Stanford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stanford clear sky chart from 2005-11-06 to 2024-01-15.

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	6% 4 2 4 4 2 4 4 5% 7% 59%	74705
	Night	6% 5% 2 4 4 3 4 4 5% 8% 55%	75095
Transparency	Day	25% 6% 11% 20% 23% 15%	74426
	Night	29% 7% 11% 19% 20% 14%	74896
Seeing	Day	7% 9% 24% 32% 21% 7%	14309
	Night	8% 5% 15% 34% 31% 7%	23767

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	15% 7% 3 6% 5% 3 5% 5% 6% 8% 37%	1637
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 4 5% 8% 48%	1660
	21st to end	Day	11% 7% 2 5% 6% 4 4 5% 6% 8% 42%	1782
February	1st to 10th	Day	11% 5% 3 5% 5% 3 5% 5% 5% 6% 47%	1620
	11th to 20th	Day	11% 6% 3 5% 5% 2 4 5% 6% 8% 45%	1660
	21st to end	Day	11% 5% 3 5% 5% 4 4 4 5% 7% 47%	1480
March	1st to 10th	Day	11% 7% 3 6% 5% 3 5% 6% 7% 9% 37%	1948
	11th to 20th	Day	10% 6% 3 7% 6% 4 6% 7% 6% 8% 38%	1980
	21st to end	Day	10% 4 2 5% 6% 4 5% 5% 6% 8% 46%	2204
April	1st to 10th	Day	10% 7% 3 5% 4 2 4 5% 5% 8% 46%	2160
	11th to 20th	Day	8% 5% 2 4 3 3 3 4 4 7% 56%	2129
	21st to end	Day	6% 4 2 3 4 2 3 4 5% 8% 60%	2160
May	1st to 10th	Day	3 4 2 3 3 2 4 3 5% 8% 62%	2219
	11th to 20th	Day	4 3 1 4 4 2 3 3 4 7% 65%	2333
	21st to end	Day	4 5% 2 4 3 2 4 3 5% 8% 60%	2723
June	1st to 10th	Day	4 4 2 3 2 2 3 3 5% 8% 63%	2520
	11th to 20th	Day	2 1 1 1 2 1 2 3 4 7% 77%	2520
	21st to end	Day	2 2 1 2 2 1 3 2 3 6% 76%	2520
July	1st to 10th	Day	1 2 1 2 2 1 2 3 3 5% 78%	2513
	11th to 20th	Day	1 2 1 2 2 2 3 4 4 7% 73%	2340
	21st to end	Day	1 1 1 2 2 1 2 3 3 7% 77%	2546
August	1st to 10th	Day	2 3 1 3 3 2 3 3 5% 6% 71%	2328
	11th to 20th	Day	1 1 1 3 2 1 2 3 4 7% 76%	2183
	21st to end	Day	1 2 1 2 3 1 3 3 4 7% 73%	2376
September	1st to 10th	Day	1 2 1 2 2 1 2 2 3 4 79%	2160
	11th to 20th	Day	2 3 2 4 3 2 3 3 4 7% 67%	2160
	21st to end	Day	2 2 1 2 2 1 2 2 4 5% 76%	1908
October	1st to 10th	Day	4 2 1 4 4 2 3 3 3 7% 68%	1800
	11th to 20th	Day	4 4 1 3 3 2 4 3 5% 7% 64%	1800
	21st to end	Day	4 5% 2 4 4 2 3 4 5% 7% 61%	1980
November	1st to 10th	Day	6% 4 2 4 4 3 4 5% 5% 8% 54%	1836
	11th to 20th	Day	6% 6% 2 5% 5% 3 5% 5% 5% 7% 51%	1710
	21st to end	Day	8% 4 2 4 4 3 4 4 5% 8% 53%	1568
December	1st to 10th	Day	8% 5% 3 7% 7% 3 4 5% 6% 7% 44%	1520
	11th to 20th	Day	13% 9% 4 8% 8% 4 5% 5% 7% 6% 32%	1928
	21st to end	Day	11% 10% 4 8% 6% 3 6% 5% 6% 8% 33%	2794
January	1st to 10th	Night	12% 8% 3 6% 5% 3 5% 5% 7% 9% 38%	2524
	11th to 20th	Night	10% 7% 3 5% 6% 3 4 4 6% 8% 44%	2405
	21st to end	Night	13% 6% 2 5% 5% 3 5% 5% 5% 8% 44%	2574
February	1st to 10th	Night	10% 6% 3 5% 5% 3 4 4 4 6% 50%	2340
	11th to 20th	Night	9% 5% 2 5% 6% 3 5% 5% 6% 9% 46%	2187
	21st to end	Night	9% 5% 3 5% 6% 3 3 5% 4 7% 51%	1776
March	1st to 10th	Night	12% 6% 3 7% 6% 3 5% 5% 6% 6% 41%	2009
