The Crystal Spring
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Crystal Spring clear sky chart from 2005-11-06 to 2026-07-16.

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	17% 8% 3 6% 4 4 4 3 5% 7% 39%	137405
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 39%	135288
Transparency	Day	30% 36% 8% 15% 10% 2	140955
	Night	31% 35% 7% 14% 11% 2	139827
Seeing	Day	20% 14% 33% 23% 9% 2	17340
	Night	21% 14% 31% 25% 8% 1	26190

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	11% 9% 3 6% 4 3 3 3 5% 6% 46%	4661
	11th to 20th	Day	16% 9% 3 5% 3 3 3 3 4 4 47%	4650
	21st to end	Day	14% 10% 3 5% 3 3 3 3 4 5% 47%	5163
February	1st to 10th	Day	18% 9% 3 6% 4 5% 4 4 5% 6% 36%	2011
	11th to 20th	Day	15% 7% 3 6% 4 3 4 4 5% 6% 43%	2048
	21st to end	Day	18% 10% 3 6% 3 4 4 4 4 5% 39%	1730
March	1st to 10th	Day	21% 13% 3 5% 3 3 3 3 3 5% 38%	4576
	11th to 20th	Day	14% 10% 3 6% 4 4 4 3 5% 7% 40%	4940
	21st to end	Day	25% 10% 2 7% 5% 4 4 3 4 5% 31%	5809
April	1st to 10th	Day	20% 10% 2 6% 4 3 4 3 4 6% 38%	2520
	11th to 20th	Day	19% 7% 2 6% 4 4 3 3 5% 6% 40%	2521
	21st to end	Day	19% 7% 3 5% 4 5% 4 3 5% 7% 38%	2730
May	1st to 10th	Day	25% 11% 4 7% 4 4 4 3 5% 7% 26%	7639
	11th to 20th	Day	19% 10% 3 7% 4 4 4 3 4 6% 36%	7625
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 8% 30%	8936
June	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 6% 8% 35%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 5% 6% 8% 38%	2940
	21st to end	Day	15% 7% 2 6% 4 3 5% 4 7% 8% 40%	2940
July	1st to 10th	Day	17% 9% 3 6% 4 4 4 4 6% 9% 33%	6500
	11th to 20th	Day	13% 8% 3 7% 4 5% 5% 5% 7% 10% 33%	6430
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 8% 42%	6841
August	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4235
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	4147
	21st to end	Day	8% 4 2 5% 4 4 4 4 7% 10% 49%	4312
September	1st to 10th	Day	14% 6% 3 6% 4 4 4 3 5% 5% 47%	2367
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 5% 7% 50%	2200
	21st to end	Day	20% 6% 2 6% 4 4 3 3 6% 5% 41%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 3 3 3 5% 6% 44%	2200
	11th to 20th	Day	17% 6% 2 4 3 3 3 3 5% 7% 47%	1985
	21st to end	Day	19% 6% 2 5% 4 4 3 3 4 5% 45%	1980
November	1st to 10th	Day	17% 6% 2 5% 4 3 3 3 4 6% 46%	1851
	11th to 20th	Day	16% 6% 2 5% 4 3 3 4 5% 7% 46%	1890
	21st to end	Day	17% 7% 3 5% 4 4 3 3 5% 6% 44%	1890
December	1st to 10th	Day	15% 9% 2 5% 3 3 3 2 4 6% 49%	2884
	11th to 20th	Day	18% 8% 2 6% 3 3 2 2 4 5% 48%	3066
	21st to end	Day	19% 11% 3 7% 4 4 3 3 4 5% 36%	4048
January	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 4 5% 50%	8234
	11th to 20th	Night	17% 10% 3 7% 4 3 4 3 4 5% 41%	8235
	21st to end	Night	15% 10% 4 7% 4 5% 4 3 4 4 39%	9015
February	1st to 10th	Night	17% 8% 3 7% 4 4 4 2 4 6% 40%	2604
