Standeford and Andreas Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Standeford and Andreas Observatories clear sky chart from 2005-11-06 to 2025-03-27.

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	12% 9% 3 8% 5% 4 5% 4 6% 7% 37%	109259
	Night	12% 9% 4 9% 5% 4 4 3 5% 6% 39%	110285
Transparency	Day	45% 7% 13% 23% 11% 1	112894
	Night	44% 6% 13% 23% 13% 1	114790
Seeing	Day	17% 10% 31% 26% 12% 3	16405
	Night	16% 9% 33% 31% 10% 1	26493

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	7% 15% 6% 19% 10% 7% 5% 3 4 6% 17%	3617
	11th to 20th	Day	6% 12% 6% 11% 7% 7% 9% 7% 10% 7% 17%	3850
	21st to end	Day	6% 15% 8% 15% 6% 5% 5% 3 4 4 28%	4488
February	1st to 10th	Day	8% 12% 5% 11% 8% 7% 6% 5% 6% 7% 25%	1800
	11th to 20th	Day	9% 12% 4 10% 7% 5% 6% 5% 6% 7% 30%	1754
	21st to end	Day	13% 10% 3 10% 7% 4 5% 3 6% 7% 32%	1527
March	1st to 10th	Day	14% 9% 3 7% 4 3 4 3 5% 8% 39%	3546
	11th to 20th	Day	11% 9% 3 6% 5% 4 5% 3 5% 8% 41%	3590
	21st to end	Day	26% 13% 4 11% 6% 3 4 3 5% 5% 20%	3880
April	1st to 10th	Day	18% 11% 3 7% 4 4 4 4 6% 7% 32%	2266
	11th to 20th	Day	19% 11% 3 7% 4 4 4 3 5% 6% 32%	2287
	21st to end	Day	24% 10% 3 6% 5% 4 4 4 6% 6% 29%	2470
May	1st to 10th	Day	18% 7% 2 6% 5% 4 5% 5% 6% 8% 35%	4136
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 6% 8% 44%	4399
	21st to end	Day	18% 6% 3 7% 5% 4 5% 4 5% 7% 37%	4824
June	1st to 10th	Day	12% 6% 2 6% 3 4 4 4 5% 9% 46%	2660
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 4 7% 7% 41%	2660
	21st to end	Day	10% 5% 2 6% 5% 4 4 5% 7% 9% 44%	2660
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 8% 10% 41%	4440
	11th to 20th	Day	5% 3 2 4 4 3 3 3 6% 8% 58%	4440
	21st to end	Day	6% 3 1 5% 4 5% 5% 5% 7% 9% 48%	4600
August	1st to 10th	Day	12% 5% 2 7% 5% 3 4 3 6% 7% 45%	4104
	11th to 20th	Day	12% 6% 3 7% 4 4 4 4 6% 9% 41%	4041
	21st to end	Day	8% 5% 2 6% 4 4 4 3 5% 7% 50%	4355
September	1st to 10th	Day	13% 5% 2 5% 3 3 3 2 5% 7% 50%	2280
	11th to 20th	Day	12% 5% 1 5% 4 4 4 3 4 7% 51%	2200
	21st to end	Day	13% 6% 2 6% 3 3 4 3 5% 7% 47%	2028
October	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 5% 5% 45%	1900
	11th to 20th	Day	16% 8% 3 5% 3 4 4 3 4 6% 45%	1900
	21st to end	Day	19% 9% 4 6% 3 3 3 3 4 6% 39%	2090
November	1st to 10th	Day	11% 10% 4 7% 5% 4 4 3 6% 7% 41%	1667
	11th to 20th	Day	10% 14% 6% 11% 5% 4 4 3 6% 6% 32%	1600
	21st to end	Day	14% 10% 4 8% 5% 3 5% 3 5% 8% 35%	1600
December	1st to 10th	Day	7% 10% 3 12% 10% 7% 6% 4 5% 7% 29%	2610
	11th to 20th	Day	12% 15% 6% 12% 8% 7% 5% 4 5% 6% 22%	3008
	21st to end	Day	19% 18% 7% 20% 5% 4 3 2 3 4 15%	3982
January	1st to 10th	Night	6% 11% 6% 19% 8% 8% 6% 3 5% 5% 22%	6348
	11th to 20th	Night	6% 10% 4 13% 8% 6% 10% 7% 8% 6% 21%	6114
	21st to end	Night	8% 15% 7% 11% 6% 4 5% 3 5% 5% 32%	6490
February	1st to 10th	Night	9% 13% 4 11% 6% 6% 4 4 4 7% 33%	2600
