Holiday
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Holiday clear sky chart from 2005-11-06 to 2025-09-19.

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	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 42%	126115
	Night	8% 5% 2 6% 4 5% 5% 4 7% 10% 43%	122928
Transparency	Day	34% 10% 18% 20% 14% 5%	129888
	Night	35% 8% 17% 20% 15% 5%	127224
Seeing	Day	8% 4 15% 26% 29% 19%	16575
	Night	8% 3 16% 33% 29% 10%	26581

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	8% 6% 2 6% 3 4 4 3 6% 8% 49%	4056
	11th to 20th	Day	19% 11% 3 8% 4 4 4 4 5% 7% 31%	4060
	21st to end	Day	12% 7% 2 5% 3 4 4 4 7% 9% 45%	4897
February	1st to 10th	Day	13% 5% 2 7% 4 3 5% 3 5% 8% 45%	2000
	11th to 20th	Day	9% 5% 1 5% 3 4 4 3 6% 7% 54%	1948
	21st to end	Day	8% 6% 1 5% 3 4 4 4 6% 8% 51%	1676
March	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 4 6% 9% 47%	4935
	11th to 20th	Day	6% 3 2 4 3 2 3 3 6% 9% 59%	4980
	21st to end	Day	8% 4 1 4 4 4 5% 4 6% 9% 50%	5465
April	1st to 10th	Day	6% 3 1 5% 4 4 5% 4 7% 9% 52%	2200
	11th to 20th	Day	8% 4 2 6% 4 4 4 4 7% 8% 49%	2176
	21st to end	Day	5% 2 1 4 4 3 5% 4 6% 9% 57%	2424
May	1st to 10th	Day	3 3 1 4 3 4 5% 4 7% 11% 57%	5862
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 10% 48%	5861
	21st to end	Day	4 3 1 3 3 4 4 4 8% 13% 53%	6433
June	1st to 10th	Day	10% 5% 3 7% 5% 6% 6% 7% 8% 10% 32%	2600
	11th to 20th	Day	5% 5% 2 6% 6% 6% 7% 8% 11% 14% 31%	2600
	21st to end	Day	5% 7% 3 8% 6% 6% 6% 6% 9% 13% 31%	2600
July	1st to 10th	Day	5% 5% 2 8% 6% 7% 9% 9% 12% 16% 21%	5900
	11th to 20th	Day	3 3 2 8% 7% 8% 9% 8% 12% 15% 25%	5900
	21st to end	Day	3 3 2 6% 6% 7% 8% 8% 11% 16% 29%	6424
August	1st to 10th	Day	7% 4 2 7% 6% 7% 7% 7% 10% 13% 30%	4152
	11th to 20th	Day	5% 3 2 5% 5% 5% 7% 6% 8% 14% 40%	3849
	21st to end	Day	6% 5% 2 8% 7% 7% 9% 7% 10% 14% 25%	3987
September	1st to 10th	Day	7% 6% 3 9% 7% 8% 8% 7% 11% 12% 23%	2200
	11th to 20th	Day	9% 6% 3 9% 6% 6% 7% 6% 9% 10% 29%	2178
	21st to end	Day	8% 5% 2 8% 5% 6% 7% 6% 8% 10% 34%	2090
October	1st to 10th	Day	9% 6% 3 8% 5% 6% 6% 4 7% 9% 36%	2090
	11th to 20th	Day	5% 4 1 5% 5% 4 5% 4 6% 9% 51%	1960
	21st to end	Day	7% 4 2 6% 4 4 3 4 7% 9% 50%	2090
November	1st to 10th	Day	8% 5% 1 5% 4 4 4 3 7% 7% 51%	1957
	11th to 20th	Day	8% 6% 2 6% 5% 4 4 4 5% 8% 48%	1996
	21st to end	Day	8% 5% 2 6% 4 4 4 5% 7% 9% 47%	1800
December	1st to 10th	Day	4 3 2 4 3 3 4 4 6% 8% 58%	2930
	11th to 20th	Day	8% 6% 2 7% 4 4 5% 4 5% 7% 47%	3373
	21st to end	Day	12% 8% 3 8% 5% 5% 4 4 6% 8% 38%	4466
January	1st to 10th	Night	8% 5% 3 6% 3 4 3 3 5% 8% 51%	5889
	11th to 20th	Night	19% 10% 4 8% 5% 5% 5% 3 6% 7% 27%	5900
	21st to end	Night	15% 6% 3 7% 4 4 4 4 6% 8% 40%	6057
February	1st to 10th	Night	11% 8% 2 6% 3 3 4 3 6% 7% 45%	2400
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 5% 8% 50%	2338
