Cal Poly Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cal Poly Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	5% 3 1 3 2 2 3 3 4 6% 68%	134464
	Night	6% 4 2 4 3 3 3 3 5% 6% 62%	132425
Transparency	Day	19% 4 8% 20% 27% 22%	129497
	Night	25% 5% 8% 19% 23% 21%	128370
Seeing	Day	8% 9% 24% 35% 20% 5%	16799
	Night	8% 4 14% 33% 33% 8%	26353

By Month

Percentage Cloud Cover SW (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	7% 3 2 3 2 3 3 3 4 7% 63%	3897
	11th to 20th	Day	9% 5% 1 3 3 2 3 3 4 5% 61%	4100
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 8% 46%	4510
February	1st to 10th	Day	16% 4 2 4 3 2 2 3 4 5% 54%	1871
	11th to 20th	Day	11% 5% 2 4 4 3 3 2 5% 7% 52%	1950
	21st to end	Day	12% 5% 2 5% 4 3 4 3 5% 6% 51%	1650
March	1st to 10th	Day	14% 6% 2 6% 4 3 4 4 6% 8% 43%	4666
	11th to 20th	Day	10% 5% 2 5% 3 3 3 2 4 6% 57%	5000
	21st to end	Day	11% 6% 2 5% 4 4 6% 4 7% 9% 42%	5722
April	1st to 10th	Day	9% 5% 2 4 3 3 3 3 4 7% 57%	2400
	11th to 20th	Day	7% 6% 1 4 3 2 3 3 5% 7% 58%	2369
	21st to end	Day	7% 3 2 4 3 3 3 3 5% 6% 62%	2400
May	1st to 10th	Day	4 3 1 2 1 1 2 1 3 5% 78%	5440
	11th to 20th	Day	1 3 1 4 2 3 3 3 4 7% 66%	5603
	21st to end	Day	2 2 1 2 1 2 2 2 3 6% 76%	6902
June	1st to 10th	Day	4 3 1 3 3 3 3 3 5% 9% 65%	2800
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 6% 78%	2800
	21st to end	Day	1 2 1 2 2 2 2 2 3 6% 80%	2800
July	1st to 10th	Day	1 1 1 1 1 2 2 4 88%	6319
	11th to 20th	Day	1 1 1 2 2 2 3 2 3 5% 79%	5887
	21st to end	Day	1 1 2 2 2 2 2 5% 6% 78%	6402
August	1st to 10th	Day	1 1 1 1 1 2 2 3 6% 81%	5527
	11th to 20th	Day	1 1 1 1 1 2 3 91%	5364
	21st to end	Day	1 1 1 1 2 2 4 5% 83%	5984
September	1st to 10th	Day	1 1 1 1 1 2 2 3 7% 79%	2400
	11th to 20th	Day	3 2 1 2 1 1 2 2 3 6% 76%	2398
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 74%	2198
October	1st to 10th	Day	2 2 1 1 2 2 3 2 4 4 77%	3211
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 4 77%	3161
	21st to end	Day	3 2 2 4 2 3 3 3 4 6% 68%	3448
November	1st to 10th	Day	5% 4 1 3 3 2 3 2 4 6% 67%	1887
	11th to 20th	Day	6% 3 1 3 2 3 3 2 4 6% 66%	1800
	21st to end	Day	9% 4 2 4 4 3 4 3 4 6% 58%	1800
December	1st to 10th	Day	6% 5% 2 5% 4 4 4 3 4 6% 57%	2766
	11th to 20th	Day	18% 5% 2 4 3 3 4 4 5% 6% 47%	3028
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 9% 41%	4004
January	1st to 10th	Night	8% 5% 2 4 2 3 3 3 4 6% 60%	6042
	11th to 20th	Night	9% 6% 2 4 2 2 3 3 4 5% 60%	5880
	21st to end	Night	13% 7% 2 4 4 4 4 3 4 6% 48%	6468
February	1st to 10th	Night	14% 6% 2 4 3 3 2 3 4 5% 55%	2526
	11th to 20th	Night	10% 5% 1 4 3 3 3 3 5% 7% 55%	2338
	21st to end	Night	11% 7% 2 5% 3 3 3 2 3 5% 57%	1980
March	1st to 10th	Night	14% 9% 3 6% 4 4 4 4 6% 6% 41%	5212
	11th to 20th	Night	9% 6% 2 5% 3 3 3 3 4 6% 56%	5000
