Cruz Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cruz Observatory 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	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	126392
	Night	16% 6% 2 6% 4 4 4 3 5% 7% 42%	122652
Transparency	Day	41% 5% 12% 24% 16% 3	129983
	Night	41% 4 11% 24% 17% 3	127147
Seeing	Day	18% 10% 25% 24% 16% 7%	16046
	Night	18% 7% 27% 33% 13% 1	25373

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	12% 6% 2 7% 5% 5% 5% 4 5% 6% 44%	3597
	11th to 20th	Day	13% 9% 3 9% 5% 4 4 4 5% 5% 38%	3862
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 5% 6% 37%	4466
February	1st to 10th	Day	15% 9% 3 6% 4 5% 4 6% 6% 6% 37%	1800
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 5% 6% 39%	1834
	21st to end	Day	19% 9% 4 6% 4 4 4 4 4 6% 38%	1650
March	1st to 10th	Day	20% 5% 1 4 4 3 5% 3 5% 6% 44%	4678
	11th to 20th	Day	10% 7% 2 6% 5% 4 5% 4 6% 7% 44%	4980
	21st to end	Day	17% 7% 2 5% 5% 4 4 3 5% 6% 42%	5664
April	1st to 10th	Day	17% 6% 3 7% 5% 3 5% 3 6% 7% 39%	2400
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 4 5% 7% 43%	2374
	21st to end	Day	17% 5% 2 6% 4 3 5% 4 6% 7% 41%	2400
May	1st to 10th	Day	25% 6% 2 6% 3 4 4 4 6% 8% 31%	5833
	11th to 20th	Day	25% 7% 2 6% 4 3 4 3 5% 8% 32%	6318
	21st to end	Day	18% 5% 2 5% 4 4 4 4 6% 9% 38%	6949
June	1st to 10th	Day	12% 6% 2 5% 5% 5% 4 5% 7% 9% 41%	2800
	11th to 20th	Day	13% 6% 2 6% 4 4 6% 5% 6% 9% 39%	2800
	21st to end	Day	10% 4 2 6% 5% 4 5% 4 6% 10% 44%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 11% 41%	6360
	11th to 20th	Day	10% 6% 3 9% 5% 6% 6% 5% 8% 10% 32%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	6569
August	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 4 7% 7% 34%	4234
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 41%	3959
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 8% 49%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 5% 3 3 4 6% 46%	2400
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 5% 7% 50%	2376
	21st to end	Day	22% 7% 3 6% 3 3 4 3 5% 6% 38%	2026
October	1st to 10th	Day	17% 6% 1 6% 3 3 4 4 5% 6% 45%	1900
	11th to 20th	Day	17% 6% 2 5% 4 3 4 2 4 6% 47%	1900
	21st to end	Day	16% 6% 2 5% 4 2 4 4 5% 7% 45%	2090
November	1st to 10th	Day	12% 7% 2 5% 3 3 3 3 4 9% 51%	1766
	11th to 20th	Day	16% 5% 2 5% 4 4 4 3 5% 6% 47%	1789
	21st to end	Day	15% 6% 2 6% 4 3 5% 3 4 7% 44%	1600
December	1st to 10th	Day	14% 8% 3 6% 4 4 4 4 6% 6% 43%	2600
	11th to 20th	Day	21% 8% 2 4 4 3 3 3 5% 7% 40%	2986
	21st to end	Day	16% 7% 2 7% 5% 5% 5% 5% 6% 7% 35%	3960
January	1st to 10th	Night	12% 7% 2 7% 4 4 4 4 6% 7% 43%	6349
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 7% 45%	6020
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 43%	6490
February	1st to 10th	Night	17% 8% 3 5% 4 4 4 4 5% 8% 38%	2600
	11th to 20th	Night	17% 7% 3 6% 4 4 4 3 5% 5% 41%	2450
