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 2025-03-12.

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% 40%	108526
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 39%	110475
Transparency	Day	25% 44% 7% 13% 9% 2	112122
	Night	27% 42% 6% 13% 10% 2	115043
Seeing	Day	20% 15% 32% 22% 9% 2	15545
	Night	21% 14% 31% 25% 8% 1	24560

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	12% 9% 3 6% 3 3 3 3 5% 6% 48%	3597
	11th to 20th	Day	16% 9% 3 4 3 3 3 3 4 4 49%	3570
	21st to end	Day	17% 9% 4 7% 4 4 4 3 4 5% 41%	3985
February	1st to 10th	Day	19% 9% 3 6% 4 4 4 4 5% 6% 36%	1915
	11th to 20th	Day	15% 7% 3 6% 4 3 5% 4 5% 6% 43%	1948
	21st to end	Day	19% 9% 3 6% 4 4 4 4 4 5% 39%	1650
March	1st to 10th	Day	25% 15% 4 6% 4 3 3 2 3 5% 31%	3216
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 6% 8% 46%	3385
	21st to end	Day	24% 10% 2 7% 4 4 4 3 4 4 35%	3964
April	1st to 10th	Day	20% 10% 2 6% 4 3 4 4 4 6% 38%	2280
	11th to 20th	Day	19% 7% 2 6% 5% 4 3 3 5% 6% 40%	2278
	21st to end	Day	20% 7% 3 5% 4 5% 4 3 5% 7% 37%	2470
May	1st to 10th	Day	26% 12% 4 7% 3 4 3 3 4 6% 27%	4106
	11th to 20th	Day	23% 11% 3 7% 4 3 3 3 4 7% 32%	4081
	21st to end	Day	16% 7% 2 5% 4 4 4 4 5% 8% 42%	4769
June	1st to 10th	Day	20% 7% 2 5% 5% 5% 5% 4 6% 8% 35%	2660
	11th to 20th	Day	16% 6% 2 5% 5% 4 5% 5% 6% 9% 38%	2660
	21st to end	Day	15% 7% 2 6% 4 3 5% 4 7% 8% 39%	2660
July	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	4440
	11th to 20th	Day	14% 7% 2 7% 4 5% 5% 4 6% 10% 37%	4440
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 8% 32%	4757
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 6% 8% 32%	4105
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	4018
	21st to end	Day	8% 4 2 5% 4 4 4 5% 7% 10% 48%	4180
September	1st to 10th	Day	14% 6% 3 6% 4 4 4 3 5% 5% 47%	2249
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 5% 7% 49%	2090
	21st to end	Day	21% 6% 2 6% 3 4 3 3 6% 5% 42%	2090
October	1st to 10th	Day	21% 6% 2 5% 4 3 3 3 5% 6% 42%	2090
	11th to 20th	Day	16% 6% 2 4 3 3 3 3 5% 7% 48%	1887
	21st to end	Day	19% 6% 2 5% 4 3 3 4 5% 5% 45%	1881
November	1st to 10th	Day	17% 6% 2 5% 4 3 3 3 4 6% 46%	1761
	11th to 20th	Day	16% 6% 2 5% 4 3 3 4 5% 7% 46%	1800
	21st to end	Day	18% 7% 2 5% 4 4 3 3 5% 6% 44%	1800
December	1st to 10th	Day	15% 9% 2 5% 3 3 3 2 4 6% 49%	2798
	11th to 20th	Day	18% 8% 2 6% 3 3 2 2 4 5% 48%	2986
	21st to end	Day	19% 12% 4 7% 4 4 3 3 4 5% 37%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 6% 52%	6349
	11th to 20th	Night	15% 10% 4 7% 4 3 4 2 4 5% 42%	6315
	21st to end	Night	17% 10% 4 8% 4 4 4 3 5% 4 36%	6968
February	1st to 10th	Night	18% 9% 3 7% 4 4 4 2 4 6% 39%	2480
	11th to 20th	Night	19% 9% 3 5% 4 3 4 4 5% 5% 40%	2338
	21st to end	Night	17% 9% 2 7% 5% 3 4 3 4 5% 41%	1980