	11th to 20th	Night	10% 7% 3 6% 5% 4 5% 4 5% 8% 43%	1980
	21st to end	Night	9% 7% 2 5% 3 3 4 4 4 6% 51%	2150
April	1st to 10th	Night	8% 7% 2 6% 5% 3 4 4 7% 8% 47%	1800
	11th to 20th	Night	6% 5% 2 5% 4 2 3 4 5% 7% 56%	1779
	21st to end	Night	4 3 1 3 3 2 4 4 5% 8% 63%	1800
May	1st to 10th	Night	2 5% 1 3 4 2 5% 3 5% 7% 60%	1738
	11th to 20th	Night	3 4 3 4 4 2 4 5% 5% 10% 58%	1558
	21st to end	Night	3 4 2 4 4 3 4 6% 5% 7% 59%	1552
June	1st to 10th	Night	3 3 2 4 4 2 3 3 5% 9% 62%	1440
	11th to 20th	Night	1 1 2 2 2 1 3 4 6% 77%	1440
	21st to end	Night	1 3 1 3 2 1 3 2 3 5% 75%	1440
July	1st to 10th	Night	3 1 1 2 1 2 2 3 8% 76%	1443
	11th to 20th	Night	1 1 1 1 2 1 3 4 4 8% 73%	1620
	21st to end	Night	2 1 3 2 2 4 4 7% 11% 63%	1773
August	1st to 10th	Night	2 4 1 4 3 2 4 4 4 9% 63%	1611
	11th to 20th	Night	2 1 2 3 2 3 5% 6% 10% 67%	1718
	21st to end	Night	2 1 3 3 2 4 4 4 9% 68%	1980
September	1st to 10th	Night	1 2 1 2 2 2 3 3 4 7% 73%	1800
	11th to 20th	Night	3 5% 2 3 3 2 3 4 4 7% 64%	1800
	21st to end	Night	2 2 1 2 2 1 2 2 4 6% 75%	2048
October	1st to 10th	Night	3 3 2 3 3 2 3 4 4 6% 67%	2160
	11th to 20th	Night	5% 4 2 4 4 1 3 3 4 7% 62%	2160
	21st to end	Night	4 4 2 4 3 2 3 4 5% 6% 62%	2376
November	1st to 10th	Night	6% 5% 2 4 4 2 4 5% 5% 7% 56%	2239
	11th to 20th	Night	7% 6% 3 5% 4 3 4 4 5% 8% 51%	2470
	21st to end	Night	6% 4 2 3 4 3 4 4 4 7% 57%	2607
December	1st to 10th	Night	9% 6% 3 6% 5% 3 5% 4 4 8% 47%	2660
	11th to 20th	Night	16% 10% 4 8% 7% 3 4 4 5% 8% 31%	3276
	21st to end	Night	11% 7% 3 7% 7% 4 6% 6% 7% 7% 34%	4862

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	44% 8% 13% 20% 10% 5%	1637
	11th to 20th	Day	34% 8% 14% 14% 18% 13%	1660
	21st to end	Day	39% 9% 8% 13% 22% 8%	1782
February	1st to 10th	Day	38% 5% 9% 20% 19% 10%	1620
	11th to 20th	Day	38% 6% 11% 18% 21% 7%	1669
	21st to end	Day	38% 6% 9% 22% 18% 7%	1480
March	1st to 10th	Day	43% 8% 12% 19% 14% 4	1948
	11th to 20th	Day	43% 8% 11% 19% 16% 4	1980
	21st to end	Day	37% 8% 14% 21% 17% 3	2204
April	1st to 10th	Day	37% 7% 14% 19% 18% 4	2160
	11th to 20th	Day	29% 7% 15% 24% 21% 5%	2132
	21st to end	Day	23% 7% 14% 29% 22% 5%	2160
May	1st to 10th	Day	22% 7% 12% 25% 28% 6%	2219
	11th to 20th	Day	20% 6% 15% 27% 25% 8%	2336
	21st to end	Day	25% 8% 18% 25% 21% 3	2724
June	1st to 10th	Day	22% 8% 14% 25% 23% 8%	2520
	11th to 20th	Day	10% 5% 12% 24% 34% 15%	2520
	21st to end	Day	11% 4 8% 17% 33% 27%	2520
July	1st to 10th	Day	9% 4 9% 15% 25% 38%	2444
	11th to 20th	Day	11% 4 10% 16% 24% 35%	2271
	21st to end	Day	8% 3 7% 15% 32% 35%	2405
August	1st to 10th	Day	15% 5% 9% 18% 30% 24%	2312
	11th to 20th	Day	9% 4 8% 17% 32% 31%	2183
	21st to end	Day	10% 5% 7% 19% 33% 26%	2376
September	1st to 10th	Day	10% 3 5% 14% 29% 39%	2160
	11th to 20th	Day	18% 5% 10% 23% 26% 17%	2160
	21st to end	Day	11% 4 8% 22% 33% 22%	1908
