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 2024-07-03.

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% 13% 4 9% 5% 4 4 3 5% 6% 28%	89593
	Night	18% 15% 5% 10% 5% 4 4 3 4 5% 26%	89626
Transparency	Day	58% 6% 10% 17% 8% 1	89312
	Night	61% 6% 10% 15% 8% 1	89461
Seeing	Day	29% 17% 29% 17% 7% 2	14115
	Night	28% 17% 29% 20% 6% 1	23204

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	15% 23% 8% 16% 9% 5% 4 3 4 3 12%	2517
	11th to 20th	Day	13% 19% 7% 20% 10% 5% 5% 2 3 3 13%	2520
	21st to end	Day	14% 23% 8% 14% 6% 7% 5% 3 4 4 12%	2832
February	1st to 10th	Day	15% 20% 6% 14% 8% 5% 4 3 4 3 17%	1710
	11th to 20th	Day	13% 18% 6% 16% 8% 6% 4 3 4 4 19%	1737
	21st to end	Day	17% 18% 6% 11% 7% 6% 3 3 6% 4 19%	1570
March	1st to 10th	Day	23% 15% 4 10% 6% 4 4 3 4 5% 24%	3116
	11th to 20th	Day	13% 25% 6% 10% 6% 4 4 3 4 4 23%	3374
	21st to end	Day	16% 12% 4 9% 5% 4 4 3 3 5% 35%	4100
April	1st to 10th	Day	20% 17% 4 7% 5% 3 3 3 4 5% 29%	2280
	11th to 20th	Day	21% 13% 3 7% 4 4 4 3 4 5% 33%	2254
	21st to end	Day	25% 11% 3 6% 4 4 4 3 4 5% 30%	2370
May	1st to 10th	Day	22% 12% 3 8% 5% 4 4 3 4 6% 27%	4412
	11th to 20th	Day	18% 11% 4 8% 5% 5% 5% 4 5% 6% 29%	4398
	21st to end	Day	16% 8% 3 6% 5% 4 5% 5% 6% 8% 35%	4837
June	1st to 10th	Day	26% 7% 2 6% 5% 4 5% 4 6% 6% 28%	2660
	11th to 20th	Day	19% 7% 3 6% 5% 4 4 4 7% 8% 33%	2660
	21st to end	Day	23% 8% 2 6% 5% 5% 4 4 7% 7% 28%	2660
July	1st to 10th	Day	16% 7% 3 6% 4 4 4 4 6% 7% 39%	2548
	11th to 20th	Day	15% 7% 2 6% 6% 5% 5% 5% 7% 8% 34%	2520
	21st to end	Day	14% 7% 3 6% 5% 5% 5% 5% 7% 9% 34%	2744
August	1st to 10th	Day	15% 6% 3 7% 4 5% 6% 4 7% 8% 34%	2506
	11th to 20th	Day	13% 7% 2 8% 5% 5% 5% 5% 7% 9% 34%	2221
	21st to end	Day	17% 6% 2 7% 5% 5% 5% 4 6% 8% 35%	2376
September	1st to 10th	Day	17% 6% 2 6% 5% 4 4 3 6% 8% 39%	2160
	11th to 20th	Day	18% 7% 2 4 4 3 3 3 6% 7% 43%	2116
	21st to end	Day	18% 6% 3 6% 4 5% 5% 4 6% 7% 36%	1980
October	1st to 10th	Day	24% 9% 2 7% 5% 5% 5% 3 5% 6% 30%	1813
	11th to 20th	Day	21% 15% 4 8% 5% 4 4 4 5% 5% 25%	1800
	21st to end	Day	28% 17% 3 7% 5% 3 3 3 4 4 23%	1796
November	1st to 10th	Day	17% 15% 4 9% 5% 4 4 3 5% 6% 28%	1671
	11th to 20th	Day	16% 18% 4 10% 4 5% 5% 3 4 4 26%	1617
	21st to end	Day	17% 20% 6% 13% 6% 4 5% 3 4 4 18%	1520
December	1st to 10th	Day	20% 26% 6% 12% 6% 4 3 3 5% 3 11%	1520
	11th to 20th	Day	19% 20% 7% 11% 6% 4 4 3 4 4 18%	1906
	21st to end	Day	23% 21% 6% 13% 5% 4 4 3 3 3 16%	2772
January	1st to 10th	Night	15% 23% 9% 16% 6% 4 3 2 3 3 15%	4429
	11th to 20th	Night	11% 21% 8% 21% 8% 6% 4 3 3 3 11%	4440
	21st to end	Night	18% 21% 8% 15% 6% 6% 4 2 3 3 14%	4820
February	1st to 10th	Night	15% 20% 7% 16% 8% 4 4 3 4 4 15%	2470
	11th to 20th	Night	13% 17% 6% 14% 7% 6% 4 3 4 4 20%	2327
	21st to end	Night	15% 17% 6% 13% 7% 4 3 2 4 5% 23%	1884