	21st to end	Night	14% 9% 3 5% 3 3 3 3 4 6% 48%	5253
April	1st to 10th	Night	7% 5% 2 4 3 3 3 2 5% 7% 59%	2000
	11th to 20th	Night	5% 6% 2 4 4 3 2 2 4 4 64%	1979
	21st to end	Night	5% 7% 2 3 2 3 2 3 4 6% 62%	2000
May	1st to 10th	Night	2 3 1 3 2 2 2 3 4 6% 72%	4560
	11th to 20th	Night	1 5% 4 8% 4 3 4 4 4 6% 57%	4314
	21st to end	Night	1 5% 2 6% 3 3 3 4 4 6% 64%	3972
June	1st to 10th	Night	2 4 2 5% 3 3 4 4 5% 8% 61%	1600
	11th to 20th	Night	1 3 1 4 2 2 3 3 4 7% 71%	1600
	21st to end	Night	1 2 1 3 2 1 2 2 3 6% 76%	1600
July	1st to 10th	Night	1 2 1 1 1 2 3 6% 81%	3640
	11th to 20th	Night	2 2 3 2 2 2 6% 9% 8% 64%	4088
	21st to end	Night	3 3 8% 3 3 4 5% 7% 7% 55%	4472
August	1st to 10th	Night	1 1 2 1 2 2 2 5% 6% 78%	4294
	11th to 20th	Night	1 1 1 1 1 2 3 5% 85%	4506
	21st to end	Night	1 2 2 1 2 3 5% 6% 77%	5015
September	1st to 10th	Night	2 2 1 2 2 2 2 2 4 4 77%	2000
	11th to 20th	Night	3 4 1 4 1 1 3 3 3 5% 73%	2000
	21st to end	Night	1 3 1 2 2 2 2 2 3 5% 76%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 3 3 5% 78%	3834
	11th to 20th	Night	2 2 1 2 2 3 3 3 4 6% 73%	3777
	21st to end	Night	4 4 1 3 2 2 2 2 4 6% 70%	4121
November	1st to 10th	Night	5% 4 2 2 3 3 2 3 5% 7% 66%	2410
	11th to 20th	Night	3 4 2 3 2 3 3 3 5% 7% 66%	2600
	21st to end	Night	6% 6% 2 3 4 3 4 4 5% 5% 58%	2600
December	1st to 10th	Night	7% 4 1 4 3 4 4 3 5% 6% 60%	4416
	11th to 20th	Night	16% 5% 2 4 4 3 4 4 6% 7% 45%	5176
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 7% 9% 36%	6952

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	21% 5% 9% 33% 21% 12%	3897
	11th to 20th	Day	25% 5% 8% 14% 21% 28%	4100
	21st to end	Day	33% 5% 9% 15% 23% 15%	4510
February	1st to 10th	Day	33% 3 6% 19% 22% 17%	1871
	11th to 20th	Day	29% 6% 10% 23% 23% 10%	1960
	21st to end	Day	32% 6% 9% 21% 24% 8%	1650
March	1st to 10th	Day	34% 7% 12% 17% 24% 5%	4666
	11th to 20th	Day	29% 5% 9% 33% 16% 9%	5000
	21st to end	Day	33% 8% 15% 26% 15% 3	5722
April	1st to 10th	Day	27% 6% 12% 27% 21% 8%	2400
	11th to 20th	Day	25% 6% 13% 26% 24% 7%	2372
	21st to end	Day	22% 6% 13% 27% 26% 7%	2400
May	1st to 10th	Day	14% 3 7% 31% 36% 10%	5440
	11th to 20th	Day	17% 3 11% 30% 31% 9%	5606
	21st to end	Day	12% 5% 10% 27% 39% 8%	6904
June	1st to 10th	Day	18% 6% 11% 28% 26% 11%	2800
	11th to 20th	Day	12% 3 10% 17% 36% 22%	2800
	21st to end	Day	12% 2 7% 16% 31% 33%	2800
July	1st to 10th	Day	5% 2 3 8% 24% 59%	6251
	11th to 20th	Day	10% 5% 4 7% 26% 49%	5817
	21st to end	Day	10% 3 4 21% 30% 32%	6217
August	1st to 10th	Day	12% 1 4 17% 33% 33%	4022
	11th to 20th	Day	6% 1 5% 9% 33% 45%	3880
	21st to end	Day	8% 1 5% 16% 31% 39%	4311
September	1st to 10th	Day	10% 2 7% 15% 31% 35%	2400
	11th to 20th	Day	14% 2 7% 20% 37% 19%	2398
	21st to end	Day	13% 4 7% 17% 34% 26%	2198