	11th to 20th	Night	8% 10% 4 8% 6% 6% 5% 4 5% 7% 38%	2533
	21st to end	Night	11% 9% 3 9% 7% 4 5% 4 5% 7% 37%	2107
March	1st to 10th	Night	10% 8% 3 7% 4 4 4 3 5% 6% 48%	3540
	11th to 20th	Night	9% 8% 2 6% 5% 5% 4 4 6% 9% 41%	3590
	21st to end	Night	27% 12% 3 10% 3 3 3 2 4 5% 29%	3889
April	1st to 10th	Night	14% 12% 3 6% 5% 4 4 4 5% 6% 38%	1912
	11th to 20th	Night	20% 9% 4 7% 4 3 4 3 4 5% 36%	1839
	21st to end	Night	18% 10% 3 7% 4 3 4 3 5% 7% 37%	1710
May	1st to 10th	Night	15% 7% 2 6% 5% 3 4 3 4 6% 44%	2768
	11th to 20th	Night	12% 5% 1 5% 4 3 4 3 5% 8% 49%	2506
	21st to end	Night	15% 6% 3 6% 4 4 3 3 5% 6% 45%	2743
June	1st to 10th	Night	10% 5% 2 3 4 4 5% 4 5% 7% 52%	1520
	11th to 20th	Night	10% 5% 3 5% 4 4 4 4 8% 7% 45%	1520
	21st to end	Night	9% 4 2 4 3 3 3 3 5% 8% 55%	1520
July	1st to 10th	Night	10% 4 2 5% 3 5% 5% 5% 7% 9% 45%	2530
	11th to 20th	Night	4 2 1 4 3 3 4 3 5% 7% 64%	2530
	21st to end	Night	4 3 1 5% 3 3 4 3 6% 9% 58%	2960
August	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 4 7% 49%	2836
	11th to 20th	Night	13% 5% 1 4 3 3 3 3 5% 9% 51%	2798
	21st to end	Night	9% 5% 2 6% 4 3 3 3 4 7% 53%	3304
September	1st to 10th	Night	10% 4 2 5% 2 3 3 2 4 6% 59%	1900
	11th to 20th	Night	11% 4 1 4 3 3 3 4 6% 8% 52%	1976
	21st to end	Night	11% 5% 2 5% 4 3 3 3 5% 6% 53%	2132
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 5% 51%	2280
	11th to 20th	Night	15% 9% 2 5% 2 3 4 3 5% 6% 45%	2280
	21st to end	Night	18% 11% 3 6% 3 3 3 2 5% 6% 40%	2508
November	1st to 10th	Night	13% 8% 3 6% 4 3 4 3 5% 5% 45%	2627
	11th to 20th	Night	12% 12% 5% 8% 5% 4 4 3 5% 4 38%	2800
	21st to end	Night	13% 10% 3 8% 5% 4 4 3 5% 7% 39%	2800
December	1st to 10th	Night	6% 8% 4 12% 7% 7% 5% 4 5% 7% 33%	4580
	11th to 20th	Night	11% 14% 6% 13% 8% 5% 4 3 5% 5% 27%	5199
	21st to end	Night	21% 14% 8% 17% 5% 4 4 3 4 4 16%	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	68% 5% 10% 13% 4	3618
	11th to 20th	Day	60% 4 11% 15% 10%	3850
	21st to end	Day	60% 5% 15% 13% 8%	4488
February	1st to 10th	Day	57% 7% 13% 15% 8% 1	1800
	11th to 20th	Day	51% 7% 14% 20% 6% 1	1763
	21st to end	Day	50% 7% 13% 19% 10%	1527
March	1st to 10th	Day	43% 7% 13% 27% 9% 1	3546
	11th to 20th	Day	40% 7% 12% 29% 11% 2	3590
	21st to end	Day	65% 7% 10% 14% 4	3880
April	1st to 10th	Day	49% 7% 13% 20% 9% 1	2266
	11th to 20th	Day	52% 6% 11% 20% 10% 1	2287
	21st to end	Day	54% 7% 11% 20% 7% 1	2470
May	1st to 10th	Day	42% 9% 14% 23% 11%	4174
	11th to 20th	Day	36% 8% 12% 28% 13% 2	4403
	21st to end	Day	43% 8% 13% 24% 10% 1	4837
June	1st to 10th	Day	34% 8% 15% 27% 15% 1	2660
	11th to 20th	Day	39% 8% 13% 21% 16% 1	2660
	21st to end	Day	35% 9% 16% 26% 13% 1	2660
July	1st to 10th	Day	36% 8% 21% 24% 10%	4371
	11th to 20th	Day	24% 6% 15% 35% 18% 2	4365
	21st to end	Day	28% 10% 15% 30% 15% 2	4455
