Horizon Planetary Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horizon Planetary Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	16% 10% 3 8% 5% 4 4 4 5% 7% 33%	108658
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 33%	110357
Transparency	Day	49% 5% 10% 22% 13% 2	112333
	Night	50% 5% 9% 20% 13% 2	114821
Seeing	Day	24% 12% 23% 22% 14% 6%	15277
	Night	23% 9% 21% 29% 15% 2	25711

By Month

Percentage Cloud Cover SE (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	17% 21% 9% 16% 6% 3 3 2 3 5% 15%	4056
	11th to 20th	Day	18% 23% 8% 11% 7% 4 4 2 3 3 18%	4060
	21st to end	Day	20% 16% 4 10% 6% 4 3 3 4 5% 24%	4466
February	1st to 10th	Day	22% 15% 5% 9% 5% 3 4 3 3 5% 27%	1800
	11th to 20th	Day	22% 15% 4 11% 4 4 4 3 4 4 25%	1814
	21st to end	Day	22% 15% 4 9% 4 3 3 3 4 3 30%	1660
March	1st to 10th	Day	21% 15% 3 8% 4 3 3 3 4 6% 29%	3545
	11th to 20th	Day	16% 8% 4 5% 5% 3 3 4 5% 7% 39%	3491
	21st to end	Day	23% 9% 3 7% 4 5% 4 4 5% 7% 30%	3815
April	1st to 10th	Day	16% 12% 3 8% 5% 4 4 5% 5% 7% 32%	2137
	11th to 20th	Day	17% 8% 2 6% 4 4 4 3 5% 7% 40%	2301
	21st to end	Day	20% 9% 2 6% 4 4 4 4 5% 7% 34%	2470
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 5% 6% 27%	4082
	11th to 20th	Day	20% 8% 3 8% 5% 4 5% 3 6% 6% 33%	4081
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 4 7% 10% 37%	4476
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 5% 6% 8% 38%	2470
	11th to 20th	Day	12% 5% 3 6% 5% 6% 6% 5% 6% 9% 36%	2470
	21st to end	Day	13% 6% 2 6% 4 4 5% 5% 7% 10% 38%	2470
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 8% 42%	4120
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	4120
	21st to end	Day	18% 9% 3 8% 6% 5% 5% 4 6% 8% 30%	4466
August	1st to 10th	Day	11% 7% 2 7% 5% 6% 6% 5% 7% 10% 33%	4104
	11th to 20th	Day	12% 5% 3 7% 5% 6% 6% 5% 7% 9% 36%	4044
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 6% 10% 48%	4340
September	1st to 10th	Day	14% 6% 2 5% 4 4 3 3 5% 8% 47%	2200
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 6% 7% 44%	2090
	21st to end	Day	18% 8% 3 7% 4 3 4 3 5% 6% 39%	2090
October	1st to 10th	Day	17% 6% 2 6% 3 3 4 2 6% 8% 44%	1943
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 5% 6% 41%	1900
	21st to end	Day	22% 7% 2 6% 3 3 4 3 4 6% 40%	2090
November	1st to 10th	Day	14% 8% 2 5% 4 3 4 3 4 6% 48%	1877
	11th to 20th	Day	15% 13% 3 6% 4 4 4 4 7% 6% 34%	1800
	21st to end	Day	18% 14% 4 7% 4 3 4 3 5% 6% 32%	1685
December	1st to 10th	Day	21% 13% 4 16% 5% 4 3 3 3 4 23%	2600
	11th to 20th	Day	24% 15% 5% 7% 4 4 4 3 5% 5% 24%	3059
	21st to end	Day	19% 16% 5% 10% 5% 5% 5% 4 6% 6% 19%	4466
January	1st to 10th	Night	18% 20% 7% 17% 4 3 3 2 3 3 19%	5889
	11th to 20th	Night	20% 16% 6% 14% 7% 4 3 2 3 4 19%	5900
	21st to end	Night	22% 13% 4 9% 5% 4 4 3 3 4 29%	6490
February	1st to 10th	Night	21% 14% 4 8% 5% 4 3 3 4 6% 27%	2600
