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 2024-11-05.

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% 7% 40%	101349
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 41%	97697
Transparency	Day	41% 5% 12% 24% 16% 3	104951
	Night	41% 4 12% 24% 16% 3	102183
Seeing	Day	18% 9% 25% 25% 16% 7%	14933
	Night	18% 7% 27% 34% 13% 1	24283

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	14% 8% 2 6% 4 4 4 4 4 6% 44%	2517
	11th to 20th	Day	14% 11% 4 9% 5% 4 4 4 6% 6% 33%	2718
	21st to end	Day	19% 10% 4 8% 5% 4 5% 3 5% 6% 33%	3124
February	1st to 10th	Day	15% 9% 3 6% 4 5% 4 5% 6% 6% 37%	1710
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 4 5% 7% 40%	1739
	21st to end	Day	19% 9% 4 6% 4 4 4 4 4 5% 37%	1570
March	1st to 10th	Day	25% 6% 2 5% 4 3 5% 3 5% 6% 37%	3262
	11th to 20th	Day	10% 6% 2 5% 5% 5% 5% 4 7% 8% 44%	3480
	21st to end	Day	21% 8% 2 5% 4 3 3 2 4 5% 43%	3958
April	1st to 10th	Day	16% 6% 3 6% 5% 3 5% 3 6% 7% 39%	2280
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 4 5% 7% 43%	2254
	21st to end	Day	18% 5% 2 6% 4 3 5% 4 6% 7% 40%	2280
May	1st to 10th	Day	28% 6% 2 6% 3 3 5% 4 5% 7% 31%	4067
	11th to 20th	Day	24% 8% 3 7% 5% 4 4 3 5% 7% 29%	4398
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	4837
June	1st to 10th	Day	12% 6% 2 5% 5% 5% 4 5% 7% 9% 40%	2660
	11th to 20th	Day	12% 6% 2 7% 4 4 6% 5% 6% 9% 40%	2660
	21st to end	Day	10% 4 2 6% 5% 4 5% 5% 6% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 6% 8% 10% 39%	4440
	11th to 20th	Day	9% 5% 2 7% 4 5% 6% 5% 8% 10% 40%	4440
	21st to end	Day	10% 7% 3 8% 6% 6% 5% 4 7% 7% 36%	4569
August	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 4 7% 7% 34%	4104
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 5% 7% 9% 41%	3835
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 8% 48%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 5% 3 3 4 6% 46%	2280
	11th to 20th	Day	13% 5% 2 4 4 3 4 3 5% 7% 50%	2280
	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% 50%	1713
	11th to 20th	Day	16% 5% 2 5% 4 4 4 3 5% 6% 47%	1700
	21st to end	Day	15% 6% 2 6% 5% 3 5% 3 4 7% 44%	1520
December	1st to 10th	Day	18% 9% 3 6% 4 4 4 4 6% 6% 36%	1520
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 6% 8% 39%	1906
	21st to end	Day	17% 7% 2 8% 6% 6% 5% 5% 6% 6% 33%	2772
January	1st to 10th	Night	15% 7% 2 7% 4 4 4 4 6% 7% 40%	4429
	11th to 20th	Night	18% 8% 2 6% 4 3 4 3 5% 6% 41%	4239
	21st to end	Night	22% 9% 2 6% 4 4 3 3 5% 5% 37%	4532
February	1st to 10th	Night	17% 8% 3 5% 4 4 4 4 5% 7% 38%	2470
	11th to 20th	Night	17% 7% 3 6% 4 4 4 3 5% 5% 41%	2325