March	1st to 10th	Night	19% 14% 4 9% 6% 4 3 3 3 4 31%	3724
	11th to 20th	Night	16% 24% 6% 10% 4 3 3 2 3 4 24%	3582
	21st to end	Night	16% 16% 4 10% 5% 3 3 2 3 3 34%	3531
April	1st to 10th	Night	20% 16% 4 9% 5% 3 4 3 4 5% 28%	1900
	11th to 20th	Night	22% 13% 3 8% 5% 4 4 3 3 5% 31%	1875
	21st to end	Night	22% 13% 3 7% 4 3 3 2 4 5% 33%	1809
May	1st to 10th	Night	24% 13% 3 6% 4 4 4 4 5% 6% 25%	2556
	11th to 20th	Night	23% 10% 3 6% 5% 3 4 3 5% 6% 32%	2495
	21st to end	Night	15% 8% 3 7% 4 3 4 4 6% 9% 37%	2738
June	1st to 10th	Night	22% 8% 4 7% 5% 4 3 4 7% 8% 29%	1520
	11th to 20th	Night	18% 7% 2 5% 4 4 5% 3 7% 8% 37%	1520
	21st to end	Night	21% 9% 2 7% 5% 4 5% 4 6% 7% 31%	1520
July	1st to 10th	Night	14% 4 3 5% 4 4 4 4 5% 8% 45%	1456
	11th to 20th	Night	11% 5% 3 6% 5% 5% 5% 5% 7% 9% 41%	1440
	21st to end	Night	12% 8% 2 6% 5% 4 5% 4 7% 7% 41%	1574
August	1st to 10th	Night	13% 6% 2 7% 5% 5% 4 4 6% 8% 41%	1432
	11th to 20th	Night	16% 7% 3 5% 3 4 3 3 7% 7% 40%	1657
	21st to end	Night	15% 7% 2 8% 4 4 5% 3 5% 7% 40%	1980
September	1st to 10th	Night	17% 6% 3 6% 4 4 4 3 5% 7% 41%	1800
	11th to 20th	Night	17% 7% 2 4 3 3 4 3 4 6% 47%	1843
	21st to end	Night	19% 7% 2 5% 5% 3 4 3 5% 7% 39%	1980
October	1st to 10th	Night	23% 9% 2 7% 4 4 4 4 5% 6% 32%	2145
	11th to 20th	Night	22% 11% 3 7% 5% 5% 5% 3 4 5% 30%	2160
	21st to end	Night	28% 16% 4 6% 5% 4 3 3 4 5% 23%	2560
November	1st to 10th	Night	20% 15% 3 8% 5% 4 5% 3 5% 5% 28%	2407
	11th to 20th	Night	18% 16% 5% 9% 5% 4 4 3 5% 5% 27%	2560
	21st to end	Night	20% 18% 6% 9% 6% 5% 4 3 4 5% 20%	2660
December	1st to 10th	Night	19% 24% 8% 13% 6% 4 3 2 4 4 12%	2660
	11th to 20th	Night	20% 22% 6% 12% 5% 4 3 3 3 3 20%	3288
	21st to end	Night	25% 19% 6% 15% 5% 4 3 2 3 3 13%	4884

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	77% 5% 8% 7% 2	2518
	11th to 20th	Day	78% 5% 7% 5% 4 1	2520
	21st to end	Day	76% 5% 8% 6% 3 1	2832
February	1st to 10th	Day	73% 3 6% 14% 3 1	1710
	11th to 20th	Day	71% 3 8% 11% 6% 1	1746
	21st to end	Day	69% 5% 8% 10% 7% 1	1570
March	1st to 10th	Day	64% 4 9% 15% 7% 1	3119
	11th to 20th	Day	66% 4 11% 10% 7% 1	3374
	21st to end	Day	53% 3 8% 18% 15% 2	4100
April	1st to 10th	Day	60% 3 7% 18% 10% 2	2280
	11th to 20th	Day	55% 4 8% 18% 13% 2	2254
	21st to end	Day	57% 5% 9% 15% 12% 3	2370
May	1st to 10th	Day	57% 6% 10% 18% 8% 1	4426
	11th to 20th	Day	53% 10% 11% 18% 8% 1	4402
	21st to end	Day	46% 7% 11% 24% 11% 1	4837
June	1st to 10th	Day	56% 6% 11% 16% 11%	2660
	11th to 20th	Day	49% 6% 11% 24% 9% 2	2660
	21st to end	Day	54% 7% 11% 18% 9%	2660
July	1st to 10th	Day	43% 7% 12% 25% 12% 1	2478
	11th to 20th	Day	45% 8% 15% 25% 7%	2445
	21st to end	Day	44% 9% 15% 22% 8%	2591
August	1st to 10th	Day	46% 9% 15% 22% 8%	2492
	11th to 20th	Day	45% 9% 15% 21% 9%	2221
