Christel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Christel Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	18% 12% 4 10% 6% 5% 4 3 5% 6% 27%	131162
	Night	17% 15% 5% 11% 6% 5% 4 3 4 5% 25%	130027
Transparency	Day	58% 5% 10% 18% 8% 1	134602
	Night	62% 5% 10% 15% 8% 1	134674
Seeing	Day	28% 18% 29% 17% 6% 2	16160
	Night	28% 17% 29% 19% 6% 1	25339

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% 21% 9% 20% 8% 6% 4 3 4 4 10%	4661
	11th to 20th	Day	9% 20% 9% 21% 11% 6% 4 2 3 3 11%	4650
	21st to end	Day	10% 18% 8% 15% 10% 11% 5% 3 3 3 13%	5240
February	1st to 10th	Day	14% 20% 6% 15% 8% 5% 5% 3 4 3 17%	1890
	11th to 20th	Day	13% 18% 6% 16% 8% 6% 4 3 4 4 18%	1925
	21st to end	Day	17% 18% 6% 12% 7% 6% 3 3 5% 4 17%	1730
March	1st to 10th	Day	20% 16% 6% 13% 5% 4 4 3 3 4 22%	4496
	11th to 20th	Day	10% 19% 5% 9% 6% 5% 5% 4 5% 5% 27%	4832
	21st to end	Day	19% 14% 4 9% 5% 4 4 2 3 4 31%	5876
April	1st to 10th	Day	20% 17% 4 8% 5% 3 3 3 4 5% 28%	2400
	11th to 20th	Day	20% 14% 3 7% 4 4 4 3 3 5% 33%	2374
	21st to end	Day	25% 12% 3 6% 4 4 4 3 4 5% 30%	2495
May	1st to 10th	Day	27% 13% 4 9% 5% 5% 4 3 4 5% 21%	6332
	11th to 20th	Day	20% 11% 4 8% 5% 5% 5% 4 5% 6% 27%	6318
	21st to end	Day	24% 9% 3 7% 5% 4 4 4 5% 6% 30%	6949
June	1st to 10th	Day	25% 8% 2 6% 5% 4 5% 4 6% 6% 28%	2800
	11th to 20th	Day	18% 7% 3 6% 5% 4 4 5% 7% 8% 33%	2800
	21st to end	Day	23% 8% 2 6% 5% 5% 4 4 7% 7% 29%	2800
July	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 4 7% 9% 33%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 4 6% 8% 38%	6360
	21st to end	Day	14% 6% 2 5% 4 4 4 4 6% 9% 40%	6926
August	1st to 10th	Day	18% 7% 3 7% 4 4 5% 4 6% 9% 32%	4563
	11th to 20th	Day	17% 7% 2 8% 5% 4 4 4 5% 8% 35%	4003
	21st to end	Day	19% 5% 2 6% 4 5% 4 4 6% 9% 37%	4312
September	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 6% 8% 41%	2400
	11th to 20th	Day	16% 6% 2 4 4 3 3 4 6% 6% 45%	2350
	21st to end	Day	18% 6% 2 6% 4 5% 4 4 6% 7% 36%	2200
October	1st to 10th	Day	22% 10% 2 6% 5% 4 4 3 4 6% 33%	2013
	11th to 20th	Day	20% 15% 4 8% 5% 4 4 4 5% 5% 27%	2000
	21st to end	Day	26% 16% 3 7% 5% 3 4 3 4 4 25%	1995
November	1st to 10th	Day	18% 16% 4 9% 5% 4 4 3 5% 6% 27%	1851
	11th to 20th	Day	15% 19% 5% 10% 4 5% 4 3 4 4 26%	1787
	21st to end	Day	18% 21% 7% 13% 5% 4 5% 3 4 4 17%	1680
December	1st to 10th	Day	16% 24% 7% 19% 8% 4 3 3 4 3 9%	2680
	11th to 20th	Day	17% 18% 7% 14% 5% 4 4 3 4 4 19%	3066
	21st to end	Day	17% 20% 6% 13% 6% 4 3 4 4 4 20%	4048
January	1st to 10th	Night	11% 21% 9% 21% 8% 4 3 2 3 3 13%	8234
	11th to 20th	Night	10% 22% 9% 22% 9% 6% 4 3 3 3 9%	8235
	21st to end	Night	11% 16% 7% 16% 10% 9% 6% 3 4 4 13%	8937
February	1st to 10th	Night	14% 19% 7% 17% 8% 5% 4 3 4 4 16%	2730
	11th to 20th	Night	13% 17% 7% 15% 8% 6% 4 3 4 4 20%	2579