	11th to 20th	Night	22% 14% 4 9% 5% 4 4 3 4 5% 24%	2468
	21st to end	Night	23% 14% 3 8% 4 3 4 4 4 5% 28%	1970
March	1st to 10th	Night	24% 12% 2 6% 5% 4 4 3 4 5% 31%	3528
	11th to 20th	Night	16% 13% 5% 6% 4 4 4 2 4 7% 36%	3498
	21st to end	Night	18% 9% 3 6% 4 4 4 4 5% 5% 39%	3817
April	1st to 10th	Night	20% 11% 2 7% 4 3 4 3 4 6% 35%	2043
	11th to 20th	Night	16% 8% 2 6% 5% 4 3 3 6% 8% 40%	1843
	21st to end	Night	16% 7% 2 7% 5% 5% 4 3 6% 5% 39%	1710
May	1st to 10th	Night	28% 10% 3 7% 5% 5% 5% 3 5% 6% 25%	2813
	11th to 20th	Night	19% 9% 3 7% 4 4 4 3 5% 6% 35%	2812
	21st to end	Night	14% 8% 2 7% 5% 4 5% 4 6% 8% 37%	3079
June	1st to 10th	Night	11% 6% 2 6% 5% 3 6% 4 7% 8% 42%	1710
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	1710
	21st to end	Night	12% 6% 2 5% 5% 3 4 4 7% 10% 42%	1710
July	1st to 10th	Night	11% 6% 3 7% 5% 5% 5% 4 8% 10% 37%	2840
	11th to 20th	Night	12% 6% 2 7% 4 5% 6% 3 5% 8% 40%	2840
	21st to end	Night	13% 9% 3 7% 5% 5% 5% 4 6% 8% 34%	3087
August	1st to 10th	Night	13% 7% 3 8% 6% 5% 5% 4 7% 9% 32%	2825
	11th to 20th	Night	14% 7% 3 8% 5% 4 4 3 6% 7% 39%	2785
	21st to end	Night	7% 4 2 6% 5% 5% 4 4 5% 10% 48%	3282
September	1st to 10th	Night	12% 5% 2 7% 4 4 4 3 6% 7% 45%	1976
	11th to 20th	Night	12% 5% 1 5% 4 4 4 3 5% 6% 50%	2090
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 5% 44%	2090
October	1st to 10th	Night	14% 5% 2 6% 4 4 4 3 6% 7% 45%	2237
	11th to 20th	Night	15% 6% 2 7% 4 3 4 3 5% 6% 44%	2280
	21st to end	Night	22% 6% 2 5% 3 4 4 2 4 6% 41%	2508
November	1st to 10th	Night	13% 8% 2 6% 4 3 4 3 5% 5% 47%	2416
	11th to 20th	Night	19% 12% 3 6% 4 4 4 4 5% 6% 34%	2600
	21st to end	Night	19% 13% 3 7% 4 4 4 3 4 5% 33%	2710
December	1st to 10th	Night	19% 13% 4 15% 4 3 4 3 4 5% 25%	4580
	11th to 20th	Night	21% 16% 7% 9% 5% 4 3 3 3 4 25%	5131
	21st to end	Night	19% 16% 5% 10% 5% 5% 4 4 4 5% 22%	6490

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	73% 3 10% 10% 4	4057
	11th to 20th	Day	73% 3 5% 11% 7%	4060
	21st to end	Day	63% 2 8% 17% 8% 1	4466
February	1st to 10th	Day	62% 3 9% 19% 7% 1	1800
	11th to 20th	Day	64% 3 8% 16% 9% 1	1823
	21st to end	Day	59% 4 8% 18% 10% 1	1660
March	1st to 10th	Day	57% 4 8% 21% 10% 1	3545
	11th to 20th	Day	44% 4 8% 29% 14% 1	3491
	21st to end	Day	53% 4 7% 22% 11% 2	3815
April	1st to 10th	Day	50% 5% 10% 19% 13% 3	2137
	11th to 20th	Day	44% 4 11% 20% 18% 4	2301
	21st to end	Day	49% 5% 11% 21% 12% 2	2470
May	1st to 10th	Day	57% 5% 9% 20% 8% 1	4120
	11th to 20th	Day	50% 5% 11% 20% 13% 1	4085
	21st to end	Day	39% 8% 15% 22% 12% 4	4489
June	1st to 10th	Day	42% 6% 13% 22% 16% 1	2470
	11th to 20th	Day	43% 7% 13% 22% 13% 1	2470
	21st to end	Day	39% 7% 15% 23% 14% 1	2470
July	1st to 10th	Day	37% 8% 13% 25% 14% 3	4055
	11th to 20th	Day	38% 6% 12% 29% 13% 2	4051
	21st to end	Day	51% 6% 10% 17% 15% 1	4324
August	1st to 10th	Day	43% 8% 13% 26% 9%	4091