	21st to end	Day	46% 8% 12% 23% 10%	2376
September	1st to 10th	Day	44% 6% 14% 25% 11% 1	2160
	11th to 20th	Day	41% 6% 11% 29% 13% 1	2116
	21st to end	Day	48% 6% 14% 24% 8%	1980
October	1st to 10th	Day	57% 5% 10% 18% 10%	1813
	11th to 20th	Day	60% 6% 13% 14% 6% 1	1800
	21st to end	Day	66% 5% 10% 12% 6% 1	1796
November	1st to 10th	Day	59% 5% 12% 18% 6%	1671
	11th to 20th	Day	63% 5% 11% 12% 6% 3	1617
	21st to end	Day	71% 5% 10% 9% 4	1520
December	1st to 10th	Day	78% 4 8% 7% 2	1520
	11th to 20th	Day	72% 5% 9% 10% 3	1906
	21st to end	Day	74% 6% 6% 11% 2	2772
January	1st to 10th	Night	75% 5% 10% 7% 2	4429
	11th to 20th	Night	78% 6% 7% 6% 3	4440
	21st to end	Night	77% 4 7% 8% 4 1	4820
February	1st to 10th	Night	75% 4 7% 9% 6%	2470
	11th to 20th	Night	68% 4 9% 13% 6%	2329
	21st to end	Night	66% 4 8% 14% 8%	1884
March	1st to 10th	Night	59% 4 11% 16% 9% 1	3734
	11th to 20th	Night	63% 6% 13% 11% 6% 1	3583
	21st to end	Night	56% 3 6% 19% 13% 2	3531
April	1st to 10th	Night	60% 5% 8% 17% 10% 1	1900
	11th to 20th	Night	58% 5% 8% 17% 12% 1	1875
	21st to end	Night	55% 4 9% 17% 13% 3	1809
May	1st to 10th	Night	58% 7% 10% 14% 9% 1	2562
	11th to 20th	Night	52% 8% 11% 18% 9% 1	2496
	21st to end	Night	43% 6% 12% 21% 16% 1	2739
June	1st to 10th	Night	52% 8% 11% 18% 10% 1	1520
	11th to 20th	Night	44% 6% 15% 20% 15% 1	1520
	21st to end	Night	51% 7% 13% 20% 9%	1520
July	1st to 10th	Night	36% 8% 13% 27% 16% 1	1416
	11th to 20th	Night	38% 7% 14% 30% 11%	1392
	21st to end	Night	40% 9% 16% 23% 11% 1	1486
August	1st to 10th	Night	40% 10% 17% 24% 10%	1422
	11th to 20th	Night	42% 8% 14% 23% 11% 1	1657
	21st to end	Night	44% 7% 13% 26% 10%	1980
September	1st to 10th	Night	42% 8% 12% 26% 11% 1	1800
	11th to 20th	Night	38% 5% 14% 31% 11%	1843
	21st to end	Night	44% 7% 11% 26% 12% 1	1980
October	1st to 10th	Night	53% 6% 12% 19% 9% 1	2145
	11th to 20th	Night	57% 6% 13% 16% 7% 1	2160
	21st to end	Night	66% 5% 11% 11% 6% 1	2560
November	1st to 10th	Night	58% 5% 13% 14% 8% 1	2407
	11th to 20th	Night	61% 5% 11% 14% 6% 3	2560
	21st to end	Night	69% 6% 10% 11% 4	2660
December	1st to 10th	Night	78% 5% 7% 8% 2	2660
	11th to 20th	Night	72% 4 8% 11% 5%	3288
	21st to end	Night	77% 6% 6% 10% 1	4884

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	34% 18% 29% 14% 4 1	180
	11th to 20th	Day	29% 25% 31% 11% 4	180
	21st to end	Day	25% 25% 31% 17% 1	214
February	1st to 10th	Day	39% 31% 26% 4	270
	11th to 20th	Day	31% 30% 33% 5% 1	344
	21st to end	Day	36% 24% 30% 9% 1	382
March	1st to 10th	Day	32% 22% 29% 13% 3	460
	11th to 20th	Day	33% 26% 28% 11% 2 1	460
	21st to end	Day	37% 23% 23% 12% 5%	506
April	1st to 10th	Day	37% 18% 27% 14% 3	460
	11th to 20th	Day	37% 20% 22% 16% 4 2	452
	21st to end	Day	37% 19% 27% 10% 6% 1	460
May	1st to 10th	Day	31% 18% 28% 15% 6% 2	463
