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 2024-03-28.

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% 10% 5% 8% 7% 4 5% 5% 5% 7% 30%	78715
	Night	15% 11% 5% 8% 7% 3 5% 5% 5% 6% 30%	81278
Transparency	Day	53% 7% 11% 19% 10% 1	78425
	Night	53% 7% 11% 18% 9% 1	81102
Seeing	Day	24% 17% 28% 19% 9% 3	13816
	Night	22% 15% 28% 24% 10% 1	23292

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% 17% 9% 10% 7% 3 5% 4 4 5% 22%	2517
	11th to 20th	Day	12% 15% 9% 17% 9% 3 4 4 4 6% 18%	2626
	21st to end	Day	19% 15% 9% 11% 8% 4 5% 4 4 4 15%	3124
February	1st to 10th	Day	15% 14% 7% 11% 9% 5% 6% 4 5% 4 21%	1710
	11th to 20th	Day	13% 12% 7% 13% 9% 4 6% 5% 5% 5% 23%	1665
	21st to end	Day	18% 13% 4 10% 8% 3 4 4 5% 5% 26%	1570
March	1st to 10th	Day	15% 12% 7% 11% 5% 3 6% 4 4 6% 28%	1999
	11th to 20th	Day	15% 15% 6% 8% 6% 3 4 4 4 5% 30%	2090
	21st to end	Day	18% 12% 5% 8% 7% 3 5% 4 5% 6% 27%	2309
April	1st to 10th	Day	14% 13% 5% 8% 7% 3 3 4 5% 6% 31%	2160
	11th to 20th	Day	17% 11% 3 6% 5% 3 5% 6% 6% 7% 32%	2134
	21st to end	Day	19% 10% 4 6% 5% 3 4 4 4 8% 32%	2214
May	1st to 10th	Day	17% 11% 4 7% 6% 3 5% 4 5% 6% 32%	2407
	11th to 20th	Day	18% 7% 4 6% 6% 4 5% 4 6% 7% 32%	2478
	21st to end	Day	13% 7% 3 6% 7% 4 4 5% 6% 7% 38%	2725
June	1st to 10th	Day	15% 7% 3 7% 6% 4 6% 5% 6% 8% 34%	2520
	11th to 20th	Day	14% 6% 3 6% 6% 3 6% 5% 6% 8% 36%	2520
	21st to end	Day	11% 7% 3 7% 6% 3 6% 6% 6% 9% 36%	2520
July	1st to 10th	Day	8% 6% 3 7% 7% 4 5% 6% 7% 10% 36%	2520
	11th to 20th	Day	10% 5% 2 6% 6% 4 6% 6% 8% 10% 37%	2520
	21st to end	Day	9% 6% 3 7% 7% 4 7% 8% 6% 8% 36%	2664
August	1st to 10th	Day	10% 5% 3 6% 7% 5% 6% 6% 7% 10% 36%	2327
	11th to 20th	Day	13% 7% 3 7% 7% 4 6% 6% 7% 8% 32%	2267
	21st to end	Day	15% 6% 2 5% 7% 4 7% 6% 7% 8% 33%	2376
September	1st to 10th	Day	16% 6% 3 5% 5% 4 5% 5% 5% 8% 38%	2160
	11th to 20th	Day	14% 7% 3 6% 5% 3 4 4 5% 7% 41%	2160
	21st to end	Day	18% 6% 3 6% 6% 4 5% 5% 6% 7% 34%	1899
October	1st to 10th	Day	16% 8% 3 7% 5% 3 6% 5% 6% 8% 33%	1800
	11th to 20th	Day	17% 9% 3 9% 7% 4 4 5% 4 7% 31%	1800
	21st to end	Day	21% 12% 4 8% 6% 4 5% 5% 4 6% 25%	1968
November	1st to 10th	Day	13% 11% 4 7% 6% 3 4 5% 5% 6% 37%	1671