	21st to end	Night	7% 5% 3 7% 4 4 3 4 5% 8% 49%	1951
March	1st to 10th	Night	10% 7% 3 6% 5% 5% 5% 3 5% 7% 45%	4918
	11th to 20th	Night	5% 4 1 4 3 3 4 3 7% 10% 56%	4980
	21st to end	Night	9% 5% 2 6% 3 4 4 4 5% 8% 49%	5467
April	1st to 10th	Night	7% 3 2 5% 4 4 4 4 7% 10% 51%	2200
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 9% 52%	2173
	21st to end	Night	5% 3 1 4 4 3 4 3 4 7% 59%	1988
May	1st to 10th	Night	3 3 1 4 3 4 4 4 7% 10% 56%	4033
	11th to 20th	Night	8% 4 2 5% 4 4 4 3 6% 10% 51%	4032
	21st to end	Night	3 2 1 4 3 3 4 4 7% 10% 57%	4421
June	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 5% 6% 10% 41%	1800
	11th to 20th	Night	5% 5% 2 6% 5% 6% 6% 6% 9% 13% 38%	1800
	21st to end	Night	5% 5% 2 8% 5% 6% 6% 6% 9% 14% 33%	1800
July	1st to 10th	Night	4 5% 2 10% 7% 7% 9% 7% 10% 14% 23%	4058
	11th to 20th	Night	3 4 2 7% 6% 7% 7% 8% 12% 15% 28%	4057
	21st to end	Night	3 5% 2 9% 7% 8% 9% 8% 11% 14% 23%	4427
August	1st to 10th	Night	5% 4 2 6% 6% 7% 6% 6% 10% 13% 36%	2977
	11th to 20th	Night	3 3 1 5% 5% 5% 7% 6% 9% 18% 39%	3200
	21st to end	Night	6% 6% 3 9% 6% 7% 9% 6% 9% 13% 27%	3875
September	1st to 10th	Night	6% 6% 3 9% 7% 7% 8% 7% 9% 13% 26%	2200
	11th to 20th	Night	9% 7% 3 8% 6% 6% 7% 5% 9% 10% 30%	2179
	21st to end	Night	8% 6% 2 7% 6% 5% 5% 4 8% 9% 39%	2090
October	1st to 10th	Night	9% 6% 3 6% 5% 5% 5% 5% 7% 9% 39%	2090
	11th to 20th	Night	5% 5% 1 4 4 4 4 3 7% 8% 54%	2218
	21st to end	Night	7% 6% 2 6% 4 4 4 3 6% 8% 50%	2508
November	1st to 10th	Night	8% 4 2 5% 4 4 4 3 5% 7% 52%	2341
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 6% 8% 49%	2400
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 7% 8% 47%	2599
December	1st to 10th	Night	5% 3 1 4 2 3 4 4 6% 8% 61%	4250
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 51%	4822
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	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	32% 5% 10% 16% 25% 12%	4057
	11th to 20th	Day	51% 4 6% 17% 15% 6%	4060
	21st to end	Day	33% 5% 11% 25% 19% 6%	4897
February	1st to 10th	Day	37% 6% 11% 16% 19% 11%	2000
	11th to 20th	Day	30% 5% 11% 19% 26% 10%	1958
	21st to end	Day	28% 6% 14% 18% 22% 11%	1676
March	1st to 10th	Day	32% 7% 13% 22% 19% 7%	4935
	11th to 20th	Day	22% 5% 10% 25% 21% 16%	4980
	21st to end	Day	28% 5% 10% 30% 21% 6%	5465
April	1st to 10th	Day	26% 9% 15% 21% 21% 7%	2200
	11th to 20th	Day	31% 9% 12% 17% 24% 9%	2176
	21st to end	Day	23% 7% 13% 25% 27% 6%	2424
May	1st to 10th	Day	19% 9% 23% 29% 15% 4	5897
	11th to 20th	Day	29% 8% 18% 29% 14% 1	5865
	21st to end	Day	21% 8% 20% 34% 15% 1	6447
June	1st to 10th	Day	41% 13% 19% 20% 6%	2600
	11th to 20th	Day	37% 17% 23% 20% 4	2600
	21st to end	Day	40% 17% 24% 15% 4	2600
July	1st to 10th	Day	45% 14% 36% 5% 1	5835
	11th to 20th	Day	42% 15% 36% 7%	5831
	21st to end	Day	38% 16% 28% 18%	6240
August	1st to 10th	Day	40% 15% 28% 14% 3	4139