August	1st to 10th	Day	38% 6% 12% 30% 12% 2	4090
	11th to 20th	Day	38% 7% 17% 30% 8%	4080
	21st to end	Day	33% 7% 16% 29% 14% 1	4354
September	1st to 10th	Day	34% 6% 12% 31% 16%	2280
	11th to 20th	Day	33% 7% 17% 30% 13% 1	2200
	21st to end	Day	37% 6% 13% 30% 14% 1	2028
October	1st to 10th	Day	29% 4 7% 34% 25% 1	3160
	11th to 20th	Day	37% 4 12% 24% 21% 1	3140
	21st to end	Day	41% 4 14% 30% 11%	3428
November	1st to 10th	Day	42% 7% 14% 20% 15% 3	1667
	11th to 20th	Day	53% 6% 13% 19% 9% 1	1600
	21st to end	Day	46% 7% 19% 20% 8% 1	1600
December	1st to 10th	Day	55% 7% 15% 17% 6% 1	2610
	11th to 20th	Day	65% 5% 15% 12% 4	3005
	21st to end	Day	74% 4 9% 9% 3	3982
January	1st to 10th	Night	63% 6% 10% 16% 5%	6348
	11th to 20th	Night	58% 6% 11% 14% 11% 1	6114
	21st to end	Night	54% 5% 13% 19% 10%	6490
February	1st to 10th	Night	52% 5% 13% 21% 8% 1	2600
	11th to 20th	Night	46% 6% 14% 24% 9% 2	2534
	21st to end	Night	46% 6% 13% 21% 12% 2	2107
March	1st to 10th	Night	36% 6% 12% 29% 15% 1	3540
	11th to 20th	Night	37% 6% 11% 29% 16% 1	3590
	21st to end	Night	59% 5% 10% 18% 7% 2	3889
April	1st to 10th	Night	45% 6% 13% 23% 12% 1	1912
	11th to 20th	Night	48% 6% 9% 17% 17% 3	1839
	21st to end	Night	47% 6% 10% 21% 14% 2	1710
May	1st to 10th	Night	39% 7% 13% 18% 22% 1	2795
	11th to 20th	Night	32% 7% 13% 24% 18% 6%	2507
	21st to end	Night	38% 7% 12% 25% 16% 1	2751
June	1st to 10th	Night	31% 7% 15% 30% 16% 2	1520
	11th to 20th	Night	35% 7% 14% 22% 19% 3	1520
	21st to end	Night	27% 6% 16% 30% 18% 3	1520
July	1st to 10th	Night	34% 7% 19% 26% 13% 1	2490
	11th to 20th	Night	20% 7% 13% 36% 22% 2	2482
	21st to end	Night	21% 7% 17% 31% 21% 2	2866
August	1st to 10th	Night	35% 6% 12% 29% 16% 3	2826
	11th to 20th	Night	32% 6% 16% 31% 14% 1	2825
	21st to end	Night	31% 6% 18% 26% 17% 2	3304
September	1st to 10th	Night	27% 4 13% 32% 21% 1	1900
	11th to 20th	Night	29% 7% 16% 30% 17% 1	1976
	21st to end	Night	31% 6% 12% 29% 19% 2	2132
October	1st to 10th	Night	23% 3 6% 32% 31% 4	3800
	11th to 20th	Night	29% 4 13% 26% 23% 4	3779
	21st to end	Night	42% 4 11% 29% 13% 1	4120
November	1st to 10th	Night	40% 5% 15% 23% 15% 2	2627
	11th to 20th	Night	48% 7% 12% 20% 12% 1	2800
	21st to end	Night	46% 6% 16% 23% 9% 1	2800
December	1st to 10th	Night	49% 5% 19% 20% 6%	4580
	11th to 20th	Night	59% 6% 13% 18% 4	5201
	21st to end	Night	71% 5% 9% 12% 3	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	15% 14% 35% 23% 11% 2	300
	11th to 20th	Day	21% 13% 37% 23% 5% 1	300
	21st to end	Day	14% 12% 40% 25% 8% 2	330
February	1st to 10th	Day	19% 14% 42% 21% 4	300
	11th to 20th	Day	13% 18% 39% 25% 4	300
	21st to end	Day	22% 15% 37% 25% 1	249
March	1st to 10th	Day	20% 13% 44% 20% 3	300
	11th to 20th	Day	25% 13% 34% 22% 5% 1	300
	21st to end	Day	31% 12% 29% 19% 7% 1	315
April	1st to 10th	Day	28% 14% 32% 22% 4	446
	11th to 20th	Day	30% 13% 25% 22% 9% 1	485