	11th to 20th	Night	18% 9% 3 5% 4 3 4 3 5% 6% 40%	2458
	21st to end	Night	17% 9% 2 8% 5% 3 4 3 4 5% 41%	2076
March	1st to 10th	Night	24% 10% 2 5% 3 3 3 3 4 6% 37%	5484
	11th to 20th	Night	16% 11% 4 8% 4 4 4 3 5% 5% 37%	5238
	21st to end	Night	24% 11% 3 6% 5% 4 4 3 4 4 32%	5360
April	1st to 10th	Night	19% 9% 3 7% 6% 4 3 3 4 6% 37%	2100
	11th to 20th	Night	19% 10% 2 6% 4 4 4 3 5% 7% 38%	2046
	21st to end	Night	20% 8% 2 6% 5% 4 4 3 4 6% 38%	1890
May	1st to 10th	Night	26% 10% 3 7% 4 4 3 3 5% 6% 29%	5259
	11th to 20th	Night	19% 11% 3 7% 5% 4 4 3 6% 6% 31%	5235
	21st to end	Night	22% 9% 3 9% 5% 5% 5% 4 6% 6% 25%	5293
June	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 6% 8% 36%	1680
	11th to 20th	Night	19% 7% 3 6% 4 4 4 3 5% 7% 38%	1680
	21st to end	Night	18% 8% 2 6% 4 4 3 3 7% 8% 37%	1680
July	1st to 10th	Night	19% 13% 4 8% 4 4 3 3 5% 7% 29%	3680
	11th to 20th	Night	18% 12% 3 6% 4 4 4 4 7% 8% 30%	3640
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 5% 7% 39%	3966
August	1st to 10th	Night	22% 8% 3 7% 5% 4 5% 4 7% 7% 29%	2915
	11th to 20th	Night	9% 7% 2 7% 6% 6% 6% 5% 7% 8% 36%	2896
	21st to end	Night	9% 4 2 4 4 3 4 3 7% 8% 52%	3586
September	1st to 10th	Night	13% 7% 3 5% 4 4 4 3 6% 7% 44%	2032
	11th to 20th	Night	15% 6% 2 4 3 4 3 3 5% 6% 50%	2200
	21st to end	Night	20% 7% 2 5% 4 3 4 4 5% 5% 40%	2200
October	1st to 10th	Night	20% 7% 2 6% 4 3 4 3 4 6% 41%	2200
	11th to 20th	Night	16% 5% 2 5% 4 3 3 3 5% 7% 47%	2410
	21st to end	Night	18% 7% 3 6% 4 3 4 4 5% 6% 40%	2860
November	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 6% 45%	2667
	11th to 20th	Night	16% 8% 2 5% 4 3 4 3 5% 5% 46%	2730
	21st to end	Night	15% 6% 2 6% 4 3 4 3 5% 7% 45%	2730
December	1st to 10th	Night	18% 8% 2 5% 4 3 3 2 3 5% 47%	4511
	11th to 20th	Night	20% 9% 2 5% 4 3 3 2 4 5% 42%	5348
	21st to end	Night	17% 10% 4 7% 4 4 3 3 4 5% 40%	7150

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	31% 21% 8% 19% 18% 3	4662
	11th to 20th	Day	32% 21% 7% 17% 16% 7%	4680
	21st to end	Day	34% 22% 8% 17% 17% 1	5163
February	1st to 10th	Day	21% 60% 6% 7% 6% 1	2011
	11th to 20th	Day	20% 58% 3 10% 8% 1	2057
	21st to end	Day	18% 59% 5% 9% 7% 2	1730
March	1st to 10th	Day	39% 28% 8% 7% 8% 10%	4579
	11th to 20th	Day	29% 31% 13% 19% 8% 1	4940
	21st to end	Day	43% 30% 4 9% 9% 5%	5809
April	1st to 10th	Day	22% 59% 4 7% 7% 1	2520
	11th to 20th	Day	17% 60% 5% 9% 8% 1	2521
	21st to end	Day	19% 60% 4 8% 8% 1	2730
May	1st to 10th	Day	46% 26% 10% 12% 6%	7651
	11th to 20th	Day	40% 25% 12% 17% 6%	7629
	21st to end	Day	44% 25% 8% 15% 9%	8937
June	1st to 10th	Day	17% 60% 4 10% 8%	2940
	11th to 20th	Day	18% 60% 5% 10% 6% 1	2940
	21st to end	Day	16% 58% 6% 11% 9%	2940
July	1st to 10th	Day	39% 29% 9% 13% 9% 1	6430
	11th to 20th	Day	36% 29% 12% 16% 7%	6355
	21st to end	Day	31% 27% 8% 23% 10% 1	6644
August	1st to 10th	Day	34% 38% 11% 13% 5%	4222