	11th to 20th	Day	32% 13% 29% 23% 4	3885
	21st to end	Day	43% 16% 27% 14% 1	3987
September	1st to 10th	Day	46% 19% 21% 11% 2	2200
	11th to 20th	Day	45% 14% 22% 14% 5%	2178
	21st to end	Day	40% 13% 15% 21% 9% 3	2090
October	1st to 10th	Day	55% 11% 11% 10% 7% 5%	3480
	11th to 20th	Day	23% 6% 7% 22% 35% 7%	3239
	21st to end	Day	26% 6% 8% 24% 29% 7%	3428
November	1st to 10th	Day	29% 7% 11% 24% 20% 10%	1957
	11th to 20th	Day	33% 7% 11% 19% 21% 9%	1996
	21st to end	Day	32% 8% 10% 21% 21% 8%	1800
December	1st to 10th	Day	23% 4 16% 26% 22% 8%	2930
	11th to 20th	Day	34% 6% 11% 18% 21% 10%	3370
	21st to end	Day	43% 5% 12% 20% 15% 5%	4466
January	1st to 10th	Night	32% 5% 8% 17% 31% 7%	5889
	11th to 20th	Night	55% 5% 9% 14% 14% 3	5900
	21st to end	Night	41% 5% 10% 19% 17% 7%	6057
February	1st to 10th	Night	37% 6% 11% 17% 19% 9%	2400
	11th to 20th	Night	33% 5% 11% 19% 23% 9%	2339
	21st to end	Night	32% 6% 14% 18% 23% 7%	1951
March	1st to 10th	Night	38% 6% 11% 19% 17% 8%	4918
	11th to 20th	Night	22% 7% 16% 21% 18% 17%	4980
	21st to end	Night	32% 5% 10% 26% 22% 5%	5467
April	1st to 10th	Night	27% 7% 18% 24% 18% 6%	2200
	11th to 20th	Night	28% 6% 14% 22% 22% 7%	2173
	21st to end	Night	25% 6% 12% 28% 25% 5%	1988
May	1st to 10th	Night	20% 8% 23% 32% 13% 3	4060
	11th to 20th	Night	28% 8% 18% 28% 17% 2	4033
	21st to end	Night	19% 9% 20% 37% 15% 1	4429
June	1st to 10th	Night	36% 11% 23% 24% 6%	1800
	11th to 20th	Night	34% 13% 27% 22% 4	1800
	21st to end	Night	38% 16% 24% 16% 6%	1800
July	1st to 10th	Night	47% 14% 33% 5% 1	4015
	11th to 20th	Night	39% 15% 39% 6%	4006
	21st to end	Night	44% 15% 28% 13%	4297
August	1st to 10th	Night	35% 14% 34% 13% 4	2966
	11th to 20th	Night	27% 15% 34% 24% 1	3230
	21st to end	Night	45% 14% 28% 11% 3	3875
September	1st to 10th	Night	44% 18% 25% 11% 2	2200
	11th to 20th	Night	44% 15% 21% 16% 3	2179
	21st to end	Night	39% 11% 18% 20% 9% 2	2090
October	1st to 10th	Night	56% 10% 11% 11% 7% 4	3480
	11th to 20th	Night	22% 6% 9% 26% 33% 4	3678
	21st to end	Night	30% 5% 9% 21% 29% 5%	4120
November	1st to 10th	Night	29% 7% 12% 21% 21% 10%	2341
	11th to 20th	Night	34% 6% 10% 21% 21% 9%	2400
	21st to end	Night	32% 7% 11% 23% 18% 9%	2599
December	1st to 10th	Night	22% 4 18% 28% 17% 11%	4250
	11th to 20th	Night	33% 5% 12% 20% 20% 9%	4824
	21st to end	Night	44% 6% 13% 18% 15% 3	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	9% 7% 38% 30% 15% 1	300
	11th to 20th	Day	8% 13% 27% 33% 16% 3	300
	21st to end	Day	15% 10% 32% 32% 10%	330
February	1st to 10th	Day	9% 7% 24% 35% 22% 3	300
	11th to 20th	Day	8% 8% 27% 33% 18% 6%	300
	21st to end	Day	5% 6% 29% 35% 21% 4	275
March	1st to 10th	Day	6% 5% 33% 38% 17% 1	500
	11th to 20th	Day	10% 6% 27% 31% 23% 3	500
	21st to end	Day	7% 4 25% 38% 23% 3	550
April	1st to 10th	Day	7% 6% 26% 36% 23% 2	500