	21st to end	Night	17% 8% 3 6% 4 5% 4 4 5% 6% 39%	1884
March	1st to 10th	Night	22% 10% 3 5% 4 3 3 2 5% 5% 38%	3574
	11th to 20th	Night	11% 6% 1 5% 5% 3 4 3 5% 6% 50%	3479
	21st to end	Night	15% 8% 3 6% 3 3 4 3 4 5% 46%	3672
April	1st to 10th	Night	18% 6% 2 6% 5% 4 4 3 5% 9% 37%	1900
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 6% 8% 43%	1875
	21st to end	Night	18% 6% 2 6% 3 3 4 3 5% 7% 44%	1900
May	1st to 10th	Night	25% 7% 3 6% 5% 5% 3 4 5% 7% 32%	2887
	11th to 20th	Night	25% 9% 3 7% 4 3 4 3 6% 6% 29%	2495
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 11% 42%	2738
June	1st to 10th	Night	14% 6% 2 5% 4 5% 4 3 6% 8% 44%	1520
	11th to 20th	Night	13% 5% 1 6% 4 5% 4 5% 6% 11% 41%	1520
	21st to end	Night	10% 5% 2 5% 5% 4 5% 5% 8% 10% 42%	1520
July	1st to 10th	Night	6% 5% 2 7% 6% 6% 5% 4 8% 9% 41%	2520
	11th to 20th	Night	8% 5% 3 8% 6% 6% 6% 5% 8% 10% 37%	2520
	21st to end	Night	14% 7% 2 6% 5% 4 4 4 6% 8% 38%	2981
August	1st to 10th	Night	20% 5% 2 6% 5% 4 5% 4 7% 8% 35%	2825
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 6% 7% 42%	2962
	21st to end	Night	13% 5% 2 5% 4 5% 4 4 6% 8% 46%	3476
September	1st to 10th	Night	14% 5% 2 7% 3 4 4 3 5% 7% 47%	1900
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 7% 52%	1900
	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	11% 5% 2 5% 3 3 4 3 5% 6% 53%	2465
	11th to 20th	Night	15% 8% 2 5% 4 3 4 3 4 6% 46%	2478
	21st to end	Night	14% 5% 2 5% 5% 4 4 3 5% 7% 45%	2660
December	1st to 10th	Night	19% 7% 2 6% 5% 4 4 4 6% 6% 37%	2660
	11th to 20th	Night	18% 5% 2 6% 4 4 5% 4 5% 6% 41%	3288
	21st to end	Night	17% 7% 3 7% 4 5% 5% 5% 6% 8% 34%	4884

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	42% 3 13% 27% 11% 3	2518
	11th to 20th	Day	50% 3 11% 19% 14% 3	2718
	21st to end	Day	52% 4 11% 22% 9% 1	3124
February	1st to 10th	Day	45% 4 13% 25% 11% 2	1710
	11th to 20th	Day	44% 3 10% 26% 14% 3	1748
	21st to end	Day	49% 3 9% 22% 13% 4	1570
March	1st to 10th	Day	48% 4 9% 27% 10% 3	3265
	11th to 20th	Day	36% 4 10% 28% 18% 4	3480
	21st to end	Day	46% 3 9% 21% 18% 3	3958
April	1st to 10th	Day	43% 4 11% 20% 17% 5%	2280
	11th to 20th	Day	39% 4 11% 24% 17% 5%	2254
	21st to end	Day	42% 5% 10% 22% 18% 4	2280
May	1st to 10th	Day	53% 3 10% 21% 11% 2	4079
	11th to 20th	Day	54% 4 14% 18% 8% 1	4402
	21st to end	Day	34% 5% 15% 26% 17% 2	4837
June	1st to 10th	Day	38% 7% 14% 25% 15% 2	2660
	11th to 20th	Day	39% 6% 13% 25% 13% 3	2660
	21st to end	Day	34% 6% 16% 26% 17% 1	2660
July	1st to 10th	Day	36% 8% 16% 22% 14% 3	4370
	11th to 20th	Day	36% 8% 16% 26% 14% 1	4365