	11th to 20th	Day	41% 7% 10% 29% 13%	4083
	21st to end	Day	30% 6% 14% 29% 19% 2	4340
September	1st to 10th	Day	36% 5% 10% 22% 22% 4	2200
	11th to 20th	Day	39% 5% 9% 26% 19% 3	2090
	21st to end	Day	47% 4 8% 21% 18% 1	2090
October	1st to 10th	Day	30% 4 8% 24% 31% 3	3229
	11th to 20th	Day	37% 4 10% 25% 19% 6%	3140
	21st to end	Day	31% 3 8% 33% 20% 5%	3428
November	1st to 10th	Day	39% 4 8% 27% 18% 4	1877
	11th to 20th	Day	49% 4 9% 19% 17% 3	1800
	21st to end	Day	54% 4 9% 20% 14%	1685
December	1st to 10th	Day	66% 3 8% 19% 3 1	2600
	11th to 20th	Day	62% 4 10% 17% 7%	3055
	21st to end	Day	64% 5% 11% 16% 5%	4466
January	1st to 10th	Night	71% 3 7% 14% 4	5889
	11th to 20th	Night	69% 4 7% 13% 6% 1	5900
	21st to end	Night	61% 3 9% 19% 8% 1	6490
February	1st to 10th	Night	59% 3 10% 18% 9%	2600
	11th to 20th	Night	64% 3 7% 16% 10% 1	2470
	21st to end	Night	58% 4 8% 17% 10% 3	1970
March	1st to 10th	Night	54% 4 6% 23% 11% 1	3528
	11th to 20th	Night	49% 5% 8% 24% 13% 1	3498
	21st to end	Night	47% 5% 7% 24% 16% 2	3817
April	1st to 10th	Night	50% 5% 7% 20% 15% 2	2043
	11th to 20th	Night	43% 3 12% 20% 16% 6%	1843
	21st to end	Night	45% 4 11% 21% 16% 2	1710
May	1st to 10th	Night	61% 5% 7% 15% 10% 2	2840
	11th to 20th	Night	48% 7% 10% 23% 11% 1	2813
	21st to end	Night	42% 8% 13% 22% 10% 4	3087
June	1st to 10th	Night	37% 8% 11% 26% 15% 2	1710
	11th to 20th	Night	39% 9% 14% 20% 16% 2	1710
	21st to end	Night	36% 8% 15% 24% 15% 2	1710
July	1st to 10th	Night	40% 8% 12% 22% 16% 3	2795
	11th to 20th	Night	39% 8% 11% 27% 13% 2	2786
	21st to end	Night	45% 8% 10% 18% 16% 3	2988
August	1st to 10th	Night	45% 8% 14% 24% 8% 1	2814
	11th to 20th	Night	42% 7% 10% 21% 19% 1	2812
	21st to end	Night	31% 6% 12% 31% 18% 2	3282
September	1st to 10th	Night	36% 6% 13% 20% 19% 6%	1976
	11th to 20th	Night	34% 5% 10% 25% 23% 3	2090
	21st to end	Night	42% 5% 10% 23% 20% 1	2090
October	1st to 10th	Night	32% 4 9% 25% 24% 5%	3731
	11th to 20th	Night	35% 5% 9% 21% 23% 8%	3779
	21st to end	Night	33% 3 5% 28% 25% 6%	4120
November	1st to 10th	Night	38% 4 10% 21% 20% 6%	2416
	11th to 20th	Night	51% 4 7% 18% 17% 4	2600
	21st to end	Night	54% 4 8% 20% 13% 1	2710
December	1st to 10th	Night	61% 4 10% 19% 5% 1	4580
	11th to 20th	Night	64% 4 11% 15% 7%	5134
	21st to end	Night	63% 5% 9% 15% 7%	6490

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	34% 24% 25% 11% 5% 1	300
	11th to 20th	Day	42% 24% 27% 7% 1	300
	21st to end	Day	37% 25% 27% 8% 3 1	330
February	1st to 10th	Day	35% 23% 30% 8% 4	300
	11th to 20th	Day	52% 18% 24% 5% 1	300
	21st to end	Day	40% 19% 31% 9%	259
March	1st to 10th	Day	32% 18% 25% 19% 5%	500
	11th to 20th	Day	29% 23% 27% 14% 6% 1	464
	21st to end	Day	34% 16% 25% 20% 5%	506
April	1st to 10th	Day	33% 17% 23% 18% 8% 1	460
	11th to 20th	Day	27% 14% 27% 18% 10% 4	452
	21st to end	Day	30% 15% 27% 18% 8% 2	460