March	1st to 10th	Night	29% 12% 2 4 2 3 3 3 4 5% 33%	3844
	11th to 20th	Night	10% 9% 3 8% 4 5% 4 4 6% 6% 40%	3602
	21st to end	Night	25% 9% 2 6% 5% 4 5% 3 4 4 32%	3664
April	1st to 10th	Night	18% 9% 3 7% 6% 4 3 3 4 6% 37%	1900
	11th to 20th	Night	19% 9% 2 6% 4 4 4 3 5% 7% 38%	1849
	21st to end	Night	20% 8% 2 6% 6% 4 4 3 4 6% 38%	1710
May	1st to 10th	Night	25% 11% 4 10% 5% 4 4 3 5% 6% 24%	2834
	11th to 20th	Night	25% 12% 3 8% 5% 4 5% 4 6% 6% 24%	2813
	21st to end	Night	17% 8% 3 7% 4 5% 5% 5% 6% 7% 33%	2833
June	1st to 10th	Night	18% 8% 3 7% 4 4 4 3 6% 8% 35%	1520
	11th to 20th	Night	19% 7% 3 5% 4 4 4 3 5% 7% 39%	1520
	21st to end	Night	18% 8% 2 6% 4 4 4 3 7% 9% 35%	1520
July	1st to 10th	Night	19% 12% 4 7% 5% 4 4 3 5% 7% 30%	2520
	11th to 20th	Night	18% 8% 2 6% 4 4 4 5% 7% 9% 33%	2520
	21st to end	Night	18% 9% 3 7% 5% 5% 4 4 5% 8% 30%	2765
August	1st to 10th	Night	23% 8% 3 7% 5% 4 5% 4 7% 7% 28%	2825
	11th to 20th	Night	9% 7% 3 8% 6% 6% 6% 5% 7% 9% 36%	2805
	21st to end	Night	9% 4 2 4 4 3 4 3 7% 8% 53%	3476
September	1st to 10th	Night	13% 7% 3 5% 5% 4 5% 3 5% 6% 44%	1930
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 5% 6% 50%	2090
	21st to end	Night	20% 7% 2 5% 4 4 5% 4 5% 6% 40%	2090
October	1st to 10th	Night	20% 7% 2 6% 4 3 4 4 4 7% 38%	2090
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 5% 7% 47%	2290
	21st to end	Night	19% 8% 3 6% 3 3 4 4 5% 6% 39%	2717
November	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 6% 45%	2537
	11th to 20th	Night	16% 8% 2 5% 4 3 4 3 5% 5% 45%	2600
	21st to end	Night	15% 6% 2 6% 4 3 4 3 5% 7% 45%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 3 3 2 3 5% 47%	4377
	11th to 20th	Night	21% 9% 2 5% 4 3 3 2 3 5% 42%	5208
	21st to end	Night	17% 10% 4 7% 4 4 3 3 4 5% 40%	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	26% 27% 5% 20% 20% 3	3598
	11th to 20th	Day	26% 27% 7% 17% 15% 7%	3600
	21st to end	Day	38% 29% 6% 13% 14% 2	3985
February	1st to 10th	Day	20% 63% 6% 6% 4 1	1915
	11th to 20th	Day	19% 61% 3 8% 8% 2	1957
	21st to end	Day	17% 62% 5% 8% 6% 2	1650
March	1st to 10th	Day	42% 38% 7% 5% 5% 2	3219
	11th to 20th	Day	16% 43% 12% 18% 10% 1	3385
	21st to end	Day	36% 41% 3 8% 7% 5%	3964
April	1st to 10th	Day	19% 65% 3 6% 6% 1	2280
	11th to 20th	Day	14% 66% 4 9% 7%	2278
	21st to end	Day	17% 66% 3 6% 6% 1	2470
May	1st to 10th	Day	38% 44% 8% 6% 4	4120
	11th to 20th	Day	36% 41% 11% 9% 3	4085
	21st to end	Day	24% 43% 7% 13% 13%	4769
June	1st to 10th	Day	16% 66% 4 8% 6%	2660
	11th to 20th	Day	15% 65% 5% 9% 6% 1	2660
	21st to end	Day	14% 64% 5% 10% 7%	2660
July	1st to 10th	Day	34% 39% 7% 10% 10% 1	4370
	11th to 20th	Day	28% 38% 11% 14% 8%	4365
	21st to end	Day	35% 37% 8% 11% 8% 1	4605
August	1st to 10th	Day	34% 39% 11% 12% 4	4092
	11th to 20th	Day	26% 40% 12% 19% 3	4057