	11th to 20th	Day	27% 39% 12% 19% 4	4186
	21st to end	Day	15% 38% 11% 23% 13% 1	4312
September	1st to 10th	Day	16% 58% 4 14% 8%	2367
	11th to 20th	Day	15% 58% 5% 11% 10% 2	2200
	21st to end	Day	22% 57% 4 9% 7%	2200
October	1st to 10th	Day	23% 37% 7% 20% 11% 3	3590
	11th to 20th	Day	18% 36% 3 19% 20% 3	3201
	21st to end	Day	16% 36% 10% 24% 13% 1	3180
November	1st to 10th	Day	20% 54% 7% 9% 9% 1	1851
	11th to 20th	Day	18% 54% 4 10% 12% 2	1890
	21st to end	Day	18% 54% 5% 15% 7% 2	1890
December	1st to 10th	Day	24% 35% 9% 21% 11%	2884
	11th to 20th	Day	27% 31% 9% 12% 15% 5%	3066
	21st to end	Day	38% 26% 7% 14% 11% 3	4048
January	1st to 10th	Night	29% 21% 7% 26% 16% 2	8234
	11th to 20th	Night	37% 21% 7% 17% 13% 5%	8280
	21st to end	Night	39% 23% 5% 14% 14% 4	9015
February	1st to 10th	Night	20% 59% 5% 9% 6% 1	2604
	11th to 20th	Night	19% 58% 3 9% 10% 2	2460
	21st to end	Night	20% 59% 4 9% 7% 2	2076
March	1st to 10th	Night	39% 29% 6% 10% 10% 6%	5494
	11th to 20th	Night	35% 31% 11% 12% 9% 2	5239
	21st to end	Night	44% 29% 3 6% 14% 4	5360
April	1st to 10th	Night	23% 59% 3 7% 6% 2	2100
	11th to 20th	Night	18% 61% 6% 8% 7% 1	2046
	21st to end	Night	21% 59% 4 8% 7% 1	1890
May	1st to 10th	Night	44% 25% 12% 11% 9%	5265
	11th to 20th	Night	42% 25% 11% 15% 6% 1	5236
	21st to end	Night	46% 26% 8% 14% 5%	5294
June	1st to 10th	Night	17% 59% 4 11% 8%	1680
	11th to 20th	Night	17% 59% 5% 12% 6% 1	1680
	21st to end	Night	17% 59% 4 12% 7% 1	1680
July	1st to 10th	Night	45% 26% 5% 12% 11%	3640
	11th to 20th	Night	40% 28% 11% 14% 6%	3592
	21st to end	Night	33% 27% 10% 19% 11% 1	3849
August	1st to 10th	Night	37% 39% 7% 10% 6% 1	2903
	11th to 20th	Night	26% 40% 10% 20% 4	2923
	21st to end	Night	14% 38% 12% 20% 17%	3586
September	1st to 10th	Night	16% 58% 5% 13% 7% 1	2032
	11th to 20th	Night	18% 57% 3 10% 11% 1	2200
	21st to end	Night	23% 58% 4 8% 7% 1	2200
October	1st to 10th	Night	25% 37% 7% 20% 10% 2	3590
	11th to 20th	Night	12% 36% 4 17% 28% 3	3933
	21st to end	Night	18% 36% 7% 21% 17% 1	4610
November	1st to 10th	Night	20% 54% 5% 10% 9% 1	2667
	11th to 20th	Night	17% 54% 4 10% 11% 3	2730
	21st to end	Night	17% 55% 6% 14% 6% 2	2730
December	1st to 10th	Night	27% 35% 7% 19% 12%	4511
	11th to 20th	Night	33% 31% 9% 10% 15% 2	5348
	21st to end	Night	36% 27% 7% 16% 14% 1	7150

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	20% 24% 34% 18% 4	220
	11th to 20th	Day	22% 28% 37% 10% 3	220
	21st to end	Day	18% 22% 42% 13% 4 1	255
February	1st to 10th	Day	23% 20% 38% 17% 3	493
	11th to 20th	Day	24% 25% 38% 12% 1	536
	21st to end	Day	22% 21% 38% 15% 4	446
March	1st to 10th	Day	25% 21% 33% 18% 3	540
	11th to 20th	Day	19% 26% 33% 18% 4	540
	21st to end	Day	23% 22% 35% 15% 4 1	594
April	1st to 10th	Day	26% 19% 35% 15% 4 1	540
	11th to 20th	Day	24% 20% 36% 17% 3	532