	21st to end	Day	29% 15% 26% 20% 9% 2	650
May	1st to 10th	Day	23% 12% 35% 22% 7% 2	653
	11th to 20th	Day	20% 11% 30% 23% 12% 4	675
	21st to end	Day	19% 8% 28% 29% 14% 2	734
June	1st to 10th	Day	13% 8% 26% 33% 17% 4	680
	11th to 20th	Day	14% 8% 35% 30% 11% 2	680
	21st to end	Day	13% 9% 32% 29% 13% 4	680
July	1st to 10th	Day	8% 5% 23% 32% 24% 8%	671
	11th to 20th	Day	9% 5% 27% 34% 19% 6%	668
	21st to end	Day	6% 3 26% 37% 21% 6%	739
August	1st to 10th	Day	10% 4 22% 33% 26% 5%	676
	11th to 20th	Day	13% 5% 22% 31% 23% 6%	665
	21st to end	Day	10% 6% 28% 33% 19% 5%	649
September	1st to 10th	Day	16% 8% 26% 30% 17% 3	490
	11th to 20th	Day	12% 6% 30% 31% 17% 4	410
	21st to end	Day	18% 13% 29% 25% 11% 5%	300
October	1st to 10th	Day	21% 11% 28% 25% 12% 4	300
	11th to 20th	Day	19% 17% 36% 22% 5% 1	300
	21st to end	Day	28% 15% 37% 14% 6%	330
November	1st to 10th	Day	22% 16% 33% 21% 7% 2	300
	11th to 20th	Day	20% 20% 40% 15% 6%	300
	21st to end	Day	22% 17% 39% 17% 4	300
December	1st to 10th	Day	17% 9% 46% 23% 6%	300
	11th to 20th	Day	19% 14% 41% 21% 5%	300
	21st to end	Day	27% 16% 34% 20% 3	330
January	1st to 10th	Night	12% 12% 38% 30% 8% 1	1000
	11th to 20th	Night	15% 12% 34% 32% 6% 1	1000
	21st to end	Night	18% 13% 34% 28% 6%	1100
February	1st to 10th	Night	15% 13% 37% 29% 5% 1	1000
	11th to 20th	Night	14% 13% 40% 26% 6% 1	979
	21st to end	Night	15% 11% 36% 30% 7% 1	787
March	1st to 10th	Night	17% 12% 37% 25% 8% 1	611
	11th to 20th	Night	20% 10% 37% 26% 7% 1	600
	21st to end	Night	25% 9% 30% 26% 9% 1	647
April	1st to 10th	Night	20% 11% 39% 26% 5%	570
	11th to 20th	Night	25% 11% 30% 27% 6% 1	563
	21st to end	Night	22% 10% 33% 27% 6% 1	570
May	1st to 10th	Night	19% 10% 33% 30% 8% 1	570
	11th to 20th	Night	19% 7% 33% 30% 11% 1	561
	21st to end	Night	19% 5% 29% 33% 13%	617
June	1st to 10th	Night	12% 4 30% 38% 14%	570
	11th to 20th	Night	14% 4 32% 40% 9%	570
	21st to end	Night	9% 5% 29% 41% 15% 1	570
July	1st to 10th	Night	6% 3 22% 38% 28% 3	555
	11th to 20th	Night	4 4 25% 44% 21% 2	552
	21st to end	Night	4 2 23% 49% 21% 2	624
August	1st to 10th	Night	9% 2 17% 48% 22% 1	567
	11th to 20th	Night	11% 2 17% 42% 25% 3	564
	21st to end	Night	8% 2 25% 41% 22% 2	627
September	1st to 10th	Night	12% 4 22% 43% 17% 2	570
	11th to 20th	Night	12% 5% 32% 33% 16% 2	570
	21st to end	Night	12% 10% 31% 33% 14%	612
October	1st to 10th	Night	17% 7% 32% 29% 13% 1	760
	11th to 20th	Night	20% 12% 33% 28% 6% 1	760
	21st to end	Night	24% 12% 30% 28% 6%	836
November	1st to 10th	Night	17% 11% 37% 26% 7% 1	911
	11th to 20th	Night	18% 13% 40% 23% 7%	1000
	21st to end	Night	17% 14% 38% 23% 7%	1000
December	1st to 10th	Night	16% 11% 38% 29% 6% 1	1000
	11th to 20th	Night	20% 9% 37% 27% 6% 1	1000
	21st to end	Night	26% 11% 32% 25% 5%	1100

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