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 2026-05-02.

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 8% 5% 5% 5% 4 6% 7% 33%	131496
	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	131001
Transparency	Day	48% 7% 12% 20% 11% 1	134990
	Night	48% 7% 12% 19% 12% 1	135594
Seeing	Day	24% 17% 29% 19% 9% 3	16641
	Night	21% 14% 28% 28% 9% 1	25704

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	8% 8% 4 16% 9% 8% 6% 4 6% 6% 25%	4661
	11th to 20th	Day	9% 12% 5% 13% 7% 6% 5% 4 5% 6% 27%	4650
	21st to end	Day	9% 11% 5% 10% 6% 5% 5% 4 6% 6% 32%	5124
February	1st to 10th	Day	10% 12% 3 10% 6% 7% 5% 4 6% 7% 29%	1862
	11th to 20th	Day	9% 9% 4 13% 7% 5% 6% 5% 6% 6% 30%	2048
	21st to end	Day	14% 12% 4 9% 6% 5% 5% 4 4 6% 31%	1730
March	1st to 10th	Day	16% 13% 5% 11% 6% 5% 5% 4 4 4 28%	4576
	11th to 20th	Day	13% 9% 4 8% 5% 5% 6% 4 6% 6% 33%	4750
	21st to end	Day	19% 12% 3 7% 5% 4 4 3 5% 6% 31%	6097
April	1st to 10th	Day	17% 11% 3 9% 5% 4 4 3 5% 5% 33%	2520
	11th to 20th	Day	16% 8% 2 7% 6% 4 5% 4 6% 6% 34%	2494
	21st to end	Day	20% 9% 3 8% 5% 4 4 3 5% 6% 33%	2709
May	1st to 10th	Day	30% 10% 4 9% 5% 4 4 3 4 5% 21%	5899
	11th to 20th	Day	17% 8% 3 9% 6% 5% 6% 5% 6% 7% 28%	6316
	21st to end	Day	17% 7% 2 7% 4 5% 5% 4 7% 8% 33%	6949
June	1st to 10th	Day	18% 7% 3 8% 5% 5% 6% 4 6% 8% 29%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 4 6% 9% 35%	2800
	21st to end	Day	18% 7% 3 8% 6% 6% 5% 5% 6% 7% 30%	2800
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 11% 35%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 5% 7% 5% 8% 11% 36%	6360
	21st to end	Day	13% 6% 2 5% 4 4 5% 4 7% 9% 41%	6926
August	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 34%	4479
	11th to 20th	Day	13% 6% 3 8% 5% 5% 5% 5% 7% 8% 34%	4046
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 7% 11% 42%	4312
September	1st to 10th	Day	12% 5% 1 6% 4 4 5% 4 6% 9% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 6% 6% 49%	2255
	21st to end	Day	16% 6% 2 5% 4 4 4 3 6% 8% 43%	2200
October	1st to 10th	Day	20% 6% 3 6% 4 5% 4 3 5% 6% 38%	2164
	11th to 20th	Day	13% 9% 3 8% 5% 5% 4 3 6% 7% 38%	1930
	21st to end	Day	21% 10% 3 8% 4 5% 4 3 5% 6% 31%	1980
November	1st to 10th	Day	12% 8% 3 7% 5% 4 4 4 7% 7% 38%	1851
	11th to 20th	Day	14% 9% 2 7% 5% 4 4 5% 5% 7% 37%	1890
	21st to end	Day	17% 10% 4 9% 6% 4 4 4 6% 7% 30%	1764
December	1st to 10th	Day	12% 14% 5% 13% 7% 5% 5% 4 7% 7% 22%	2680
	11th to 20th	Day	18% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	3066
	21st to end	Day	16% 12% 3 10% 6% 5% 5% 3 4 5% 30%	4048
January	1st to 10th	Night	10% 8% 3 14% 9% 6% 6% 4 6% 7% 27%	8234
	11th to 20th	Night	9% 12% 5% 13% 7% 6% 6% 5% 6% 8% 23%	8235
	21st to end	Night	11% 8% 3 9% 6% 7% 6% 5% 6% 7% 31%	9057
February	1st to 10th	Night	10% 10% 4 10% 6% 6% 6% 5% 7% 7% 29%	2753
	11th to 20th	Night	9% 8% 4 9% 7% 7% 6% 5% 6% 8% 31%	2458