	21st to end	Night	16% 8% 3 7% 4 5% 4 4 5% 6% 39%	1980
March	1st to 10th	Night	17% 8% 2 6% 4 3 3 3 5% 5% 44%	5144
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 6% 52%	4979
	21st to end	Night	14% 7% 3 7% 4 4 4 3 5% 6% 44%	5266
April	1st to 10th	Night	18% 7% 2 6% 5% 4 4 3 5% 9% 37%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 6% 8% 42%	1975
	21st to end	Night	18% 6% 2 6% 3 3 4 3 5% 7% 44%	2000
May	1st to 10th	Night	24% 6% 3 7% 5% 5% 4 4 5% 8% 29%	4135
	11th to 20th	Night	25% 8% 3 6% 4 3 3 3 6% 8% 32%	3575
	21st to end	Night	21% 6% 2 6% 4 3 3 3 5% 8% 37%	3926
June	1st to 10th	Night	14% 6% 2 5% 4 5% 4 3 6% 8% 43%	1600
	11th to 20th	Night	14% 5% 1 6% 4 5% 4 5% 6% 11% 40%	1600
	21st to end	Night	10% 4 2 5% 5% 4 5% 5% 8% 10% 42%	1600
July	1st to 10th	Night	8% 6% 3 7% 6% 5% 5% 4 7% 10% 39%	3600
	11th to 20th	Night	10% 6% 3 9% 7% 6% 6% 5% 7% 10% 30%	3600
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 5% 9% 42%	4280
August	1st to 10th	Night	19% 5% 2 6% 5% 4 5% 4 7% 8% 35%	2915
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 6% 7% 41%	3057
	21st to end	Night	13% 5% 2 5% 4 5% 4 4 6% 8% 46%	3586
September	1st to 10th	Night	14% 5% 2 7% 3 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 5% 7% 50%	1981
	21st to end	Night	20% 7% 2 6% 4 5% 4 3 4 6% 40%	2151
October	1st to 10th	Night	20% 6% 3 6% 4 3 3 2 4 5% 44%	2280
	11th to 20th	Night	15% 6% 2 6% 3 4 3 3 4 7% 49%	2280
	21st to end	Night	17% 5% 2 5% 3 3 3 3 5% 7% 48%	2508
November	1st to 10th	Night	10% 5% 2 4 3 3 4 3 4 6% 54%	2532
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 4 6% 46%	2609
	21st to end	Night	14% 5% 2 5% 5% 4 4 3 6% 7% 45%	2800
December	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 43%	4580
	11th to 20th	Night	22% 6% 2 5% 3 3 4 3 4 6% 40%	5208
	21st to end	Night	19% 8% 3 7% 4 4 5% 4 5% 7% 35%	6996

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	41% 3 10% 26% 17% 3	3598
	11th to 20th	Day	47% 3 9% 20% 17% 5%	3896
	21st to end	Day	47% 4 11% 23% 15% 1	4466
February	1st to 10th	Day	45% 4 13% 25% 12% 2	1800
	11th to 20th	Day	45% 3 9% 25% 14% 3	1843
	21st to end	Day	48% 3 9% 23% 13% 4	1650
March	1st to 10th	Day	40% 3 9% 25% 16% 7%	4681
	11th to 20th	Day	38% 4 11% 27% 15% 5%	4980
	21st to end	Day	44% 3 9% 19% 21% 5%	5664
April	1st to 10th	Day	44% 4 10% 20% 17% 5%	2400
	11th to 20th	Day	40% 4 11% 23% 17% 4	2374
	21st to end	Day	41% 5% 10% 22% 18% 4	2400
May	1st to 10th	Day	51% 4 13% 20% 11% 2	5842
	11th to 20th	Day	51% 4 11% 22% 11% 1	6322
	21st to end	Day	42% 6% 13% 23% 15% 2	6949
June	1st to 10th	Day	38% 6% 14% 25% 16% 2	2800
	11th to 20th	Day	40% 6% 13% 25% 13% 3	2800
	21st to end	Day	35% 6% 16% 26% 17% 1	2800
July	1st to 10th	Day	36% 9% 14% 22% 17% 2	6290
	11th to 20th	Day	43% 9% 16% 22% 10% 1	6285
	21st to end	Day	38% 6% 11% 22% 22% 1	6381
August	1st to 10th	Day	47% 6% 11% 25% 10%	4221