	21st to end	Day	44% 6% 11% 20% 17% 1	4423
August	1st to 10th	Day	47% 6% 11% 25% 10%	4091
	11th to 20th	Day	37% 6% 11% 29% 16% 1	3874
	21st to end	Day	34% 6% 10% 28% 20% 2	4180
September	1st to 10th	Day	41% 3 11% 21% 20% 5%	2280
	11th to 20th	Day	33% 4 9% 27% 23% 4	2280
	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	34% 3 13% 26% 20% 4	1713
	11th to 20th	Day	38% 4 11% 23% 20% 2	1700
	21st to end	Day	41% 4 11% 31% 11% 2	1520
December	1st to 10th	Day	47% 4 16% 20% 12% 1	1520
	11th to 20th	Day	44% 3 12% 30% 10% 1	1906
	21st to end	Day	49% 5% 15% 20% 10% 1	2772
January	1st to 10th	Night	43% 5% 11% 27% 11% 3	4429
	11th to 20th	Night	44% 4 12% 25% 13% 2	4239
	21st to end	Night	49% 3 8% 28% 10% 2	4532
February	1st to 10th	Night	45% 3 13% 25% 12% 2	2470
	11th to 20th	Night	44% 4 9% 22% 18% 3	2327
	21st to end	Night	47% 3 10% 21% 15% 5%	1884
March	1st to 10th	Night	48% 3 10% 24% 13% 1	3584
	11th to 20th	Night	35% 4 11% 31% 17% 3	3480
	21st to end	Night	42% 3 7% 21% 23% 4	3672
April	1st to 10th	Night	44% 4 10% 21% 17% 4	1900
	11th to 20th	Night	39% 4 11% 22% 20% 4	1875
	21st to end	Night	40% 5% 9% 22% 18% 5%	1900
May	1st to 10th	Night	53% 4 9% 19% 13% 2	2894
	11th to 20th	Night	55% 5% 14% 14% 10% 2	2496
	21st to end	Night	37% 5% 16% 24% 14% 4	2739
June	1st to 10th	Night	37% 7% 11% 24% 20% 2	1520
	11th to 20th	Night	37% 6% 15% 23% 17% 2	1520
	21st to end	Night	35% 6% 14% 26% 17% 2	1520
July	1st to 10th	Night	39% 8% 17% 18% 14% 5%	2480
	11th to 20th	Night	40% 7% 15% 23% 14% 1	2472
	21st to end	Night	42% 7% 14% 18% 18% 1	2886
August	1st to 10th	Night	47% 5% 17% 22% 9%	2814
	11th to 20th	Night	40% 5% 12% 26% 15% 1	2989
	21st to end	Night	35% 5% 11% 29% 18% 2	3476
September	1st to 10th	Night	38% 5% 10% 21% 21% 4	1900
	11th to 20th	Night	32% 4 9% 26% 25% 4	1900
	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% 3 12% 27% 21% 5%	2465
	11th to 20th	Night	39% 4 9% 25% 18% 5%	2478
	21st to end	Night	38% 4 13% 28% 16% 2	2660
December	1st to 10th	Night	47% 4 12% 24% 13% 1	2660
	11th to 20th	Night	43% 4 13% 23% 15% 3	3288
	21st to end	Night	47% 5% 15% 23% 10% 1	4884

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% 16% 33% 21% 3 2	180
	11th to 20th	Day	26% 22% 33% 16% 3	238
	21st to end	Day	22% 16% 36% 18% 6% 1	297
February	1st to 10th	Day	23% 16% 35% 20% 5%	270
	11th to 20th	Day	23% 20% 41% 14% 2	346
	21st to end	Day	28% 14% 32% 21% 4	382
March	1st to 10th	Day	19% 18% 30% 26% 6%	460
	11th to 20th	Day	20% 18% 35% 19% 5% 2	460
	21st to end	Day	23% 13% 33% 23% 6% 2	506
April	1st to 10th	Day	22% 12% 34% 21% 9% 2	460
	11th to 20th	Day	16% 15% 30% 21% 12% 5%	452