	21st to end	Night	14% 18% 6% 13% 8% 4 3 2 4 5% 22%	2076
March	1st to 10th	Night	18% 16% 5% 10% 7% 5% 3 2 3 3 27%	5389
	11th to 20th	Night	14% 18% 5% 9% 4 4 4 3 5% 7% 28%	5124
	21st to end	Night	18% 17% 5% 11% 5% 4 3 3 3 3 28%	5055
April	1st to 10th	Night	21% 15% 4 10% 5% 4 3 3 4 5% 27%	2000
	11th to 20th	Night	22% 13% 3 8% 5% 4 4 3 3 5% 30%	1975
	21st to end	Night	22% 13% 3 7% 4 3 3 2 4 5% 33%	1904
May	1st to 10th	Night	29% 13% 3 7% 4 4 4 4 5% 6% 21%	3650
	11th to 20th	Night	23% 10% 4 7% 5% 4 4 3 5% 7% 29%	3575
	21st to end	Night	21% 11% 3 7% 5% 4 4 4 5% 7% 29%	3926
June	1st to 10th	Night	22% 8% 3 7% 5% 4 3 4 7% 8% 28%	1600
	11th to 20th	Night	18% 7% 2 6% 4 4 5% 3 6% 8% 36%	1600
	21st to end	Night	20% 9% 2 7% 5% 5% 5% 5% 5% 7% 31%	1600
July	1st to 10th	Night	16% 6% 2 7% 5% 5% 5% 5% 6% 8% 36%	3600
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 5% 7% 41%	3600
	21st to end	Night	12% 8% 3 7% 4 4 4 3 6% 7% 42%	3926
August	1st to 10th	Night	18% 8% 3 7% 4 4 4 3 5% 6% 38%	2586
	11th to 20th	Night	18% 9% 4 6% 4 4 4 3 6% 7% 36%	3007
	21st to end	Night	16% 7% 2 7% 4 4 5% 4 6% 7% 40%	3586
September	1st to 10th	Night	16% 6% 3 6% 4 4 4 3 5% 7% 42%	2000
	11th to 20th	Night	15% 6% 2 4 3 3 4 3 4 7% 49%	2049
	21st to end	Night	19% 7% 2 6% 5% 4 4 3 5% 7% 39%	2200
October	1st to 10th	Night	21% 9% 2 7% 4 4 4 3 5% 6% 34%	2384
	11th to 20th	Night	21% 11% 3 6% 5% 4 4 3 4 5% 32%	2400
	21st to end	Night	27% 15% 3 6% 5% 4 3 3 4 5% 24%	2845
November	1st to 10th	Night	21% 15% 3 8% 4 4 5% 3 4 5% 28%	2667
	11th to 20th	Night	18% 17% 5% 8% 5% 4 4 3 5% 5% 27%	2830
	21st to end	Night	20% 20% 7% 10% 5% 5% 4 3 4 5% 19%	2940
December	1st to 10th	Night	15% 28% 10% 16% 5% 4 3 2 3 3 9%	4720
	11th to 20th	Night	18% 21% 8% 14% 5% 4 3 3 3 3 19%	5348
	21st to end	Night	20% 18% 6% 14% 5% 5% 5% 2 3 3 18%	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	80% 4 6% 9% 2	4662
	11th to 20th	Day	80% 3 5% 5% 6% 1	4680
	21st to end	Day	78% 3 6% 7% 6%	5240
February	1st to 10th	Day	73% 3 6% 14% 3 1	1890
	11th to 20th	Day	71% 3 8% 12% 6%	1934
	21st to end	Day	71% 4 7% 10% 6% 1	1730
March	1st to 10th	Day	68% 3 8% 11% 7% 3	4499
	11th to 20th	Day	59% 5% 13% 12% 8% 2	4832
	21st to end	Day	59% 3 7% 14% 16% 2	5876
April	1st to 10th	Day	61% 3 7% 18% 10% 2	2400
	11th to 20th	Day	55% 4 7% 18% 14% 2	2374
	21st to end	Day	57% 5% 8% 15% 12% 3	2495
May	1st to 10th	Day	65% 5% 8% 15% 6% 1	6343
	11th to 20th	Day	56% 8% 10% 16% 9% 1	6322
	21st to end	Day	55% 6% 10% 21% 8% 1	6949
June	1st to 10th	Day	55% 6% 11% 17% 11%	2800
	11th to 20th	Day	48% 6% 11% 24% 9% 2	2800
	21st to end	Day	53% 7% 11% 19% 10% 1	2800
July	1st to 10th	Day	46% 6% 14% 25% 9%	6290
	11th to 20th	Day	42% 7% 14% 27% 10%	6285
	21st to end	Day	40% 7% 12% 36% 6%	6728
August	1st to 10th	Day	48% 7% 14% 22% 8%	4549