	11th to 20th	Day	38% 7% 11% 28% 15% 1	3998
	21st to end	Day	33% 5% 10% 29% 20% 2	4312
September	1st to 10th	Day	40% 3 11% 21% 20% 5%	2400
	11th to 20th	Day	34% 4 9% 27% 23% 4	2376
	21st to end	Day	46% 4 8% 20% 19% 3	2026
October	1st to 10th	Day	37% 3 9% 19% 27% 5%	3160
	11th to 20th	Day	28% 3 12% 28% 22% 7%	3140
	21st to end	Day	25% 3 12% 28% 28% 3	3428
November	1st to 10th	Day	33% 3 13% 25% 21% 5%	1766
	11th to 20th	Day	38% 4 11% 24% 20% 2	1789
	21st to end	Day	41% 4 12% 30% 11% 2	1600
December	1st to 10th	Day	41% 3 17% 25% 11% 3	2600
	11th to 20th	Day	45% 3 13% 30% 9% 1	2986
	21st to end	Day	47% 5% 15% 19% 13% 1	3960
January	1st to 10th	Night	40% 4 9% 26% 17% 3	6349
	11th to 20th	Night	38% 3 12% 23% 17% 6%	6061
	21st to end	Night	42% 3 10% 26% 17% 2	6490
February	1st to 10th	Night	45% 3 14% 25% 11% 2	2600
	11th to 20th	Night	45% 4 9% 23% 18% 3	2452
	21st to end	Night	47% 3 9% 21% 16% 4	1980
March	1st to 10th	Night	41% 3 10% 25% 13% 8%	5154
	11th to 20th	Night	34% 4 13% 29% 14% 6%	4980
	21st to end	Night	42% 3 8% 19% 25% 3	5266
April	1st to 10th	Night	45% 4 10% 20% 17% 4	2000
	11th to 20th	Night	40% 4 11% 22% 20% 4	1975
	21st to end	Night	40% 5% 9% 22% 19% 5%	2000
May	1st to 10th	Night	53% 5% 10% 16% 12% 3	4142
	11th to 20th	Night	51% 5% 13% 20% 11% 1	3576
	21st to end	Night	45% 4 12% 22% 13% 3	3927
June	1st to 10th	Night	38% 6% 11% 23% 20% 2	1600
	11th to 20th	Night	38% 6% 15% 23% 16% 1	1600
	21st to end	Night	35% 6% 14% 25% 17% 2	1600
July	1st to 10th	Night	40% 8% 15% 19% 15% 4	3560
	11th to 20th	Night	47% 8% 15% 19% 10% 1	3552
	21st to end	Night	37% 7% 12% 22% 21% 1	4153
August	1st to 10th	Night	46% 5% 17% 21% 10%	2904
	11th to 20th	Night	41% 5% 12% 26% 15% 1	3084
	21st to end	Night	35% 5% 11% 29% 18% 2	3586
September	1st to 10th	Night	37% 5% 10% 22% 21% 4	2000
	11th to 20th	Night	34% 4 9% 25% 25% 3	1981
	21st to end	Night	45% 5% 8% 19% 20% 3	2151
October	1st to 10th	Night	44% 4 9% 21% 22% 2	3800
	11th to 20th	Night	27% 3 10% 28% 28% 4	3779
	21st to end	Night	26% 3 11% 33% 24% 3	4120
November	1st to 10th	Night	32% 4 12% 27% 22% 5%	2532
	11th to 20th	Night	40% 4 8% 25% 18% 5%	2609
	21st to end	Night	38% 4 13% 28% 16% 2	2800
December	1st to 10th	Night	40% 4 13% 30% 12% 1	4580
	11th to 20th	Night	45% 3 13% 24% 12% 3	5208
	21st to end	Night	49% 4 11% 19% 16% 1	6996

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	28% 18% 31% 19% 3 2	200
	11th to 20th	Day	26% 25% 32% 14% 3	265
	21st to end	Day	22% 17% 38% 17% 6% 1	330
February	1st to 10th	Day	24% 17% 35% 19% 5%	300
	11th to 20th	Day	27% 20% 38% 14% 2	381
	21st to end	Day	27% 14% 34% 21% 4	414
March	1st to 10th	Day	19% 19% 29% 26% 5%	500
	11th to 20th	Day	20% 18% 35% 19% 5% 2	500
	21st to end	Day	22% 15% 33% 21% 6% 2	550
April	1st to 10th	Day	24% 13% 33% 20% 8% 2	500
	11th to 20th	Day	17% 16% 30% 20% 11% 5%	492
