Eileen Collins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eileen Collins Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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	15% 11% 4 9% 5% 5% 5% 4 6% 7% 30%	101473
	Night	16% 11% 4 9% 5% 5% 4 4 5% 6% 31%	97550
Transparency	Day	51% 6% 11% 20% 10% 1	105060
	Night	52% 6% 12% 19% 10% 1	102051
Seeing	Day	24% 17% 28% 19% 9% 3	14696
	Night	22% 15% 28% 24% 10% 1	23983

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	13% 18% 9% 12% 6% 4 5% 3 4 5% 22%	2517
	11th to 20th	Day	11% 16% 8% 19% 7% 4 4 3 4 5% 18%	2626
	21st to end	Day	18% 16% 9% 12% 7% 6% 4 4 4 4 15%	3124
February	1st to 10th	Day	14% 15% 5% 12% 8% 6% 6% 3 4 4 21%	1710
	11th to 20th	Day	13% 14% 6% 14% 7% 4 6% 4 5% 5% 23%	1665
	21st to end	Day	18% 15% 3 11% 7% 4 3 4 5% 5% 26%	1570
March	1st to 10th	Day	21% 14% 5% 10% 4 4 4 2 3 5% 27%	3296
	11th to 20th	Day	10% 18% 5% 10% 5% 4 4 3 5% 5% 31%	3480
	21st to end	Day	15% 11% 4 11% 6% 4 5% 3 5% 6% 29%	3924
April	1st to 10th	Day	15% 16% 3 9% 6% 4 3 3 5% 6% 30%	2280
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 5% 6% 7% 31%	2254
	21st to end	Day	18% 11% 3 7% 4 5% 3 3 5% 7% 32%	2337
May	1st to 10th	Day	23% 10% 3 7% 4 4 4 3 4 6% 33%	4231
	11th to 20th	Day	19% 10% 4 8% 6% 5% 5% 4 7% 8% 25%	4398
	21st to end	Day	11% 7% 3 7% 5% 5% 4 4 6% 8% 41%	4837
June	1st to 10th	Day	15% 8% 2 8% 5% 5% 6% 4 7% 8% 33%	2660
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	2660
	21st to end	Day	11% 7% 3 7% 5% 5% 6% 5% 6% 10% 35%	2660
July	1st to 10th	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 35%	4440
	11th to 20th	Day	7% 5% 2 7% 6% 6% 6% 6% 9% 12% 34%	4440
	21st to end	Day	8% 7% 3 8% 6% 6% 6% 5% 7% 8% 34%	4667
August	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 7% 9% 31%	4104
	11th to 20th	Day	15% 8% 3 8% 6% 6% 6% 5% 8% 9% 27%	3980
	21st to end	Day	13% 8% 3 6% 4 5% 5% 4 7% 9% 36%	4180
September	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 4 5% 8% 39%	2280
	11th to 20th	Day	14% 7% 3 6% 4 5% 3 4 5% 8% 42%	2280
	21st to end	Day	20% 8% 3 6% 5% 5% 5% 4 6% 6% 33%	2003
October	1st to 10th	Day	15% 8% 3 7% 4 6% 5% 5% 6% 8% 33%	1900
	11th to 20th	Day	17% 10% 3 10% 5% 5% 4 4 4 6% 32%	1900
	21st to end	Day	20% 12% 4 8% 5% 5% 5% 4 4 7% 26%	2077
November	1st to 10th	Day	13% 11% 4 8% 5% 4 4 4 5% 6% 37%	1698
	11th to 20th	Day	15% 16% 4 9% 5% 4 4 3 6% 6% 28%	1577
	21st to end	Day	16% 15% 4 10% 7% 5% 4 3 5% 7% 25%	1520
December	1st to 10th	Day	18% 18% 6% 12% 5% 5% 4 4 5% 6% 17%	1520
	11th to 20th	Day	17% 16% 8% 11% 5% 4 4 3 5% 6% 22%	1906
	21st to end	Day	22% 19% 5% 13% 6% 5% 4 3 4 4 14%	2772
January	1st to 10th	Night	15% 15% 7% 13% 7% 5% 4 3 5% 6% 21%	4429
	11th to 20th	Night	12% 15% 7% 15% 7% 6% 5% 3 4 5% 19%	4330
	21st to end	Night	20% 16% 7% 14% 8% 5% 4 2 4 3 15%	4532
February	1st to 10th	Night	14% 15% 4 11% 7% 6% 5% 4 5% 6% 23%	2470