October	1st to 10th	Day	12% 3 5% 14% 25% 42%	3211
	11th to 20th	Day	13% 2 5% 21% 37% 21%	3161
	21st to end	Day	20% 2 3 14% 34% 27%	3448
November	1st to 10th	Day	20% 4 8% 19% 29% 20%	1887
	11th to 20th	Day	20% 5% 8% 21% 27% 20%	1800
	21st to end	Day	27% 5% 6% 17% 25% 21%	1800
December	1st to 10th	Day	28% 3 5% 15% 29% 20%	2766
	11th to 20th	Day	38% 3 9% 19% 21% 10%	3028
	21st to end	Day	38% 5% 12% 26% 12% 6%	4004
January	1st to 10th	Night	25% 4 9% 26% 25% 11%	6042
	11th to 20th	Night	27% 6% 6% 16% 19% 26%	5880
	21st to end	Night	37% 4 7% 14% 20% 17%	6468
February	1st to 10th	Night	32% 3 8% 17% 23% 16%	2526
	11th to 20th	Night	28% 5% 9% 23% 23% 13%	2339
	21st to end	Night	30% 5% 8% 22% 25% 10%	1980
March	1st to 10th	Night	38% 7% 10% 17% 22% 6%	5212
	11th to 20th	Night	29% 5% 13% 27% 18% 8%	5000
	21st to end	Night	36% 6% 11% 26% 17% 3	5253
April	1st to 10th	Night	26% 5% 13% 27% 19% 10%	2000
	11th to 20th	Night	25% 3 12% 28% 23% 9%	1979
	21st to end	Night	25% 4 12% 27% 26% 7%	2000
May	1st to 10th	Night	16% 4 10% 25% 31% 14%	4557
	11th to 20th	Night	27% 3 14% 22% 24% 10%	4316
	21st to end	Night	21% 5% 11% 22% 33% 9%	3972
June	1st to 10th	Night	23% 6% 13% 22% 24% 12%	1600
	11th to 20th	Night	17% 3 8% 18% 29% 24%	1600
	21st to end	Night	14% 2 10% 14% 28% 32%	1600
July	1st to 10th	Night	7% 3 3 8% 18% 61%	3600
	11th to 20th	Night	14% 15% 4 8% 15% 44%	4035
	21st to end	Night	27% 6% 4 14% 26% 22%	4345
August	1st to 10th	Night	14% 1 6% 18% 28% 33%	3116
	11th to 20th	Night	7% 2 6% 11% 24% 50%	3256
	21st to end	Night	10% 4 6% 14% 25% 41%	3608
September	1st to 10th	Night	13% 2 7% 14% 25% 40%	2000
	11th to 20th	Night	18% 2 9% 17% 29% 25%	2000
	21st to end	Night	13% 4 7% 16% 33% 26%	2200
October	1st to 10th	Night	11% 2 5% 12% 24% 45%	3834
	11th to 20th	Night	16% 2 5% 20% 33% 23%	3777
	21st to end	Night	19% 1 4 13% 34% 29%	4121
November	1st to 10th	Night	19% 4 9% 20% 26% 23%	2410
	11th to 20th	Night	20% 4 9% 19% 27% 21%	2600
	21st to end	Night	27% 5% 9% 16% 21% 22%	2600
December	1st to 10th	Night	26% 3 4 15% 30% 22%	4416
	11th to 20th	Night	38% 4 11% 17% 20% 10%	5176
	21st to end	Night	43% 6% 11% 24% 11% 5%	6952

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	23% 12% 31% 18% 14% 1	315
	11th to 20th	Day	14% 13% 30% 27% 16% 1	400
	21st to end	Day	11% 15% 30% 27% 14% 3	440
February	1st to 10th	Day	17% 12% 29% 26% 13% 3	436
	11th to 20th	Day	15% 15% 31% 26% 12% 2	495
	21st to end	Day	10% 18% 32% 29% 9% 2	414
March	1st to 10th	Day	17% 17% 34% 25% 6%	500
	11th to 20th	Day	17% 15% 30% 27% 9% 1	500
	21st to end	Day	15% 20% 31% 25% 7% 1	550
April	1st to 10th	Day	13% 13% 34% 25% 13% 2	500
	11th to 20th	Day	11% 16% 29% 29% 11% 2	492
	21st to end	Day	7% 14% 33% 29% 14% 4	500