	21st to end	Night	12% 10% 3 8% 5% 5% 5% 5% 6% 7% 34%	2076
March	1st to 10th	Night	18% 11% 2 7% 6% 5% 5% 4 6% 7% 29%	5484
	11th to 20th	Night	12% 10% 4 8% 6% 6% 5% 4 5% 6% 35%	5429
	21st to end	Night	18% 12% 3 8% 5% 4 4 3 5% 5% 31%	5072
April	1st to 10th	Night	17% 8% 3 8% 5% 6% 5% 4 5% 7% 33%	2100
	11th to 20th	Night	16% 9% 3 7% 5% 5% 4 4 5% 6% 36%	2075
	21st to end	Night	18% 8% 3 7% 4 4 3 4 6% 6% 36%	1911
May	1st to 10th	Night	31% 11% 3 6% 4 4 4 4 4 6% 22%	4064
	11th to 20th	Night	14% 11% 4 10% 6% 5% 5% 4 6% 6% 30%	3607
	21st to end	Night	17% 7% 2 8% 5% 6% 5% 5% 7% 7% 32%	3926
June	1st to 10th	Night	17% 7% 2 7% 5% 4 5% 4 6% 7% 36%	1600
	11th to 20th	Night	18% 5% 2 6% 4 4 4 4 5% 7% 40%	1600
	21st to end	Night	15% 6% 2 6% 5% 5% 5% 4 7% 8% 37%	1600
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	3600
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 4 8% 8% 39%	3600
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 7% 49%	3926
August	1st to 10th	Night	17% 8% 3 8% 5% 4 5% 3 5% 7% 37%	2637
	11th to 20th	Night	15% 6% 3 6% 4 4 4 3 6% 8% 40%	2998
	21st to end	Night	8% 4 2 5% 3 4 4 3 8% 10% 50%	3586
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 50%	2000
	11th to 20th	Night	11% 5% 2 5% 3 4 3 4 5% 7% 52%	2143
	21st to end	Night	14% 6% 2 4 4 4 4 3 6% 7% 46%	2200
October	1st to 10th	Night	19% 7% 2 7% 5% 3 4 4 6% 7% 38%	2234
	11th to 20th	Night	15% 7% 2 6% 4 5% 6% 4 5% 8% 38%	2470
	21st to end	Night	20% 9% 2 7% 4 4 5% 4 7% 6% 32%	2860
November	1st to 10th	Night	13% 8% 2 7% 4 6% 5% 4 6% 5% 39%	2667
	11th to 20th	Night	14% 7% 2 7% 4 5% 5% 4 6% 7% 37%	2730
	21st to end	Night	13% 10% 3 7% 6% 5% 4 4 7% 8% 33%	2851
December	1st to 10th	Night	12% 13% 4 12% 7% 6% 5% 4 6% 8% 24%	4720
	11th to 20th	Night	20% 10% 4 9% 5% 4 4 3 4 5% 31%	5348
	21st to end	Night	17% 12% 4 11% 6% 5% 4 3 5% 5% 27%	7150

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	56% 8% 12% 19% 6%	4662
	11th to 20th	Day	55% 6% 9% 16% 11% 2	4680
	21st to end	Day	51% 6% 8% 21% 13% 2	5124
February	1st to 10th	Day	53% 7% 13% 17% 9% 2	1862
	11th to 20th	Day	52% 8% 12% 18% 9% 2	2057
	21st to end	Day	54% 6% 11% 15% 12% 1	1730
March	1st to 10th	Day	59% 6% 8% 11% 9% 7%	4579
	11th to 20th	Day	47% 6% 17% 19% 9% 2	4750
	21st to end	Day	52% 5% 8% 14% 17% 4	6097
April	1st to 10th	Day	52% 5% 9% 19% 12% 3	2520
	11th to 20th	Day	48% 7% 11% 17% 15% 2	2494
	21st to end	Day	52% 5% 9% 18% 13% 3	2709
May	1st to 10th	Day	64% 7% 8% 15% 5% 1	5910
	11th to 20th	Day	53% 8% 14% 15% 9% 1	6320
	21st to end	Day	46% 7% 15% 22% 9% 1	6949
June	1st to 10th	Day	51% 9% 14% 17% 10%	2800
	11th to 20th	Day	45% 7% 14% 19% 14% 2	2800
	21st to end	Day	51% 8% 11% 18% 11% 1	2800
July	1st to 10th	Day	42% 6% 16% 22% 13%	6290
	11th to 20th	Day	39% 7% 17% 24% 12% 1	6285
	21st to end	Day	38% 6% 15% 29% 12%	6728
August	1st to 10th	Day	44% 8% 16% 21% 11%	4465