	21st to end	Day	23% 15% 34% 21% 6% 1	540
May	1st to 10th	Day	28% 17% 32% 17% 5%	543
	11th to 20th	Day	21% 12% 42% 18% 5% 2	567
	21st to end	Day	23% 10% 31% 26% 8% 2	617
June	1st to 10th	Day	19% 9% 31% 29% 11% 1	570
	11th to 20th	Day	16% 10% 37% 28% 9% 1	570
	21st to end	Day	14% 9% 33% 30% 12% 1	570
July	1st to 10th	Day	17% 6% 25% 33% 17% 2	565
	11th to 20th	Day	11% 6% 29% 33% 18% 3	544
	21st to end	Day	14% 6% 32% 34% 13% 2	576
August	1st to 10th	Day	13% 7% 29% 36% 14% 2	526
	11th to 20th	Day	14% 7% 24% 34% 18% 4	519
	21st to end	Day	13% 6% 30% 32% 14% 6%	583
September	1st to 10th	Day	18% 6% 28% 28% 15% 5%	530
	11th to 20th	Day	15% 8% 29% 25% 16% 7%	530
	21st to end	Day	25% 9% 28% 24% 10% 4	530
October	1st to 10th	Day	22% 12% 25% 23% 13% 6%	530
	11th to 20th	Day	20% 12% 34% 20% 12% 2	459
	21st to end	Day	22% 17% 28% 21% 9% 3	462
November	1st to 10th	Day	15% 16% 36% 21% 10% 2	425
	11th to 20th	Day	20% 21% 34% 19% 6%	430
	21st to end	Day	20% 23% 31% 18% 7%	430
December	1st to 10th	Day	19% 18% 38% 19% 5%	346
	11th to 20th	Day	25% 27% 33% 11% 3	220
	21st to end	Day	27% 19% 35% 12% 5% 2	242
January	1st to 10th	Night	21% 19% 40% 16% 4	840
	11th to 20th	Night	27% 21% 33% 15% 2	840
	21st to end	Night	20% 23% 35% 18% 3	924
February	1st to 10th	Night	21% 17% 38% 21% 2	840
	11th to 20th	Night	24% 21% 39% 15% 1	823
	21st to end	Night	21% 22% 35% 19% 3	692
March	1st to 10th	Night	23% 17% 35% 20% 4	840
	11th to 20th	Night	21% 16% 37% 20% 5%	840
	21st to end	Night	24% 17% 33% 21% 5%	924
April	1st to 10th	Night	23% 15% 33% 24% 4	840
	11th to 20th	Night	22% 15% 35% 23% 4	802
	21st to end	Night	22% 13% 33% 25% 6%	630
May	1st to 10th	Night	27% 11% 31% 24% 7%	630
	11th to 20th	Night	21% 12% 39% 23% 5% 1	622
	21st to end	Night	24% 7% 23% 37% 8% 1	517
June	1st to 10th	Night	21% 5% 29% 32% 12%	420
	11th to 20th	Night	22% 5% 25% 38% 10%	420
	21st to end	Night	23% 5% 21% 40% 12%	420
July	1st to 10th	Night	20% 6% 23% 37% 13% 1	410
	11th to 20th	Night	15% 4 22% 40% 17% 2	398
	21st to end	Night	16% 4 24% 43% 12% 1	453
August	1st to 10th	Night	13% 5% 23% 45% 14% 1	598
	11th to 20th	Night	13% 5% 21% 44% 16% 1	603
	21st to end	Night	16% 4 20% 37% 20% 3	880
September	1st to 10th	Night	16% 4 25% 35% 18% 2	800
	11th to 20th	Night	18% 5% 25% 28% 21% 4	800
	21st to end	Night	24% 6% 22% 31% 14% 2	800
October	1st to 10th	Night	24% 10% 23% 26% 14% 3	800
	11th to 20th	Night	19% 14% 29% 25% 12% 2	800
	21st to end	Night	23% 14% 32% 22% 9%	880
November	1st to 10th	Night	20% 13% 31% 25% 10% 1	820
	11th to 20th	Night	19% 22% 33% 19% 7%	840
	21st to end	Night	17% 22% 33% 22% 6% 1	840
December	1st to 10th	Night	24% 20% 33% 18% 4	840
	11th to 20th	Night	24% 23% 32% 17% 3	840
	21st to end	Night	25% 19% 37% 16% 3	924

Page generated at 2026-07-13 03:14:53. Version ac07

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