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 2005-11-05 to 2024-10-14.

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	17% 10% 3 7% 5% 4 4 4 5% 7% 34%	100510
	Night	18% 11% 3 7% 5% 4 4 3 5% 6% 34%	97991
Transparency	Day	49% 5% 10% 22% 12% 2	100322
	Night	51% 5% 9% 20% 13% 2	97854
Seeing	Day	24% 11% 23% 22% 14% 6%	14818
	Night	23% 9% 21% 29% 16% 2	25115

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	20% 20% 9% 9% 4 3 3 3 3 6% 19%	2836
	11th to 20th	Day	19% 20% 9% 12% 8% 4 2 2 3 3 18%	2840
	21st to end	Day	24% 20% 5% 9% 6% 4 3 3 4 5% 19%	3124
February	1st to 10th	Day	21% 15% 5% 9% 6% 3 4 3 3 5% 27%	1710
	11th to 20th	Day	21% 15% 4 11% 4 4 4 3 4 4 26%	1720
	21st to end	Day	23% 15% 4 8% 4 3 3 3 4 4 29%	1579
March	1st to 10th	Day	22% 15% 4 7% 4 3 3 3 4 6% 29%	3435
	11th to 20th	Day	16% 8% 4 5% 5% 3 3 4 5% 7% 39%	3480
	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% 8% 2 6% 4 4 3 3 5% 6% 41%	1830
	21st to end	Day	23% 8% 2 6% 3 3 4 3 4 6% 38%	1980
November	1st to 10th	Day	14% 8% 2 5% 3 3 4 2 4 5% 49%	1781
	11th to 20th	Day	15% 13% 3 6% 4 4 4 4 6% 6% 35%	1710
	21st to end	Day	18% 13% 3 6% 4 3 4 3 5% 6% 33%	1601
December	1st to 10th	Day	27% 15% 5% 8% 5% 4 3 4 3 5% 21%	1520
	11th to 20th	Day	22% 17% 6% 7% 4 3 4 2 4 5% 27%	1951
	21st to end	Day	21% 19% 5% 9% 4 5% 4 5% 6% 6% 17%	3124
January	1st to 10th	Night	19% 23% 6% 10% 5% 3 4 3 4 4 20%	4109
	11th to 20th	Night	22% 16% 6% 12% 6% 4 3 3 3 4 20%	4120
	21st to end	Night	28% 17% 5% 9% 5% 3 3 2 3 3 22%	4532
February	1st to 10th	Night	21% 14% 4 8% 5% 4 3 3 4 6% 27%	2470
	11th to 20th	Night	23% 14% 4 9% 5% 4 4 3 4 5% 25%	2342
	21st to end	Night	24% 14% 3 8% 4 3 4 4 4 4 28%	1875
March	1st to 10th	Night	24% 12% 2 6% 5% 4 4 3 4 5% 31%	3418
	11th to 20th	Night	16% 13% 5% 6% 4 4 4 2 4 7% 35%	3480
	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	16% 6% 2 7% 4 3 4 3 4 6% 44%	2203
	21st to end	Night	24% 6% 2 5% 3 4 4 2 4 5% 40%	2376
November	1st to 10th	Night	13% 8% 2 6% 5% 4 4 3 5% 5% 47%	2292
	11th to 20th	Night	19% 12% 3 6% 4 4 4 4 5% 6% 35%	2470
	21st to end	Night	19% 12% 3 7% 4 4 4 3 4 5% 34%	2574
December	1st to 10th	Night	24% 16% 4 9% 4 3 4 3 4 5% 23%	2660
	11th to 20th	Night	21% 17% 6% 8% 4 4 3 3 3 5% 26%	3239
	21st to end	Night	22% 17% 5% 9% 5% 5% 3 3 4 5% 22%	4532