	11th to 20th	Day	47% 8% 16% 24% 6%	4042
	21st to end	Day	43% 7% 17% 23% 9%	4312
September	1st to 10th	Day	43% 6% 13% 27% 11% 1	2400
	11th to 20th	Day	39% 5% 11% 30% 14% 1	2350
	21st to end	Day	48% 6% 14% 23% 10%	2200
October	1st to 10th	Day	47% 5% 11% 26% 11%	3273
	11th to 20th	Day	52% 4 13% 20% 10% 1	3240
	21st to end	Day	51% 4 16% 23% 5% 1	3195
November	1st to 10th	Day	60% 5% 11% 18% 6%	1851
	11th to 20th	Day	62% 4 11% 12% 8% 3	1787
	21st to end	Day	73% 5% 10% 9% 4	1680
December	1st to 10th	Day	82% 3 7% 5% 1	2680
	11th to 20th	Day	70% 5% 8% 12% 5%	3066
	21st to end	Day	69% 5% 6% 13% 6%	4048
January	1st to 10th	Night	78% 3 9% 8% 2	8234
	11th to 20th	Night	83% 4 5% 5% 3	8280
	21st to end	Night	77% 3 6% 8% 6%	8937
February	1st to 10th	Night	74% 3 6% 10% 6%	2730
	11th to 20th	Night	69% 4 9% 13% 5%	2581
	21st to end	Night	67% 4 8% 13% 8%	2076
March	1st to 10th	Night	64% 3 8% 13% 10% 1	5399
	11th to 20th	Night	56% 6% 16% 12% 7% 3	5125
	21st to end	Night	63% 3 5% 17% 11% 2	5055
April	1st to 10th	Night	60% 4 8% 17% 10% 1	2000
	11th to 20th	Night	58% 5% 8% 17% 12% 1	1975
	21st to end	Night	55% 4 9% 18% 13% 3	1904
May	1st to 10th	Night	63% 7% 9% 13% 7% 1	3656
	11th to 20th	Night	54% 7% 10% 18% 10% 1	3576
	21st to end	Night	54% 6% 10% 19% 12% 1	3927
June	1st to 10th	Night	52% 8% 12% 18% 10% 1	1600
	11th to 20th	Night	45% 6% 15% 19% 15% 1	1600
	21st to end	Night	51% 7% 12% 20% 10%	1600
July	1st to 10th	Night	44% 6% 16% 23% 10%	3560
	11th to 20th	Night	43% 6% 14% 27% 11%	3552
	21st to end	Night	41% 7% 12% 31% 9%	3809
August	1st to 10th	Night	47% 7% 14% 23% 9%	2576
	11th to 20th	Night	47% 7% 16% 21% 8% 1	3034
	21st to end	Night	41% 7% 14% 27% 10% 1	3586
September	1st to 10th	Night	41% 7% 12% 28% 11% 1	2000
	11th to 20th	Night	37% 5% 13% 32% 12%	2049
	21st to end	Night	44% 7% 11% 24% 13% 1	2200
October	1st to 10th	Night	49% 5% 11% 23% 11% 1	3904
	11th to 20th	Night	50% 4 13% 20% 11% 1	3899
	21st to end	Night	56% 4 15% 18% 6% 1	4595
November	1st to 10th	Night	59% 5% 12% 14% 8% 1	2667
	11th to 20th	Night	61% 5% 11% 14% 7% 3	2830
	21st to end	Night	70% 5% 9% 11% 4	2940
December	1st to 10th	Night	82% 4 6% 6% 1	4720
	11th to 20th	Night	74% 4 6% 10% 6%	5348
	21st to end	Night	73% 5% 5% 10% 6% 2	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	31% 21% 31% 12% 4	220
	11th to 20th	Day	31% 27% 30% 9% 3	220
	21st to end	Day	27% 27% 29% 15% 2	262
February	1st to 10th	Day	37% 29% 28% 7%	330
	11th to 20th	Day	32% 29% 33% 6%	412
	21st to end	Day	35% 25% 31% 8% 1	446
March	1st to 10th	Day	33% 23% 28% 13% 2	508
	11th to 20th	Day	33% 27% 28% 10% 2 1	500
	21st to end	Day	38% 22% 24% 11% 5%	550
April	1st to 10th	Day	39% 19% 27% 13% 3	500
	11th to 20th	Day	38% 20% 21% 15% 4 1	492
	21st to end	Day	37% 20% 27% 9% 5% 1	500