May	1st to 10th	Day	29% 13% 26% 18% 9% 5%	463
	11th to 20th	Day	23% 10% 28% 22% 11% 5%	487
	21st to end	Day	16% 8% 23% 25% 18% 9%	529
June	1st to 10th	Day	14% 9% 23% 29% 18% 7%	490
	11th to 20th	Day	17% 7% 22% 26% 19% 8%	490
	21st to end	Day	14% 5% 32% 27% 17% 5%	490
July	1st to 10th	Day	15% 4 19% 28% 19% 14%	485
	11th to 20th	Day	12% 4 20% 32% 21% 12%	484
	21st to end	Day	19% 4 17% 29% 20% 10%	532
August	1st to 10th	Day	13% 5% 19% 26% 24% 14%	486
	11th to 20th	Day	15% 6% 18% 28% 23% 10%	479
	21st to end	Day	11% 4 14% 31% 26% 14%	539
September	1st to 10th	Day	16% 5% 17% 24% 27% 11%	490
	11th to 20th	Day	13% 3 14% 28% 30% 12%	490
	21st to end	Day	25% 8% 19% 27% 15% 7%	490
October	1st to 10th	Day	17% 7% 23% 26% 21% 6%	490
	11th to 20th	Day	21% 12% 25% 23% 15% 5%	490
	21st to end	Day	27% 12% 25% 23% 11% 3	539
November	1st to 10th	Day	21% 10% 28% 24% 14% 3	415
	11th to 20th	Day	28% 17% 27% 18% 8% 2	300
	21st to end	Day	26% 20% 30% 20% 3 1	243
December	1st to 10th	Day	38% 16% 24% 17% 5% 1	200
	11th to 20th	Day	37% 16% 22% 19% 5% 1	215
	21st to end	Day	37% 15% 25% 17% 6%	330
January	1st to 10th	Night	33% 16% 26% 21% 4	800
	11th to 20th	Night	35% 17% 24% 21% 3	800
	21st to end	Night	31% 16% 29% 21% 3	880
February	1st to 10th	Night	31% 14% 26% 27% 3	800
	11th to 20th	Night	34% 19% 28% 17% 1	783
	21st to end	Night	33% 15% 29% 21% 2	660
March	1st to 10th	Night	30% 14% 24% 27% 5%	800
	11th to 20th	Night	30% 16% 25% 20% 8% 1	767
	21st to end	Night	28% 12% 30% 25% 5%	836
April	1st to 10th	Night	27% 13% 28% 26% 6% 1	760
	11th to 20th	Night	21% 10% 29% 28% 11% 1	719
	21st to end	Night	19% 8% 31% 30% 11% 1	570
May	1st to 10th	Night	23% 7% 24% 31% 14% 1	570
	11th to 20th	Night	20% 7% 25% 31% 16% 2	561
	21st to end	Night	15% 3 19% 33% 25% 5%	617
June	1st to 10th	Night	12% 4 18% 39% 24% 3	570
	11th to 20th	Night	12% 4 16% 38% 28% 3	570
	21st to end	Night	13% 5% 16% 41% 23% 2	570
July	1st to 10th	Night	14% 3 13% 34% 31% 5%	555
	11th to 20th	Night	12% 2 11% 35% 35% 4	552
	21st to end	Night	14% 3 11% 33% 33% 5%	624
August	1st to 10th	Night	11% 4 8% 38% 35% 4	567
	11th to 20th	Night	13% 5% 8% 37% 33% 4	564
	21st to end	Night	13% 3 8% 33% 37% 6%	720
September	1st to 10th	Night	13% 3 13% 36% 30% 5%	760
	11th to 20th	Night	13% 4 13% 29% 37% 5%	760
	21st to end	Night	19% 5% 19% 30% 24% 3	760
October	1st to 10th	Night	17% 5% 18% 32% 25% 4	760
	11th to 20th	Night	19% 9% 22% 32% 17% 1	760
	21st to end	Night	25% 8% 20% 33% 13% 1	836
November	1st to 10th	Night	18% 8% 22% 33% 17% 1	780
	11th to 20th	Night	28% 11% 23% 27% 11% 1	800
	21st to end	Night	28% 10% 24% 29% 8% 1	800
December	1st to 10th	Night	34% 13% 26% 22% 6%	800
	11th to 20th	Night	28% 15% 24% 28% 5%	800
	21st to end	Night	32% 10% 26% 28% 3	880

Page generated at 2025-03-09 08:08:44. Version ac07

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