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	68% 4 10% 13% 6%	2837
	11th to 20th	Day	74% 3 5% 13% 5%	2840
	21st to end	Day	70% 2 6% 16% 5% 1	3124
February	1st to 10th	Day	62% 3 9% 18% 8% 1	1710
	11th to 20th	Day	63% 3 8% 16% 9% 1	1729
	21st to end	Day	60% 4 8% 17% 10% 1	1579
March	1st to 10th	Day	57% 4 8% 21% 9% 1	3435
	11th to 20th	Day	44% 4 8% 29% 14% 1	3480
	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	39% 4 11% 23% 20% 2	1943
	11th to 20th	Day	44% 4 9% 26% 16%	1831
	21st to end	Day	48% 4 7% 24% 15% 2	1980
November	1st to 10th	Day	39% 4 8% 27% 18% 4	1781
	11th to 20th	Day	48% 4 9% 19% 17% 3	1710
	21st to end	Day	52% 4 9% 20% 15%	1601
December	1st to 10th	Day	67% 2 7% 18% 5% 1	1520
	11th to 20th	Day	61% 2 9% 18% 8% 1	1947
	21st to end	Day	66% 5% 11% 14% 4	3124
January	1st to 10th	Night	69% 3 8% 14% 6% 1	4109
	11th to 20th	Night	69% 4 8% 15% 4	4120
	21st to end	Night	69% 3 7% 16% 5% 1	4532
February	1st to 10th	Night	59% 3 9% 18% 10%	2470
	11th to 20th	Night	62% 3 7% 17% 10% 1	2344
	21st to end	Night	59% 4 9% 16% 9% 3	1875
March	1st to 10th	Night	55% 5% 7% 22% 11% 1	3418
	11th to 20th	Night	49% 5% 8% 24% 13% 1	3480
	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	38% 5% 11% 25% 18% 3	2237
	11th to 20th	Night	41% 5% 9% 23% 21% 2	2207
	21st to end	Night	47% 4 5% 24% 17% 3	2376
November	1st to 10th	Night	39% 4 10% 21% 20% 6%	2292
	11th to 20th	Night	50% 4 7% 19% 17% 3	2470
	21st to end	Night	53% 4 8% 20% 14% 2	2574
December	1st to 10th	Night	64% 3 7% 18% 6% 1	2660
	11th to 20th	Night	62% 4 9% 16% 8%	3242
	21st to end	Night	65% 5% 8% 15% 6%	4532

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	33% 23% 26% 12% 5% 1	270
	11th to 20th	Day	42% 23% 27% 7% 1	270
	21st to end	Day	38% 24% 26% 9% 3 1	297
February	1st to 10th	Day	36% 20% 31% 9% 4	270
	11th to 20th	Day	49% 19% 25% 6% 1	270
	21st to end	Day	44% 17% 30% 9%	234
March	1st to 10th	Day	32% 17% 27% 19% 5%	460
	11th to 20th	Day	30% 23% 26% 14% 6% 1	460
	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% 27% 23% 15% 3	462
	21st to end	Day	29% 12% 23% 22% 11% 2	495
November	1st to 10th	Day	22% 9% 27% 24% 15% 3	379
	11th to 20th	Day	28% 18% 25% 19% 9% 2	270
	21st to end	Day	24% 18% 32% 21% 4 1	219
December	1st to 10th	Day	37% 17% 22% 17% 5% 1	180
	11th to 20th	Day	36% 17% 22% 20% 4 2	193
	21st to end	Day	38% 15% 26% 16% 6%	297
January	1st to 10th	Night	33% 16% 25% 22% 4	760
	11th to 20th	Night	34% 16% 24% 22% 3	760
	21st to end	Night	32% 16% 28% 21% 3	836
February	1st to 10th	Night	30% 13% 26% 28% 3	760
	11th to 20th	Night	34% 19% 28% 18% 1	743
	21st to end	Night	34% 15% 28% 21% 2	628
March	1st to 10th	Night	29% 14% 24% 27% 5%	760
	11th to 20th	Night	30% 17% 25% 20% 7% 1	760
	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% 8% 22% 33% 16% 1	735
	21st to end	Night	27% 8% 20% 32% 12% 1	792
November	1st to 10th	Night	18% 8% 22% 33% 18% 1	740
	11th to 20th	Night	28% 11% 22% 27% 11% 1	760
	21st to end	Night	27% 10% 24% 30% 9% 1	760
December	1st to 10th	Night	34% 13% 25% 23% 5%	760
	11th to 20th	Night	28% 15% 24% 28% 5%	760
	21st to end	Night	33% 11% 25% 28% 3	836

Page generated at 2024-10-11 10:49:56. Version ac07

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