October	1st to 10th	Day	19% 4 8% 18% 28% 23%	1800
	11th to 20th	Day	21% 5% 11% 18% 22% 23%	1800
	21st to end	Day	23% 5% 10% 19% 23% 19%	1980
November	1st to 10th	Day	30% 6% 11% 22% 20% 12%	1836
	11th to 20th	Day	32% 7% 10% 22% 21% 7%	1710
	21st to end	Day	30% 5% 11% 17% 28% 9%	1568
December	1st to 10th	Day	37% 8% 12% 18% 16% 10%	1520
	11th to 20th	Day	52% 6% 12% 12% 13% 5%	1928
	21st to end	Day	47% 9% 12% 18% 12% 3	2794
January	1st to 10th	Night	42% 7% 14% 21% 12% 5%	2524
	11th to 20th	Night	39% 8% 13% 13% 15% 12%	2405
	21st to end	Night	41% 7% 11% 12% 17% 12%	2574
February	1st to 10th	Night	36% 5% 9% 19% 20% 11%	2340
	11th to 20th	Night	35% 7% 11% 18% 22% 7%	2189
	21st to end	Night	34% 7% 12% 28% 15% 4	1776
March	1st to 10th	Night	43% 7% 14% 17% 14% 5%	2009
	11th to 20th	Night	40% 8% 9% 21% 18% 5%	1980
	21st to end	Night	36% 8% 14% 20% 17% 5%	2150
April	1st to 10th	Night	35% 8% 17% 21% 16% 4	1800
	11th to 20th	Night	30% 8% 13% 24% 19% 7%	1779
	21st to end	Night	19% 9% 14% 32% 20% 6%	1800
May	1st to 10th	Night	24% 8% 10% 25% 24% 10%	1738
	11th to 20th	Night	21% 10% 15% 24% 23% 7%	1559
	21st to end	Night	23% 10% 19% 24% 20% 3	1552
June	1st to 10th	Night	21% 8% 14% 25% 25% 8%	1440
	11th to 20th	Night	10% 7% 10% 23% 31% 19%	1440
	21st to end	Night	13% 4 7% 17% 33% 25%	1440
July	1st to 10th	Night	9% 4 8% 15% 28% 36%	1403
	11th to 20th	Night	10% 7% 10% 17% 23% 33%	1566
	21st to end	Night	12% 5% 12% 16% 26% 28%	1674
August	1st to 10th	Night	20% 7% 9% 18% 24% 22%	1602
	11th to 20th	Night	12% 5% 9% 18% 27% 29%	1718
	21st to end	Night	12% 6% 10% 20% 27% 25%	1980
September	1st to 10th	Night	11% 5% 7% 13% 25% 39%	1800
	11th to 20th	Night	19% 6% 11% 22% 27% 15%	1800
	21st to end	Night	12% 5% 8% 21% 29% 25%	2048
October	1st to 10th	Night	18% 5% 11% 17% 25% 24%	2160
	11th to 20th	Night	23% 6% 9% 19% 20% 23%	2160
	21st to end	Night	22% 5% 10% 20% 24% 20%	2376
November	1st to 10th	Night	27% 7% 12% 24% 18% 13%	2239
	11th to 20th	Night	30% 7% 12% 23% 19% 8%	2470
	21st to end	Night	27% 6% 12% 21% 22% 13%	2607
December	1st to 10th	Night	37% 8% 10% 17% 17% 12%	2660
	11th to 20th	Night	52% 6% 10% 13% 15% 3	3276
	21st to end	Night	46% 11% 14% 15% 11% 4	4862

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	21% 13% 23% 25% 14% 3	298
	11th to 20th	Day	14% 8% 23% 31% 18% 5%	354
	21st to end	Day	11% 9% 26% 28% 20% 5%	374
February	1st to 10th	Day	13% 14% 24% 24% 18% 7%	340
	11th to 20th	Day	14% 16% 28% 25% 14% 2	373
	21st to end	Day	11% 16% 28% 30% 14% 1	346
March	1st to 10th	Day	16% 16% 30% 26% 10% 1	420
	11th to 20th	Day	16% 15% 26% 25% 14% 5%	420
	21st to end	Day	11% 17% 35% 26% 10% 2	462
April	1st to 10th	Day	12% 15% 34% 25% 12% 3	420
	11th to 20th	Day	11% 17% 31% 25% 11% 4	412
	21st to end	Day	5% 17% 30% 29% 13% 6%	420