	21st to end	Day	15% 39% 11% 22% 12%	4180
September	1st to 10th	Day	15% 60% 4 13% 7%	2249
	11th to 20th	Day	15% 61% 5% 10% 9% 2	2090
	21st to end	Day	21% 60% 3 8% 7%	2090
October	1st to 10th	Day	23% 38% 7% 19% 10% 3	3480
	11th to 20th	Day	17% 37% 3 19% 20% 3	3103
	21st to end	Day	16% 37% 10% 24% 12% 1	3081
November	1st to 10th	Day	19% 56% 6% 8% 8% 1	1761
	11th to 20th	Day	17% 57% 4 8% 11% 2	1800
	21st to end	Day	17% 56% 5% 14% 6% 2	1800
December	1st to 10th	Day	24% 36% 9% 21% 10%	2798
	11th to 20th	Day	27% 32% 9% 12% 15% 5%	2986
	21st to end	Day	38% 27% 7% 14% 11% 3	3960
January	1st to 10th	Night	23% 26% 6% 28% 15% 2	6349
	11th to 20th	Night	33% 26% 8% 16% 12% 4	6360
	21st to end	Night	39% 30% 5% 12% 10% 4	6968
February	1st to 10th	Night	19% 62% 5% 8% 4 1	2480
	11th to 20th	Night	18% 61% 3 7% 10% 2	2340
	21st to end	Night	18% 62% 4 8% 6% 2	1980
March	1st to 10th	Night	39% 40% 5% 8% 5% 2	3854
	11th to 20th	Night	22% 44% 9% 12% 12% 1	3603
	21st to end	Night	38% 41% 3 4 10% 4	3664
April	1st to 10th	Night	20% 65% 3 6% 5% 1	1900
	11th to 20th	Night	15% 66% 4 6% 7% 1	1849
	21st to end	Night	18% 65% 3 7% 6% 1	1710
May	1st to 10th	Night	38% 43% 9% 7% 2	2840
	11th to 20th	Night	40% 43% 8% 7% 2	2814
	21st to end	Night	27% 44% 9% 13% 7%	2834
June	1st to 10th	Night	16% 65% 4 9% 6%	1520
	11th to 20th	Night	14% 65% 4 11% 6% 1	1520
	21st to end	Night	15% 65% 4 10% 7% 1	1520
July	1st to 10th	Night	38% 36% 4 9% 12%	2480
	11th to 20th	Night	30% 39% 10% 14% 6%	2472
	21st to end	Night	36% 37% 9% 9% 8% 1	2677
August	1st to 10th	Night	38% 40% 7% 9% 5% 1	2813
	11th to 20th	Night	26% 41% 10% 19% 4	2832
	21st to end	Night	13% 39% 12% 20% 16%	3476
September	1st to 10th	Night	15% 61% 5% 11% 7% 1	1930
	11th to 20th	Night	17% 60% 3 9% 10% 1	2090
	21st to end	Night	21% 61% 4 8% 6% 1	2090
October	1st to 10th	Night	26% 38% 7% 19% 9% 2	3480
	11th to 20th	Night	11% 37% 4 17% 28% 3	3813
	21st to end	Night	17% 37% 7% 21% 17% 1	4467
November	1st to 10th	Night	20% 57% 5% 9% 8% 1	2537
	11th to 20th	Night	17% 57% 4 9% 11% 3	2600
	21st to end	Night	16% 57% 6% 14% 6% 2	2600
December	1st to 10th	Night	26% 36% 7% 19% 12%	4377
	11th to 20th	Night	33% 31% 9% 10% 15% 1	5208
	21st to end	Night	36% 28% 7% 16% 13% 1	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	21% 25% 35% 17% 3	200
	11th to 20th	Day	22% 27% 37% 11% 4	200
	21st to end	Day	19% 23% 40% 13% 5% 1	232
February	1st to 10th	Day	24% 21% 37% 16% 2	457
	11th to 20th	Day	25% 26% 37% 11% 1	496
	21st to end	Day	22% 23% 37% 14% 4	414
March	1st to 10th	Day	24% 22% 34% 17% 3	500
	11th to 20th	Day	19% 27% 33% 17% 3	464
	21st to end	Day	22% 23% 34% 15% 5% 2	506
April	1st to 10th	Day	25% 20% 35% 15% 4 1	460
	11th to 20th	Day	24% 20% 36% 17% 3	452
	21st to end	Day	25% 16% 34% 18% 6% 2	460
May	1st to 10th	Day	30% 17% 31% 17% 5%	463
