	Day	44% 9% 15% 22% 9%	2235
	21st to end	Day	38% 8% 15% 26% 13% 1	2376
September	1st to 10th	Day	37% 8% 13% 27% 14% 1	2160
	11th to 20th	Day	37% 6% 10% 28% 18% 2	2030
	21st to end	Day	37% 7% 15% 26% 13% 2	1980
October	1st to 10th	Day	49% 8% 11% 18% 13% 2	1948
	11th to 20th	Day	44% 7% 15% 25% 7% 1	1738
	21st to end	Day	57% 8% 10% 16% 8% 2	1782
November	1st to 10th	Day	42% 8% 14% 23% 13%	1671
	11th to 20th	Day	47% 8% 13% 17% 13% 2	1710
	21st to end	Day	52% 7% 18% 16% 6% 1	1596
December	1st to 10th	Day	60% 8% 14% 12% 5% 1	1520
	11th to 20th	Day	55% 6% 12% 16% 9% 1	1906
	21st to end	Day	64% 7% 9% 12% 8%	2772
January	1st to 10th	Night	49% 9% 13% 17% 10% 1	2649
	11th to 20th	Night	54% 11% 13% 14% 7% 1	2616
	21st to end	Night	48% 8% 13% 17% 12% 2	2772
February	1st to 10th	Night	53% 10% 13% 15% 8% 1	2360
	11th to 20th	Night	49% 9% 13% 18% 10% 1	2100
	21st to end	Night	49% 8% 14% 19% 8% 3	1776
March	1st to 10th	Night	49% 7% 13% 19% 11% 2	2160
	11th to 20th	Night	52% 6% 11% 17% 11% 2	2116
	21st to end	Night	45% 7% 10% 17% 17% 3	1980
April	1st to 10th	Night	49% 6% 10% 21% 12% 2	1800
	11th to 20th	Night	46% 7% 10% 22% 14% 1	1775
	21st to end	Night	49% 7% 11% 18% 11% 4	1638
May	1st to 10th	Night	52% 7% 8% 17% 14% 2	1620
	11th to 20th	Night	46% 7% 10% 21% 14% 2	1434
	21st to end	Night	42% 11% 14% 21% 11% 2	1551
June	1st to 10th	Night	46% 11% 14% 17% 12% 1	1440
	11th to 20th	Night	43% 9% 14% 19% 13% 2	1440
	21st to end	Night	44% 10% 14% 22% 11%	1440
July	1st to 10th	Night	37% 10% 18% 21% 13% 1	1400
	11th to 20th	Night	35% 9% 18% 27% 11%	1392
	21st to end	Night	35% 8% 16% 28% 11% 2	1486
August	1st to 10th	Night	35% 9% 15% 28% 12%	1444
	11th to 20th	Night	39% 8% 17% 25% 11%	1662
	21st to end	Night	35% 8% 14% 28% 14%	1980
September	1st to 10th	Night	33% 8% 17% 29% 13% 1	1800
	11th to 20th	Night	33% 7% 12% 29% 18% 1	1928
	21st to end	Night	36% 7% 13% 27% 15% 2	1980
October	1st to 10th	Night	47% 7% 12% 21% 10% 2	2010
	11th to 20th	Night	45% 9% 15% 23% 8% 1	2222
	21st to end	Night	53% 9% 12% 17% 8% 1	2574
November	1st to 10th	Night	44% 8% 14% 21% 11% 2	2407
	11th to 20th	Night	45% 7% 14% 20% 12% 2	2470
	21st to end	Night	48% 9% 16% 19% 7%	2579
December	1st to 10th	Night	59% 9% 13% 15% 4	2660
	11th to 20th	Night	53% 8% 14% 14% 11% 1	3288
	21st to end	Night	62% 7% 10% 16% 4	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	27% 21% 28% 18% 6% 1	180
	11th to 20th	Day	35% 21% 27% 14% 2 1	172
	21st to end	Day	19% 27% 32% 15% 6% 1	176
February	1st to 10th	Day	33% 27% 30% 8% 2	240
	11th to 20th	Day	27% 25% 31% 14% 2	416
	21st to end	Day	29% 21% 31% 16% 3	346
March	1st to 10th	Day	27% 16% 35% 18% 4	420
	11th to 20th	Day	31% 25% 27% 13% 4 1	420
	21st to end	Day	31% 21% 25% 14% 7% 3	462
April	1st to 10th	Day	35% 17% 26% 13% 7% 2	420
	11th to 20th	Day	32% 21% 24% 12% 10% 1	412