	21st to end	Day	24% 11% 28% 21% 13% 3	500
May	1st to 10th	Day	24% 12% 27% 20% 13% 3	503
	11th to 20th	Day	21% 8% 26% 24% 13% 7%	527
	21st to end	Day	20% 5% 18% 23% 21% 12%	573
June	1st to 10th	Day	14% 5% 26% 26% 18% 10%	530
	11th to 20th	Day	17% 7% 22% 28% 19% 8%	530
	21st to end	Day	13% 3 23% 30% 21% 10%	530
July	1st to 10th	Day	12% 3 13% 27% 29% 16%	525
	11th to 20th	Day	6% 2 14% 32% 31% 16%	524
	21st to end	Day	14% 3 16% 31% 27% 8%	576
August	1st to 10th	Day	12% 4 15% 29% 27% 13%	526
	11th to 20th	Day	13% 4 13% 28% 27% 15%	519
	21st to end	Day	12% 4 14% 29% 24% 16%	583
September	1st to 10th	Day	18% 5% 17% 22% 23% 15%	530
	11th to 20th	Day	14% 3 15% 27% 25% 16%	522
	21st to end	Day	26% 8% 18% 26% 16% 7%	490
October	1st to 10th	Day	23% 5% 20% 24% 20% 8%	490
	11th to 20th	Day	17% 10% 24% 28% 17% 4	490
	21st to end	Day	17% 7% 32% 30% 11% 3	539
November	1st to 10th	Day	14% 10% 29% 28% 17% 3	398
	11th to 20th	Day	15% 15% 31% 28% 10% 1	389
	21st to end	Day	21% 20% 30% 17% 11% 2	200
December	1st to 10th	Day	24% 15% 36% 18% 6% 2	200
	11th to 20th	Day	26% 14% 37% 16% 8% 1	200
	21st to end	Day	20% 13% 35% 23% 10%	220
January	1st to 10th	Night	17% 13% 40% 26% 3	800
	11th to 20th	Night	23% 13% 31% 29% 4	800
	21st to end	Night	18% 10% 40% 29% 2	880
February	1st to 10th	Night	21% 12% 36% 29% 2	800
	11th to 20th	Night	23% 15% 39% 22% 2	783
	21st to end	Night	21% 15% 38% 23% 3	660
March	1st to 10th	Night	19% 12% 38% 27% 5%	800
	11th to 20th	Night	20% 13% 35% 26% 6% 1	800
	21st to end	Night	20% 8% 36% 29% 6%	880
April	1st to 10th	Night	22% 12% 37% 25% 5%	800
	11th to 20th	Night	19% 9% 31% 31% 9%	791
	21st to end	Night	20% 7% 31% 33% 9% 1	800
May	1st to 10th	Night	21% 6% 25% 37% 10% 1	643
	11th to 20th	Night	20% 5% 28% 36% 11%	394
	21st to end	Night	14% 2 18% 43% 21% 2	433
June	1st to 10th	Night	14% 3 20% 42% 21% 1	400
	11th to 20th	Night	15% 2 18% 46% 20%	400
	21st to end	Night	12% 3 15% 51% 20% 1	400
July	1st to 10th	Night	11% 3 14% 42% 27% 3	390
	11th to 20th	Night	7% 2 10% 45% 33% 3	388
	21st to end	Night	11% 2 11% 45% 28% 2	587
August	1st to 10th	Night	12% 2 6% 47% 31% 3	597
	11th to 20th	Night	13% 2 11% 40% 32% 3	698
	21st to end	Night	13% 2 12% 40% 29% 4	880
September	1st to 10th	Night	15% 4 17% 36% 25% 4	800
	11th to 20th	Night	14% 4 11% 39% 29% 4	793
	21st to end	Night	22% 3 16% 36% 21% 2	760
October	1st to 10th	Night	22% 4 18% 33% 19% 4	760
	11th to 20th	Night	17% 6% 26% 36% 14% 1	760
	21st to end	Night	20% 5% 31% 31% 13% 1	836
November	1st to 10th	Night	12% 4 31% 37% 14% 1	780
	11th to 20th	Night	18% 11% 28% 34% 9% 1	800
	21st to end	Night	16% 12% 32% 31% 9%	800
December	1st to 10th	Night	23% 10% 34% 27% 6% 1	800
	11th to 20th	Night	19% 9% 37% 29% 6%	800
	21st to end	Night	20% 9% 35% 32% 4	880

Page generated at 2025-09-16 01:49:15. Version ac07

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