May	1st to 10th	Day	32% 18% 28% 15% 6% 2	503
	11th to 20th	Day	27% 20% 27% 18% 8%	527
	21st to end	Day	25% 18% 26% 22% 8%	573
June	1st to 10th	Day	30% 17% 30% 17% 5% 1	530
	11th to 20th	Day	23% 15% 35% 18% 6% 2	530
	21st to end	Day	28% 15% 34% 14% 7% 1	530
July	1st to 10th	Day	18% 11% 35% 24% 10% 1	525
	11th to 20th	Day	17% 13% 34% 26% 7% 2	524
	21st to end	Day	17% 13% 36% 27% 7% 1	576
August	1st to 10th	Day	18% 10% 30% 29% 12% 2	526
	11th to 20th	Day	20% 8% 27% 29% 11% 4	519
	21st to end	Day	21% 13% 30% 21% 11% 3	583
September	1st to 10th	Day	20% 12% 29% 23% 11% 6%	530
	11th to 20th	Day	16% 11% 24% 24% 18% 7%	530
	21st to end	Day	20% 13% 29% 24% 10% 3	530
October	1st to 10th	Day	29% 15% 24% 19% 9% 4	530
	11th to 20th	Day	32% 20% 27% 13% 6% 1	530
	21st to end	Day	38% 16% 27% 16% 3	470
November	1st to 10th	Day	36% 20% 25% 15% 4	425
	11th to 20th	Day	31% 23% 28% 9% 8% 1	327
	21st to end	Day	38% 22% 31% 7% 2	220
December	1st to 10th	Day	35% 17% 30% 15% 2	220
	11th to 20th	Day	37% 27% 27% 7% 1	220
	21st to end	Day	38% 19% 29% 11% 2 1	242
January	1st to 10th	Night	30% 26% 32% 11% 2	840
	11th to 20th	Night	31% 26% 26% 14% 2 1	840
	21st to end	Night	24% 23% 32% 17% 4	924
February	1st to 10th	Night	29% 23% 33% 13% 2	840
	11th to 20th	Night	27% 25% 34% 12% 1	823
	21st to end	Night	28% 21% 35% 14% 2	692
March	1st to 10th	Night	30% 20% 31% 17% 2	809
	11th to 20th	Night	32% 22% 29% 14% 4	800
	21st to end	Night	30% 20% 27% 17% 4 1	880
April	1st to 10th	Night	31% 18% 32% 17% 2	800
	11th to 20th	Night	32% 20% 26% 15% 6%	791
	21st to end	Night	33% 16% 29% 16% 6%	704
May	1st to 10th	Night	36% 10% 30% 19% 5% 1	430
	11th to 20th	Night	23% 11% 35% 25% 5% 1	394
	21st to end	Night	19% 13% 33% 25% 9%	433
June	1st to 10th	Night	25% 10% 29% 29% 7% 1	400
	11th to 20th	Night	23% 7% 31% 28% 9% 1	400
	21st to end	Night	23% 9% 35% 26% 7% 1	400
July	1st to 10th	Night	13% 10% 32% 37% 7% 1	390
	11th to 20th	Night	12% 6% 34% 35% 12% 1	388
	21st to end	Night	16% 7% 35% 32% 8% 1	438
August	1st to 10th	Night	16% 8% 26% 37% 14%	398
	11th to 20th	Night	21% 6% 22% 33% 16% 1	661
	21st to end	Night	19% 8% 30% 30% 11% 1	880
September	1st to 10th	Night	19% 11% 27% 30% 12% 1	800
	11th to 20th	Night	19% 8% 21% 31% 19% 3	800
	21st to end	Night	25% 11% 25% 24% 15% 1	800
October	1st to 10th	Night	29% 12% 26% 20% 12%	800
	11th to 20th	Night	29% 16% 28% 21% 6% 1	800
	21st to end	Night	38% 15% 26% 18% 3	880
November	1st to 10th	Night	33% 17% 28% 16% 5%	820
	11th to 20th	Night	30% 21% 28% 12% 7% 1	840
	21st to end	Night	34% 23% 26% 12% 5%	840
December	1st to 10th	Night	37% 24% 23% 13% 3	840
	11th to 20th	Night	34% 23% 28% 11% 3 1	840
	21st to end	Night	35% 23% 27% 12% 3	924

Page generated at 2026-02-28 01:40:22. Version ac07

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