May	1st to 10th	Day	4 14% 34% 30% 13% 5%	423
	11th to 20th	Day	8% 13% 33% 29% 13% 4	448
	21st to end	Day	7% 12% 28% 31% 15% 7%	485
June	1st to 10th	Day	4 10% 27% 31% 20% 6%	450
	11th to 20th	Day	2 9% 29% 37% 19% 5%	450
	21st to end	Day	1 5% 22% 40% 24% 8%	450
July	1st to 10th	Day	2 4 16% 46% 27% 5%	446
	11th to 20th	Day	1 14% 42% 32% 10%	444
	21st to end	Day	1 15% 41% 33% 10%	487
August	1st to 10th	Day	2 3 22% 40% 24% 9%	447
	11th to 20th	Day	1 1 19% 41% 26% 12%	437
	21st to end	Day	2 18% 39% 31% 9%	495
September	1st to 10th	Day	1 4 13% 40% 30% 11%	450
	11th to 20th	Day	2 6% 24% 36% 24% 9%	450
	21st to end	Day	2 8% 18% 34% 28% 9%	450
October	1st to 10th	Day	2 7% 23% 35% 21% 12%	450
	11th to 20th	Day	5% 8% 17% 31% 26% 13%	450
	21st to end	Day	7% 5% 24% 30% 24% 9%	495
November	1st to 10th	Day	7% 9% 23% 26% 25% 9%	432
	11th to 20th	Day	4 8% 17% 30% 27% 14%	270
	21st to end	Day	7% 5% 22% 28% 31% 6%	203
December	1st to 10th	Day	8% 13% 18% 28% 25% 8%	180
	11th to 20th	Day	11% 12% 20% 24% 23% 9%	180
	21st to end	Day	16% 15% 22% 27% 18% 3	198
January	1st to 10th	Night	17% 7% 17% 32% 22% 5%	760
	11th to 20th	Night	13% 6% 15% 35% 24% 7%	740
	21st to end	Night	17% 4 14% 35% 24% 5%	792
February	1st to 10th	Night	12% 7% 16% 38% 23% 5%	720
	11th to 20th	Night	12% 7% 20% 34% 22% 5%	704
	21st to end	Night	12% 8% 20% 38% 19% 3	592
March	1st to 10th	Night	14% 9% 20% 31% 23% 3	720
	11th to 20th	Night	14% 8% 17% 33% 24% 4	720
	21st to end	Night	11% 9% 22% 37% 19% 2	792
April	1st to 10th	Night	10% 7% 21% 38% 21% 3	720
	11th to 20th	Night	8% 12% 21% 33% 20% 5%	711
	21st to end	Night	5% 5% 25% 35% 24% 6%	720
May	1st to 10th	Night	5% 4 20% 39% 27% 5%	658
	11th to 20th	Night	4 4 17% 36% 33% 6%	496
	21st to end	Night	5% 7% 19% 35% 30% 5%	389
June	1st to 10th	Night	4 4 16% 42% 32% 3	360
	11th to 20th	Night	1 3 14% 47% 35% 1	360
	21st to end	Night	1 4 9% 38% 44% 3	360
July	1st to 10th	Night	1 3 7% 31% 54% 4	353
	11th to 20th	Night	1 3 30% 50% 15%	522
	21st to end	Night	1 1 6% 24% 57% 12%	591
August	1st to 10th	Night	5% 4 6% 29% 47% 9%	537
	11th to 20th	Night	1 2 9% 33% 47% 8%	650
	21st to end	Night	1 1 8% 31% 46% 11%	792
September	1st to 10th	Night	2 2 10% 31% 44% 11%	720
	11th to 20th	Night	5% 4 12% 32% 39% 8%	720
	21st to end	Night	2 2 11% 30% 43% 12%	720
October	1st to 10th	Night	4 3 11% 36% 36% 10%	720
	11th to 20th	Night	6% 4 11% 29% 39% 11%	720
	21st to end	Night	5% 5% 9% 34% 33% 13%	792
November	1st to 10th	Night	7% 4 15% 31% 34% 9%	740
	11th to 20th	Night	9% 3 12% 37% 32% 7%	760
	21st to end	Night	8% 4 14% 40% 27% 6%	760
December	1st to 10th	Night	11% 6% 17% 35% 25% 6%	760
	11th to 20th	Night	17% 6% 18% 35% 19% 4	760
	21st to end	Night	16% 9% 22% 32% 19% 2	836

Page generated at 2024-01-12 13:17:43. Version ac07

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