	11th to 20th	Day	7% 5% 23% 35% 25% 5%	492
	21st to end	Day	5% 3 15% 34% 36% 7%	500
May	1st to 10th	Day	6% 3 17% 33% 31% 11%	503
	11th to 20th	Day	7% 2 14% 27% 34% 16%	527
	21st to end	Day	7% 1 6% 22% 37% 26%	573
June	1st to 10th	Day	12% 1 7% 21% 36% 24%	530
	11th to 20th	Day	6% 2 7% 16% 39% 30%	530
	21st to end	Day	4 1 3 16% 31% 45%	530
July	1st to 10th	Day	4 2 3 11% 29% 51%	525
	11th to 20th	Day	5% 2 10% 32% 50%	524
	21st to end	Day	7% 1 3 14% 32% 43%	576
August	1st to 10th	Day	6% 1 4 13% 31% 46%	526
	11th to 20th	Day	7% 1 3 9% 29% 51%	519
	21st to end	Day	9% 2 4 14% 33% 38%	583
September	1st to 10th	Day	7% 2 5% 17% 33% 37%	530
	11th to 20th	Day	10% 2 4 19% 37% 27%	522
	21st to end	Day	8% 2 5% 17% 39% 28%	490
October	1st to 10th	Day	12% 2 12% 29% 34% 11%	490
	11th to 20th	Day	5% 3 16% 29% 34% 13%	490
	21st to end	Day	8% 3 12% 34% 36% 7%	539
November	1st to 10th	Day	8% 3 17% 32% 34% 7%	495
	11th to 20th	Day	11% 5% 17% 36% 27% 3	496
	21st to end	Day	7% 5% 24% 33% 25% 6%	300
December	1st to 10th	Day	5% 8% 25% 33% 24% 5%	300
	11th to 20th	Day	13% 2 27% 34% 20% 3	300
	21st to end	Day	8% 8% 23% 31% 26% 4	330
January	1st to 10th	Night	13% 6% 27% 37% 17% 1	800
	11th to 20th	Night	12% 6% 24% 39% 18% 2	800
	21st to end	Night	14% 7% 30% 36% 13%	880
February	1st to 10th	Night	13% 5% 24% 40% 18%	800
	11th to 20th	Night	10% 5% 28% 38% 17% 1	783
	21st to end	Night	9% 4 25% 45% 17%	660
March	1st to 10th	Night	6% 5% 29% 47% 12%	800
	11th to 20th	Night	8% 5% 23% 41% 22% 1	800
	21st to end	Night	7% 4 27% 43% 18% 1	880
April	1st to 10th	Night	7% 5% 21% 45% 21% 1	800
	11th to 20th	Night	7% 4 23% 45% 20% 1	791
	21st to end	Night	6% 2 19% 46% 26% 1	730
May	1st to 10th	Night	4 1 10% 45% 36% 4	600
	11th to 20th	Night	6% 2 12% 37% 36% 6%	591
	21st to end	Night	5% 1 7% 34% 43% 10%	650
June	1st to 10th	Night	11% 2 5% 24% 45% 14%	600
	11th to 20th	Night	6% 1 5% 20% 43% 26%	600
	21st to end	Night	6% 1 2 14% 45% 32%	600
July	1st to 10th	Night	7% 1 2 7% 40% 42%	585
	11th to 20th	Night	4 2 8% 41% 45%	582
	21st to end	Night	5% 4 8% 43% 40%	657
August	1st to 10th	Night	6% 1 3 8% 46% 37%	597
	11th to 20th	Night	5% 1 3 7% 40% 44%	594
	21st to end	Night	9% 2 5% 17% 42% 26%	832
September	1st to 10th	Night	7% 2 6% 18% 45% 23%	800
	11th to 20th	Night	11% 2 7% 23% 43% 15%	793
	21st to end	Night	8% 2 7% 24% 46% 12%	760
October	1st to 10th	Night	12% 2 14% 34% 32% 5%	760
	11th to 20th	Night	6% 3 12% 36% 39% 4	760
	21st to end	Night	10% 3 17% 38% 30% 3	836
November	1st to 10th	Night	8% 2 20% 41% 26% 3	780
	11th to 20th	Night	9% 4 20% 43% 23% 2	800
	21st to end	Night	9% 4 22% 43% 21% 1	800
December	1st to 10th	Night	9% 3 22% 37% 27% 1	800
	11th to 20th	Night	10% 4 23% 39% 23% 2	800
	21st to end	Night	10% 5% 23% 39% 21% 1	880

Page generated at 2025-09-16 01:49:17. Version ac07

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