	21st to end	Day	24% 11% 29% 22% 13% 2	460
May	1st to 10th	Day	25% 12% 25% 21% 14% 3	463
	11th to 20th	Day	21% 8% 25% 25% 14% 7%	487
	21st to end	Day	18% 5% 18% 24% 23% 12%	529
June	1st to 10th	Day	15% 5% 26% 27% 18% 10%	490
	11th to 20th	Day	15% 7% 21% 29% 20% 8%	490
	21st to end	Day	13% 3 24% 30% 20% 10%	490
July	1st to 10th	Day	12% 3 13% 27% 29% 16%	485
	11th to 20th	Day	5% 2 13% 33% 32% 15%	484
	21st to end	Day	15% 3 17% 31% 27% 8%	532
August	1st to 10th	Day	12% 4 14% 28% 28% 13%	486
	11th to 20th	Day	12% 4 14% 29% 26% 16%	479
	21st to end	Day	12% 4 13% 29% 25% 16%	539
September	1st to 10th	Day	18% 4 17% 21% 23% 16%	490
	11th to 20th	Day	13% 3 16% 26% 25% 16%	490
	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	15% 9% 27% 28% 17% 3	381
	11th to 20th	Day	16% 15% 30% 28% 11% 1	360
	21st to end	Day	19% 17% 32% 19% 11% 2	180
December	1st to 10th	Day	24% 15% 33% 19% 6% 2	180
	11th to 20th	Day	24% 14% 37% 16% 8% 1	180
	21st to end	Day	21% 12% 35% 22% 10% 1	198
January	1st to 10th	Night	17% 11% 41% 27% 4	760
	11th to 20th	Night	24% 12% 31% 29% 4	760
	21st to end	Night	19% 10% 40% 29% 2	836
February	1st to 10th	Night	21% 11% 36% 30% 2	760
	11th to 20th	Night	21% 15% 40% 23% 2	743
	21st to end	Night	22% 14% 38% 23% 3	628
March	1st to 10th	Night	19% 11% 39% 27% 5%	760
	11th to 20th	Night	20% 13% 35% 25% 6% 1	760
	21st to end	Night	20% 8% 36% 29% 6%	836
April	1st to 10th	Night	21% 12% 37% 26% 5%	760
	11th to 20th	Night	19% 9% 31% 32% 9%	751
	21st to end	Night	20% 7% 31% 33% 9% 1	760
May	1st to 10th	Night	22% 6% 25% 37% 10% 1	611
	11th to 20th	Night	20% 5% 27% 36% 12%	374
	21st to end	Night	13% 2 18% 44% 22% 2	411
June	1st to 10th	Night	14% 3 19% 42% 21% 1	380
	11th to 20th	Night	13% 2 18% 46% 21%	380
	21st to end	Night	12% 3 15% 52% 18% 1	380
July	1st to 10th	Night	10% 3 14% 42% 27% 3	370
	11th to 20th	Night	7% 2 10% 46% 33% 3	368
	21st to end	Night	12% 2 11% 45% 28% 2	557
August	1st to 10th	Night	12% 1 6% 48% 30% 3	567
	11th to 20th	Night	13% 2 11% 40% 32% 3	663
	21st to end	Night	13% 2 11% 40% 29% 4	836
September	1st to 10th	Night	15% 4 17% 36% 24% 4	760
	11th to 20th	Night	14% 4 10% 39% 30% 4	760
	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	13% 4 31% 37% 15% 1	760
	11th to 20th	Night	18% 11% 28% 34% 9% 1	760
	21st to end	Night	16% 10% 33% 32% 10%	760
December	1st to 10th	Night	23% 10% 33% 27% 6% 1	760
	11th to 20th	Night	18% 9% 38% 29% 6%	760
	21st to end	Night	20% 8% 36% 33% 3	836

Page generated at 2024-11-03 02:11:29. Version ac07

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