	11th to 20th	Night	12% 14% 5% 12% 7% 7% 5% 4 6% 5% 24%	2400
	21st to end	Night	16% 13% 5% 12% 7% 4 5% 3 6% 6% 24%	1884
March	1st to 10th	Night	18% 13% 5% 10% 6% 4 4 3 4 5% 28%	3542
	11th to 20th	Night	13% 19% 6% 10% 6% 4 4 3 4 5% 27%	3479
	21st to end	Night	16% 12% 3 10% 5% 4 4 3 4 6% 34%	3707
April	1st to 10th	Night	18% 12% 4 9% 5% 5% 4 3 5% 6% 29%	1900
	11th to 20th	Night	18% 10% 3 7% 5% 5% 4 4 4 6% 34%	1875
	21st to end	Night	17% 10% 3 6% 4 4 5% 5% 5% 6% 36%	1859
May	1st to 10th	Night	24% 11% 3 8% 4 4 4 3 4 6% 30%	2708
	11th to 20th	Night	18% 9% 4 7% 5% 5% 5% 4 6% 7% 29%	2495
	21st to end	Night	10% 7% 2 7% 5% 4 5% 4 7% 8% 41%	2738
June	1st to 10th	Night	14% 8% 2 6% 5% 4 4 4 6% 9% 38%	1520
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 6% 7% 8% 40%	1520
	21st to end	Night	10% 6% 2 7% 5% 4 4 5% 6% 7% 44%	1520
July	1st to 10th	Night	8% 8% 3 8% 6% 5% 4 4 5% 9% 40%	2520
	11th to 20th	Night	9% 4 2 5% 4 6% 5% 4 7% 10% 43%	2520
	21st to end	Night	11% 7% 3 7% 4 4 6% 4 5% 7% 43%	2883
August	1st to 10th	Night	17% 7% 3 8% 4 4 4 4 5% 7% 36%	2825
	11th to 20th	Night	16% 8% 2 6% 4 4 3 3 6% 8% 40%	2816
	21st to end	Night	11% 6% 2 6% 5% 5% 6% 4 7% 8% 40%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Night	12% 6% 3 6% 4 3 3 3 6% 7% 47%	1900
	21st to end	Night	18% 7% 2 7% 5% 4 4 4 5% 7% 38%	2175
October	1st to 10th	Night	16% 9% 2 8% 4 4 5% 3 6% 7% 36%	2280
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 6% 7% 35%	2280
	21st to end	Night	22% 12% 3 7% 5% 4 4 3 5% 6% 30%	2519
November	1st to 10th	Night	12% 10% 3 8% 5% 4 5% 4 4 6% 39%	2453
	11th to 20th	Night	17% 12% 3 9% 6% 5% 4 3 5% 6% 30%	2603
	21st to end	Night	13% 14% 4 10% 6% 5% 5% 4 6% 6% 27%	2660
December	1st to 10th	Night	16% 16% 5% 11% 7% 5% 6% 4 5% 6% 19%	2660
	11th to 20th	Night	18% 16% 5% 12% 6% 5% 4 3 4 5% 22%	3288
	21st to end	Night	24% 19% 6% 10% 5% 5% 4 3 5% 6% 15%	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	64% 6% 8% 16% 5% 1	2518
	11th to 20th	Day	68% 5% 8% 12% 5%	2626
	21st to end	Day	72% 6% 9% 11% 3	3124
February	1st to 10th	Day	66% 5% 10% 12% 6% 1	1710
	11th to 20th	Day	62% 6% 6% 17% 8%	1674
	21st to end	Day	60% 5% 8% 17% 8% 2	1570
March	1st to 10th	Day	61% 3 9% 20% 5% 1	3299
	11th to 20th	Day	54% 5% 12% 18% 9% 2	3480
	21st to end	Day	54% 4 8% 21% 11% 1	3924
April	1st to 10th	Day	55% 5% 8% 20% 10% 2	2280
	11th to 20th	Day	50% 7% 10% 18% 13% 1	2254
	21st to end	Day	51% 5% 9% 19% 14% 3	2337
May	1st to 10th	Day	52% 5% 9% 21% 12% 2	4243
	11th to 20th	Day	53% 10% 11% 19% 7% 1	4402
	21st to end	Day	39% 8% 11% 26% 14% 2	4837
June	1st to 10th	Day	47% 7% 12% 19% 14% 1	2660
	11th to 20th	Day	44% 7% 15% 22% 11% 1	2660
	21st to end	Day	42% 10% 13% 22% 13% 1	2660
July	1st to 10th	Day	40% 7% 16% 25% 11% 1	4370
	11th to 20th	Day	38% 8% 15% 25% 12% 1	4365
	21st to end	Day	44% 8% 15% 20% 13% 1	4518