	11th to 20th	Day	46% 6% 16% 25% 6%	4085
	21st to end	Day	35% 7% 17% 27% 13% 1	4312
September	1st to 10th	Day	35% 8% 13% 29% 15% 1	2400
	11th to 20th	Day	35% 6% 10% 28% 19% 1	2255
	21st to end	Day	38% 7% 14% 26% 13% 2	2200
October	1st to 10th	Day	42% 7% 10% 22% 17% 1	3528
	11th to 20th	Day	41% 5% 13% 27% 14%	3120
	21st to end	Day	40% 6% 10% 29% 14% 2	3180
November	1st to 10th	Day	42% 8% 13% 22% 14%	1851
	11th to 20th	Day	46% 8% 12% 18% 15% 2	1890
	21st to end	Day	53% 7% 17% 17% 5%	1764
December	1st to 10th	Day	61% 6% 14% 14% 4	2680
	11th to 20th	Day	53% 6% 10% 18% 12% 2	3066
	21st to end	Day	58% 5% 7% 14% 14% 2	4048
January	1st to 10th	Night	56% 8% 12% 16% 8% 1	8234
	11th to 20th	Night	57% 8% 10% 12% 10% 3	8280
	21st to end	Night	49% 7% 8% 17% 16% 3	9057
February	1st to 10th	Night	50% 10% 12% 18% 9% 1	2753
	11th to 20th	Night	48% 9% 13% 19% 10% 1	2460
	21st to end	Night	48% 8% 13% 20% 9% 3	2076
March	1st to 10th	Night	53% 6% 10% 14% 14% 4	5494
	11th to 20th	Night	50% 6% 17% 16% 9% 2	5430
	21st to end	Night	53% 5% 7% 15% 18% 2	5072
April	1st to 10th	Night	51% 6% 10% 19% 13% 1	2100
	11th to 20th	Night	48% 7% 10% 21% 14% 1	2075
	21st to end	Night	47% 6% 10% 19% 14% 3	1911
May	1st to 10th	Night	62% 7% 11% 13% 7% 1	4070
	11th to 20th	Night	53% 8% 12% 19% 8% 1	3608
	21st to end	Night	49% 8% 15% 17% 10% 1	3927
June	1st to 10th	Night	45% 10% 13% 18% 12% 1	1600
	11th to 20th	Night	43% 10% 14% 19% 13% 2	1600
	21st to end	Night	44% 9% 14% 22% 10% 1	1600
July	1st to 10th	Night	38% 5% 21% 24% 11%	3560
	11th to 20th	Night	41% 7% 15% 23% 13%	3552
	21st to end	Night	34% 5% 14% 35% 11% 1	3809
August	1st to 10th	Night	49% 6% 14% 21% 10%	2627
	11th to 20th	Night	42% 6% 19% 22% 10%	3025
	21st to end	Night	28% 7% 20% 32% 13%	3586
September	1st to 10th	Night	33% 7% 16% 30% 14% 1	2000
	11th to 20th	Night	32% 7% 12% 30% 19% 1	2143
	21st to end	Night	37% 7% 13% 27% 15% 2	2200
October	1st to 10th	Night	46% 6% 12% 23% 12% 1	3650
	11th to 20th	Night	35% 7% 13% 26% 19% 1	4019
	21st to end	Night	42% 6% 11% 30% 10% 1	4610
November	1st to 10th	Night	44% 7% 13% 22% 11% 2	2667
	11th to 20th	Night	45% 7% 13% 20% 13% 2	2730
	21st to end	Night	48% 9% 16% 19% 8%	2851
December	1st to 10th	Night	58% 8% 13% 17% 4	4720
	11th to 20th	Night	56% 5% 10% 13% 14% 1	5348
	21st to end	Night	58% 6% 9% 15% 10% 2	7150

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	26% 23% 26% 18% 6% 1	220
	11th to 20th	Day	32% 22% 30% 13% 2 1	220
	21st to end	Day	23% 25% 31% 14% 6%	242
February	1st to 10th	Day	29% 27% 32% 10% 2	327
	11th to 20th	Day	28% 24% 31% 15% 2	536
	21st to end	Day	28% 21% 33% 15% 3	446
March	1st to 10th	Day	30% 17% 33% 17% 3	540
	11th to 20th	Day	33% 24% 26% 12% 4 1	540
	21st to end	Day	34% 22% 25% 12% 6% 2	594
April	1st to 10th	Day	38% 17% 26% 11% 6% 1	540
	11th to 20th	Day	35% 21% 25% 11% 8% 1	532