	21st to end	Day	32% 20% 28% 12% 6% 2	420
May	1st to 10th	Day	32% 21% 26% 12% 6% 3	423
	11th to 20th	Day	19% 19% 34% 19% 6% 2	447
	21st to end	Day	20% 16% 31% 24% 9% 1	485
June	1st to 10th	Day	22% 16% 38% 15% 7% 1	450
	11th to 20th	Day	20% 16% 31% 21% 10% 2	450
	21st to end	Day	21% 16% 31% 20% 10% 2	450
July	1st to 10th	Day	16% 12% 32% 25% 11% 4	445
	11th to 20th	Day	14% 9% 35% 26% 13% 3	444
	21st to end	Day	14% 8% 39% 26% 11% 2	488
August	1st to 10th	Day	13% 10% 32% 23% 19% 4	446
	11th to 20th	Day	15% 10% 28% 26% 14% 7%	439
	21st to end	Day	13% 16% 25% 26% 14% 6%	495
September	1st to 10th	Day	16% 12% 24% 24% 16% 7%	450
	11th to 20th	Day	16% 14% 26% 17% 17% 10%	450
	21st to end	Day	18% 13% 27% 22% 12% 6%	450
October	1st to 10th	Day	25% 14% 23% 18% 13% 5%	450
	11th to 20th	Day	19% 19% 25% 24% 12% 2	419
	21st to end	Day	36% 13% 23% 20% 8%	396
November	1st to 10th	Day	22% 17% 23% 24% 13% 1	365
	11th to 20th	Day	29% 16% 28% 18% 8% 1	370
	21st to end	Day	30% 18% 27% 19% 4 1	256
December	1st to 10th	Day	28% 22% 29% 14% 6% 1	180
	11th to 20th	Day	34% 21% 32% 8% 5%	180
	21st to end	Day	32% 20% 26% 17% 4 1	198
January	1st to 10th	Night	21% 16% 36% 24% 3	760
	11th to 20th	Night	24% 17% 30% 25% 3	748
	21st to end	Night	17% 18% 34% 26% 4	792
February	1st to 10th	Night	19% 21% 35% 23% 2	720
	11th to 20th	Night	20% 16% 37% 23% 3	703
	21st to end	Night	22% 19% 33% 23% 3	592
March	1st to 10th	Night	22% 17% 32% 26% 3	720
	11th to 20th	Night	23% 19% 29% 23% 6%	720
	21st to end	Night	25% 15% 29% 23% 7% 1	792
April	1st to 10th	Night	23% 20% 26% 25% 5%	720
	11th to 20th	Night	20% 24% 26% 22% 8%	711
	21st to end	Night	28% 15% 30% 22% 6%	558
May	1st to 10th	Night	28% 14% 27% 24% 8%	540
	11th to 20th	Night	16% 14% 30% 33% 7% 1	372
	21st to end	Night	15% 8% 28% 37% 12% 1	389
June	1st to 10th	Night	19% 9% 28% 34% 9%	360
	11th to 20th	Night	20% 9% 24% 34% 12%	360
	21st to end	Night	16% 6% 26% 38% 13% 1	360
July	1st to 10th	Night	15% 4 21% 45% 15% 1	350
	11th to 20th	Night	15% 3 22% 40% 18% 1	348
	21st to end	Night	12% 6% 20% 45% 17% 1	394
August	1st to 10th	Night	12% 4 17% 43% 23% 1	380
	11th to 20th	Night	14% 6% 23% 35% 19% 3	598
	21st to end	Night	14% 6% 19% 37% 22% 2	792
September	1st to 10th	Night	15% 7% 24% 32% 20% 2	720
	11th to 20th	Night	16% 8% 21% 30% 21% 4	720
	21st to end	Night	17% 8% 24% 32% 18% 2	720
October	1st to 10th	Night	25% 12% 17% 28% 16% 2	720
	11th to 20th	Night	20% 12% 23% 31% 12% 1	720
	21st to end	Night	28% 14% 25% 26% 7%	792
November	1st to 10th	Night	19% 16% 27% 29% 9%	740
	11th to 20th	Night	21% 15% 29% 26% 8% 1	760
	21st to end	Night	22% 18% 26% 27% 6% 1	760
December	1st to 10th	Night	25% 16% 33% 21% 4	760
	11th to 20th	Night	24% 18% 30% 24% 4	760
	21st to end	Night	27% 16% 33% 20% 3	836

Page generated at 2024-01-14 09:55:01. Version ac07

Copyright 2024 Attilla Danko