August	1st to 10th	Day	49% 7% 13% 21% 10%	4091
	11th to 20th	Day	49% 9% 14% 22% 6%	4019
	21st to end	Day	42% 7% 11% 31% 9% 1	4180
September	1st to 10th	Day	43% 7% 10% 26% 13% 1	2280
	11th to 20th	Day	40% 7% 11% 25% 17% 1	2280
	21st to end	Day	50% 5% 10% 21% 12% 1	2003
October	1st to 10th	Day	42% 7% 12% 23% 16% 1	3160
	11th to 20th	Day	46% 5% 11% 20% 17% 1	3140
	21st to end	Day	41% 5% 15% 27% 11% 1	3403
November	1st to 10th	Day	48% 6% 11% 21% 12% 2	1698
	11th to 20th	Day	56% 5% 11% 19% 8% 2	1577
	21st to end	Day	58% 7% 12% 18% 5%	1520
December	1st to 10th	Day	67% 6% 11% 11% 5%	1520
	11th to 20th	Day	63% 5% 15% 11% 5%	1906
	21st to end	Day	72% 6% 8% 11% 2	2772
January	1st to 10th	Night	63% 7% 11% 13% 6%	4429
	11th to 20th	Night	65% 7% 10% 14% 5%	4330
	21st to end	Night	74% 5% 6% 11% 4	4532
February	1st to 10th	Night	59% 7% 12% 15% 7%	2470
	11th to 20th	Night	60% 5% 11% 15% 8% 1	2402
	21st to end	Night	59% 5% 10% 15% 9% 2	1884
March	1st to 10th	Night	59% 5% 11% 17% 7% 1	3552
	11th to 20th	Night	57% 7% 12% 17% 6% 1	3480
	21st to end	Night	51% 4 8% 22% 12% 3	3707
April	1st to 10th	Night	55% 6% 10% 18% 11% 1	1900
	11th to 20th	Night	51% 6% 11% 18% 14% 1	1875
	21st to end	Night	47% 7% 10% 19% 14% 2	1859
May	1st to 10th	Night	56% 5% 9% 20% 9% 1	2715
	11th to 20th	Night	50% 10% 14% 17% 8% 1	2496
	21st to end	Night	37% 9% 14% 20% 16% 3	2739
June	1st to 10th	Night	42% 7% 10% 25% 15% 1	1520
	11th to 20th	Night	40% 8% 14% 24% 12% 2	1520
	21st to end	Night	36% 9% 14% 26% 14% 1	1520
July	1st to 10th	Night	42% 6% 14% 23% 15% 1	2480
	11th to 20th	Night	34% 8% 17% 28% 13% 1	2472
	21st to end	Night	40% 7% 12% 22% 18% 1	2791
August	1st to 10th	Night	47% 7% 15% 23% 8%	2814
	11th to 20th	Night	42% 6% 16% 27% 9%	2843
	21st to end	Night	39% 7% 13% 30% 10% 1	3476
September	1st to 10th	Night	39% 6% 12% 28% 13% 2	1900
	11th to 20th	Night	35% 7% 13% 26% 18% 1	1900
	21st to end	Night	45% 6% 12% 22% 13% 1	2175
October	1st to 10th	Night	48% 7% 10% 22% 13%	3800
	11th to 20th	Night	41% 5% 12% 22% 16% 4	3779
	21st to end	Night	41% 5% 15% 28% 10% 1	4143
November	1st to 10th	Night	45% 6% 12% 22% 13% 2	2453
	11th to 20th	Night	54% 5% 12% 18% 9% 2	2603
	21st to end	Night	55% 7% 15% 18% 5%	2660
December	1st to 10th	Night	64% 7% 13% 13% 4	2660
	11th to 20th	Night	64% 5% 10% 15% 6%	3288
	21st to end	Night	70% 6% 9% 12% 2	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	35% 19% 31% 11% 4 1	180
	11th to 20th	Day	35% 29% 27% 8%	212
	21st to end	Day	28% 30% 29% 9% 4	297
February	1st to 10th	Day	34% 33% 26% 6% 1	270
	11th to 20th	Day	36% 27% 31% 6%	272
	21st to end	Day	40% 20% 28% 10% 2	382
March	1st to 10th	Day	35% 21% 28% 14% 3	460
	11th to 20th	Day	34% 23% 27% 11% 4	460
	21st to end	Day	33% 23% 27% 12% 5% 1	506
April	1st to 10th	Day	32% 22% 28% 14% 3 2	460
	11th to 20th	Day	29% 23% 28% 12% 6% 4	452