	21st to end	Day	31% 21% 27% 12% 6% 2	540
May	1st to 10th	Day	34% 19% 26% 13% 6% 2	507
	11th to 20th	Day	21% 18% 34% 20% 6% 2	527
	21st to end	Day	20% 16% 32% 23% 8% 1	573
June	1st to 10th	Day	22% 15% 37% 17% 8% 1	530
	11th to 20th	Day	20% 15% 32% 22% 10% 2	530
	21st to end	Day	21% 17% 31% 18% 10% 2	530
July	1st to 10th	Day	15% 11% 34% 25% 11% 4	525
	11th to 20th	Day	13% 9% 35% 27% 13% 3	524
	21st to end	Day	15% 8% 39% 25% 12% 2	576
August	1st to 10th	Day	13% 10% 30% 24% 19% 4	526
	11th to 20th	Day	15% 9% 28% 24% 14% 9%	519
	21st to end	Day	12% 14% 26% 26% 15% 7%	583
September	1st to 10th	Day	15% 12% 25% 24% 16% 8%	530
	11th to 20th	Day	14% 12% 24% 18% 20% 12%	530
	21st to end	Day	19% 12% 27% 22% 13% 7%	530
October	1st to 10th	Day	23% 14% 22% 20% 15% 6%	530
	11th to 20th	Day	21% 18% 25% 22% 12% 2	491
	21st to end	Day	33% 13% 25% 20% 8% 1	462
November	1st to 10th	Day	24% 18% 23% 22% 12% 1	425
	11th to 20th	Day	29% 16% 28% 18% 8% 1	430
	21st to end	Day	33% 18% 26% 18% 4 1	304
December	1st to 10th	Day	28% 20% 30% 15% 6%	220
	11th to 20th	Day	36% 21% 30% 9% 5%	220
	21st to end	Day	31% 19% 26% 19% 5%	242
January	1st to 10th	Night	20% 17% 35% 24% 3	840
	11th to 20th	Night	24% 16% 31% 25% 4	840
	21st to end	Night	18% 18% 35% 25% 4	924
February	1st to 10th	Night	18% 20% 36% 23% 3	840
	11th to 20th	Night	21% 16% 38% 22% 3	823
	21st to end	Night	22% 17% 33% 24% 3	692
March	1st to 10th	Night	23% 17% 33% 24% 3	840
	11th to 20th	Night	23% 19% 29% 22% 6%	840
	21st to end	Night	27% 16% 29% 22% 6% 1	924
April	1st to 10th	Night	26% 20% 26% 23% 5%	840
	11th to 20th	Night	23% 23% 27% 20% 7%	831
	21st to end	Night	25% 16% 30% 23% 6%	651
May	1st to 10th	Night	28% 13% 27% 23% 8%	604
	11th to 20th	Night	16% 14% 29% 34% 7% 1	413
	21st to end	Night	16% 8% 28% 35% 12%	433
June	1st to 10th	Night	19% 10% 27% 35% 9%	400
	11th to 20th	Night	19% 10% 25% 34% 13%	400
	21st to end	Night	17% 6% 26% 38% 13% 1	400
July	1st to 10th	Night	15% 4 20% 45% 15% 1	390
	11th to 20th	Night	15% 4 22% 39% 20% 1	388
	21st to end	Night	13% 5% 20% 46% 16% 1	438
August	1st to 10th	Night	13% 4 17% 42% 24%	423
	11th to 20th	Night	14% 6% 23% 34% 20% 3	666
	21st to end	Night	13% 5% 19% 38% 22% 2	880
September	1st to 10th	Night	15% 7% 24% 32% 20% 2	800
	11th to 20th	Night	15% 7% 19% 30% 24% 5%	800
	21st to end	Night	17% 8% 22% 33% 18% 2	800
October	1st to 10th	Night	23% 12% 17% 29% 17% 2	800
	11th to 20th	Night	21% 12% 23% 31% 12% 1	800
	21st to end	Night	27% 13% 25% 29% 7%	880
November	1st to 10th	Night	20% 15% 28% 28% 9%	820
	11th to 20th	Night	21% 16% 30% 25% 8% 1	840
	21st to end	Night	22% 18% 27% 27% 6%	840
December	1st to 10th	Night	25% 15% 34% 22% 4	840
	11th to 20th	Night	26% 17% 29% 24% 4	840
	21st to end	Night	27% 16% 33% 21% 3	924

Page generated at 2026-04-29 02:21:45. Version ac07

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