May	1st to 10th	Day	9% 10% 33% 32% 12% 3	503
	11th to 20th	Day	5% 11% 28% 38% 14% 3	528
	21st to end	Day	9% 11% 26% 33% 17% 4	573
June	1st to 10th	Day	5% 6% 21% 44% 20% 5%	530
	11th to 20th	Day	3 6% 27% 38% 22% 4	530
	21st to end	Day	2 3 19% 42% 26% 9%	530
July	1st to 10th	Day	1 1 14% 46% 31% 7%	526
	11th to 20th	Day	2 7% 45% 33% 13%	524
	21st to end	Day	1 1 8% 44% 34% 12%	575
August	1st to 10th	Day	3 1 16% 45% 27% 8%	527
	11th to 20th	Day	3 1 12% 39% 36% 10%	517
	21st to end	Day	2 2 12% 45% 31% 9%	583
September	1st to 10th	Day	3 3 15% 41% 30% 9%	530
	11th to 20th	Day	4 6% 23% 39% 23% 5%	530
	21st to end	Day	3 6% 19% 39% 27% 5%	530
October	1st to 10th	Day	3 8% 24% 41% 19% 5%	503
	11th to 20th	Day	5% 7% 25% 39% 20% 5%	490
	21st to end	Day	5% 6% 26% 38% 21% 4	539
November	1st to 10th	Day	6% 10% 27% 31% 22% 4	423
	11th to 20th	Day	8% 12% 26% 33% 18% 3	300
	21st to end	Day	12% 7% 28% 30% 19% 4	300
December	1st to 10th	Day	13% 11% 27% 31% 13% 4	246
	11th to 20th	Day	14% 12% 26% 27% 20% 3	200
	21st to end	Day	21% 15% 28% 23% 10% 3	220
January	1st to 10th	Night	15% 7% 20% 32% 20% 6%	800
	11th to 20th	Night	12% 3 19% 38% 22% 6%	800
	21st to end	Night	15% 4 18% 40% 19% 5%	880
February	1st to 10th	Night	17% 5% 16% 35% 23% 5%	800
	11th to 20th	Night	13% 8% 18% 35% 22% 4	784
	21st to end	Night	15% 6% 20% 38% 18% 3	660
March	1st to 10th	Night	15% 6% 21% 37% 18% 4	800
	11th to 20th	Night	13% 5% 18% 37% 22% 6%	800
	21st to end	Night	14% 8% 18% 35% 23% 3	880
April	1st to 10th	Night	9% 6% 19% 38% 25% 3	800
	11th to 20th	Night	9% 6% 21% 33% 26% 5%	791
	21st to end	Night	7% 5% 24% 31% 24% 9%	800
May	1st to 10th	Night	7% 4 16% 36% 29% 8%	800
	11th to 20th	Night	5% 4 14% 36% 34% 7%	701
	21st to end	Night	5% 2 15% 39% 30% 8%	433
June	1st to 10th	Night	4 2 10% 35% 40% 10%	400
	11th to 20th	Night	3 2 11% 32% 44% 8%	400
	21st to end	Night	2 1 5% 29% 48% 16%	400
July	1st to 10th	Night	1 4 19% 60% 16%	390
	11th to 20th	Night	2 1 13% 57% 26%	571
	21st to end	Night	1 1 2 18% 54% 25%	657
August	1st to 10th	Night	1 5% 29% 50% 15%	706
	11th to 20th	Night	1 1 2 24% 59% 13%	792
	21st to end	Night	1 1 3 24% 56% 15%	880
September	1st to 10th	Night	3 1 4 25% 54% 14%	800
	11th to 20th	Night	4 1 6% 34% 44% 12%	800
	21st to end	Night	4 1 8% 31% 45% 11%	800
October	1st to 10th	Night	5% 1 8% 36% 41% 8%	772
	11th to 20th	Night	5% 2 11% 35% 37% 9%	760
	21st to end	Night	7% 2 14% 36% 31% 10%	836
November	1st to 10th	Night	6% 4 14% 37% 31% 8%	780
	11th to 20th	Night	6% 3 18% 39% 28% 7%	800
	21st to end	Night	8% 5% 17% 42% 24% 4	800
December	1st to 10th	Night	13% 6% 17% 36% 24% 4	800
	11th to 20th	Night	17% 4 19% 38% 19% 4	800
	21st to end	Night	14% 7% 23% 36% 18% 3	880

Page generated at 2025-09-30 22:39:05. Version ac07

Copyright © 2025 Attilla Danko.