	21st to end	Day	30% 20% 27% 15% 6% 1	460
May	1st to 10th	Day	30% 19% 26% 15% 6% 4	463
	11th to 20th	Day	22% 17% 31% 17% 11% 3	487
	21st to end	Day	19% 12% 28% 25% 12% 5%	529
June	1st to 10th	Day	17% 18% 31% 21% 10% 3	490
	11th to 20th	Day	16% 11% 31% 27% 13% 2	490
	21st to end	Day	14% 16% 35% 24% 9% 3	490
July	1st to 10th	Day	11% 8% 30% 31% 14% 6%	485
	11th to 20th	Day	9% 11% 27% 32% 17% 5%	484
	21st to end	Day	11% 8% 36% 30% 11% 4	532
August	1st to 10th	Day	13% 7% 28% 27% 18% 8%	486
	11th to 20th	Day	16% 7% 27% 27% 15% 8%	479
	21st to end	Day	15% 11% 27% 23% 15% 9%	539
September	1st to 10th	Day	19% 10% 21% 25% 16% 9%	490
	11th to 20th	Day	16% 7% 25% 22% 19% 12%	490
	21st to end	Day	26% 13% 26% 19% 13% 4	490
October	1st to 10th	Day	26% 12% 23% 21% 13% 5%	490
	11th to 20th	Day	25% 17% 29% 20% 7% 2	490
	21st to end	Day	29% 18% 29% 19% 5% 1	532
November	1st to 10th	Day	26% 17% 28% 17% 10% 2	374
	11th to 20th	Day	35% 20% 28% 14% 3 1	237
	21st to end	Day	34% 25% 32% 8% 1	180
December	1st to 10th	Day	35% 27% 28% 6% 4 1	180
	11th to 20th	Day	36% 26% 28% 8% 2 1	180
	21st to end	Day	33% 26% 31% 9% 1	198
January	1st to 10th	Night	24% 24% 34% 15% 2	760
	11th to 20th	Night	25% 22% 30% 20% 3	760
	21st to end	Night	22% 23% 33% 19% 3	836
February	1st to 10th	Night	28% 20% 31% 19% 1	760
	11th to 20th	Night	28% 27% 29% 14% 1	743
	21st to end	Night	31% 23% 27% 17% 3	628
March	1st to 10th	Night	24% 25% 31% 17% 3	760
	11th to 20th	Night	29% 21% 29% 17% 4	760
	21st to end	Night	27% 20% 31% 16% 5% 1	836
April	1st to 10th	Night	27% 18% 31% 22% 3	760
	11th to 20th	Night	25% 19% 29% 18% 8% 1	751
	21st to end	Night	23% 13% 30% 26% 8%	719
May	1st to 10th	Night	26% 11% 24% 28% 10% 1	497
	11th to 20th	Night	17% 9% 34% 28% 11%	374
	21st to end	Night	12% 8% 25% 38% 16%	411
June	1st to 10th	Night	16% 8% 26% 34% 14% 1	380
	11th to 20th	Night	15% 7% 24% 38% 15%	380
	21st to end	Night	12% 8% 28% 38% 14%	380
July	1st to 10th	Night	9% 5% 22% 42% 21% 2	370
	11th to 20th	Night	10% 4 24% 38% 23% 2	368
	21st to end	Night	12% 4 19% 42% 21% 2	498
August	1st to 10th	Night	10% 4 22% 39% 25% 2	567
	11th to 20th	Night	13% 4 18% 34% 27% 3	582
	21st to end	Night	12% 6% 18% 37% 24% 3	836
September	1st to 10th	Night	18% 7% 23% 30% 19% 3	760
	11th to 20th	Night	15% 6% 20% 27% 26% 5%	760
	21st to end	Night	20% 10% 25% 25% 17% 3	760
October	1st to 10th	Night	23% 9% 25% 23% 18% 2	760
	11th to 20th	Night	23% 14% 29% 26% 7% 1	760
	21st to end	Night	29% 15% 28% 22% 5%	836
November	1st to 10th	Night	20% 15% 32% 23% 9%	755
	11th to 20th	Night	28% 19% 29% 18% 6% 1	760
	21st to end	Night	25% 18% 32% 21% 4 1	760
December	1st to 10th	Night	29% 21% 31% 15% 4	760
	11th to 20th	Night	28% 21% 32% 16% 3	760
	21st to end	Night	29% 20% 34% 15% 2	836

Page generated at 2024-11-01 22:47:43. Version ac07

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