	11th to 20th	Day	26% 19% 27% 18% 9%	487
	21st to end	Day	24% 18% 26% 23% 9%	529
June	1st to 10th	Day	31% 16% 29% 17% 6% 1	490
	11th to 20th	Day	23% 14% 34% 19% 7% 2	490
	21st to end	Day	30% 15% 32% 14% 8% 1	490
July	1st to 10th	Day	17% 11% 34% 26% 11% 1	453
	11th to 20th	Day	18% 12% 34% 26% 8% 2	444
	21st to end	Day	17% 13% 35% 27% 7% 1	488
August	1st to 10th	Day	18% 10% 31% 27% 11% 2	446
	11th to 20th	Day	20% 8% 27% 28% 12% 4	439
	21st to end	Day	21% 13% 30% 21% 11% 4	495
September	1st to 10th	Day	20% 12% 30% 22% 10% 7%	450
	11th to 20th	Day	19% 13% 25% 22% 16% 6%	450
	21st to end	Day	20% 13% 28% 25% 11% 3	450
October	1st to 10th	Day	32% 15% 22% 17% 10% 4	450
	11th to 20th	Day	33% 20% 27% 13% 7% 1	450
	21st to end	Day	39% 15% 28% 15% 3	403
November	1st to 10th	Day	33% 19% 25% 17% 5% 1	365
	11th to 20th	Day	33% 22% 27% 9% 7% 1	277
	21st to end	Day	34% 21% 33% 9% 2	180
December	1st to 10th	Day	39% 16% 26% 16% 2	180
	11th to 20th	Day	37% 25% 29% 8% 1 1	180
	21st to end	Day	41% 18% 29% 10% 2 1	198
January	1st to 10th	Night	31% 24% 31% 11% 2	760
	11th to 20th	Night	31% 25% 26% 14% 3 1	760
	21st to end	Night	23% 21% 34% 17% 4	836
February	1st to 10th	Night	30% 23% 33% 12% 2	760
	11th to 20th	Night	27% 25% 34% 13% 1	743
	21st to end	Night	28% 21% 34% 15% 3	628
March	1st to 10th	Night	30% 19% 31% 18% 2	760
	11th to 20th	Night	32% 22% 28% 14% 4	760
	21st to end	Night	29% 20% 27% 18% 4 1	836
April	1st to 10th	Night	31% 18% 32% 18% 2	760
	11th to 20th	Night	32% 20% 26% 16% 6%	751
	21st to end	Night	32% 16% 29% 16% 6%	669
May	1st to 10th	Night	34% 10% 31% 19% 6% 1	409
	11th to 20th	Night	23% 12% 34% 26% 5% 1	374
	21st to end	Night	18% 13% 33% 26% 10%	411
June	1st to 10th	Night	24% 11% 28% 29% 7% 1	380
	11th to 20th	Night	22% 7% 31% 29% 10% 1	380
	21st to end	Night	23% 9% 35% 25% 7% 1	380
July	1st to 10th	Night	12% 9% 31% 39% 8% 1	354
	11th to 20th	Night	11% 5% 34% 34% 13% 1	348
	21st to end	Night	15% 6% 37% 31% 9% 1	394
August	1st to 10th	Night	15% 8% 27% 35% 14%	358
	11th to 20th	Night	21% 5% 22% 34% 16% 1	593
	21st to end	Night	18% 7% 31% 30% 12% 2	792
September	1st to 10th	Night	19% 10% 27% 30% 12% 1	720
	11th to 20th	Night	21% 8% 23% 29% 16% 3	720
	21st to end	Night	24% 10% 26% 23% 15% 2	720
October	1st to 10th	Night	30% 13% 26% 19% 12%	720
	11th to 20th	Night	30% 16% 26% 21% 6% 1	720
	21st to end	Night	40% 14% 25% 19% 2	792
November	1st to 10th	Night	31% 17% 29% 17% 5% 1	740
	11th to 20th	Night	30% 21% 28% 12% 7% 1	760
	21st to end	Night	34% 22% 26% 13% 5%	760
December	1st to 10th	Night	38% 23% 23% 13% 3	760
	11th to 20th	Night	33% 23% 28% 12% 3 1	760
	21st to end	Night	35% 23% 28% 12% 3	836

Page generated at 2024-06-29 22:08:54. Version ac07

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