	11th to 20th	Day	22% 12% 41% 17% 6% 1	487
	21st to end	Day	21% 10% 31% 28% 9% 2	529
June	1st to 10th	Day	19% 10% 31% 29% 10% 1	490
	11th to 20th	Day	16% 9% 37% 29% 9% 1	490
	21st to end	Day	14% 10% 35% 29% 10% 1	490
July	1st to 10th	Day	16% 7% 27% 33% 16% 2	485
	11th to 20th	Day	11% 6% 29% 33% 17% 4	484
	21st to end	Day	15% 6% 33% 32% 13% 2	532
August	1st to 10th	Day	12% 7% 30% 35% 14% 2	486
	11th to 20th	Day	14% 8% 24% 33% 17% 4	479
	21st to end	Day	14% 5% 28% 31% 15% 6%	539
September	1st to 10th	Day	18% 6% 28% 28% 15% 5%	490
	11th to 20th	Day	16% 9% 29% 24% 15% 7%	490
	21st to end	Day	25% 10% 29% 23% 9% 4	490
October	1st to 10th	Day	23% 11% 26% 23% 12% 5%	490
	11th to 20th	Day	18% 12% 35% 20% 12% 2	426
	21st to end	Day	23% 16% 29% 21% 9% 3	429
November	1st to 10th	Day	15% 16% 36% 21% 10% 2	395
	11th to 20th	Day	21% 21% 33% 20% 5%	400
	21st to end	Day	21% 24% 30% 18% 8% 1	400
December	1st to 10th	Day	20% 18% 37% 20% 5%	320
	11th to 20th	Day	26% 27% 33% 12% 3 1	200
	21st to end	Day	26% 18% 36% 12% 6% 2	220
January	1st to 10th	Night	21% 19% 40% 17% 4	800
	11th to 20th	Night	27% 21% 34% 16% 3	800
	21st to end	Night	21% 22% 36% 18% 4	880
February	1st to 10th	Night	22% 18% 37% 21% 2	800
	11th to 20th	Night	24% 22% 39% 14% 1	783
	21st to end	Night	21% 22% 35% 19% 3	660
March	1st to 10th	Night	23% 18% 36% 19% 4	800
	11th to 20th	Night	20% 16% 39% 20% 4	767
	21st to end	Night	23% 17% 33% 21% 6%	836
April	1st to 10th	Night	22% 15% 34% 24% 4	760
	11th to 20th	Night	22% 16% 35% 23% 4	725
	21st to end	Night	22% 14% 32% 25% 6%	570
May	1st to 10th	Night	28% 12% 30% 24% 7%	570
	11th to 20th	Night	23% 12% 38% 22% 4 1	562
	21st to end	Night	23% 7% 23% 37% 9% 1	467
June	1st to 10th	Night	22% 5% 28% 32% 12%	380
	11th to 20th	Night	22% 5% 23% 39% 10% 1	380
	21st to end	Night	23% 5% 22% 40% 10%	380
July	1st to 10th	Night	19% 6% 24% 36% 14% 1	370
	11th to 20th	Night	16% 4 22% 41% 16% 1	368
	21st to end	Night	17% 4 23% 43% 13% 1	430
August	1st to 10th	Night	13% 5% 24% 45% 13% 1	568
	11th to 20th	Night	13% 5% 22% 43% 16% 1	572
	21st to end	Night	16% 4 19% 37% 20% 3	836
September	1st to 10th	Night	17% 3 26% 34% 18% 2	760
	11th to 20th	Night	18% 5% 25% 27% 20% 4	760
	21st to end	Night	24% 6% 23% 30% 14% 2	760
October	1st to 10th	Night	25% 10% 22% 27% 14% 3	760
	11th to 20th	Night	18% 14% 30% 25% 12% 2	760
	21st to end	Night	23% 14% 32% 21% 9%	836
November	1st to 10th	Night	19% 12% 32% 25% 11% 1	780
	11th to 20th	Night	20% 22% 32% 19% 7% 1	800
	21st to end	Night	17% 23% 33% 21% 6% 1	800
December	1st to 10th	Night	25% 19% 34% 18% 5% 1	800
	11th to 20th	Night	25% 22% 32% 18% 3 1	800
	21st to end	Night	25% 18% 37% 17% 3	880

Page generated at 2025-03-09 08:08:46. Version ac07

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