	11th to 20th	Day	15% 15% 5% 8% 6% 3 4 4 6% 6% 28%	1577
	21st to end	Day	16% 14% 4 9% 8% 4 4 5% 5% 7% 25%	1520
December	1st to 10th	Day	18% 17% 7% 11% 6% 4 5% 5% 5% 6% 17%	1520
	11th to 20th	Day	17% 15% 8% 10% 6% 3 4 4 5% 6% 22%	1906
	21st to end	Day	22% 18% 6% 12% 8% 4 4 4 4 4 14%	2772
January	1st to 10th	Night	15% 14% 7% 12% 9% 4 4 4 5% 6% 21%	4429
	11th to 20th	Night	12% 15% 7% 14% 9% 5% 5% 5% 4 5% 19%	4330
	21st to end	Night	21% 15% 8% 12% 10% 4 4 3 4 3 15%	4532
February	1st to 10th	Night	14% 13% 6% 10% 8% 4 6% 6% 5% 6% 23%	2470
	11th to 20th	Night	13% 13% 6% 10% 8% 5% 6% 5% 6% 5% 24%	2400
	21st to end	Night	16% 12% 5% 10% 8% 3 4 5% 6% 6% 24%	1884
March	1st to 10th	Night	13% 12% 7% 11% 8% 3 4 4 4 6% 28%	2166
	11th to 20th	Night	16% 14% 5% 9% 8% 3 4 4 5% 5% 29%	2089
	21st to end	Night	17% 12% 4 8% 6% 3 5% 5% 4 5% 32%	2201
April	1st to 10th	Night	18% 9% 5% 8% 6% 3 4 4 4 7% 30%	1800
	11th to 20th	Night	18% 9% 4 6% 6% 3 5% 4 4 6% 35%	1775
	21st to end	Night	18% 9% 3 6% 5% 2 5% 6% 5% 6% 35%	1762
May	1st to 10th	Night	18% 13% 3 7% 5% 3 5% 5% 4 5% 32%	1551
	11th to 20th	Night	13% 7% 3 5% 6% 4 7% 5% 6% 8% 35%	1415
	21st to end	Night	11% 6% 3 5% 6% 3 5% 6% 5% 7% 43%	1550
June	1st to 10th	Night	14% 7% 3 6% 5% 3 4 5% 6% 10% 38%	1440
	11th to 20th	Night	12% 7% 3 5% 6% 3 5% 6% 6% 8% 39%	1440
	21st to end	Night	10% 5% 2 6% 5% 3 4 6% 5% 7% 46%	1440
July	1st to 10th	Night	7% 6% 3 6% 7% 4 4 5% 5% 9% 45%	1440
	11th to 20th	Night	9% 3 2 4 5% 3 5% 4 7% 9% 48%	1440
	21st to end	Night	11% 5% 3 5% 4 4 6% 5% 6% 9% 42%	1651
August	1st to 10th	Night	8% 5% 2 5% 6% 3 4 6% 7% 9% 45%	1611
	11th to 20th	Night	12% 6% 3 6% 5% 3 5% 4 7% 9% 41%	1615
	21st to end	Night	10% 5% 2 5% 5% 4 6% 5% 8% 9% 41%	1980
September	1st to 10th	Night	15% 6% 2 5% 5% 3 4 4 5% 6% 46%	1800
	11th to 20th	Night	13% 6% 3 5% 4 3 3 5% 6% 7% 47%	1800
	21st to end	Night	15% 6% 2 6% 5% 3 4 5% 5% 7% 40%	2059
October	1st to 10th	Night	16% 8% 2 7% 5% 3 5% 4 5% 8% 37%	2160
	11th to 20th	Night	19% 9% 3 6% 5% 3 4 4 6% 7% 35%	2160
	21st to end	Night	23% 12% 3 7% 6% 3 4 4 5% 5% 28%	2386
November	1st to 10th	Night	12% 10% 3 7% 6% 3 5% 4 4 6% 40%	2407
	11th to 20th	Night	17% 11% 4 8% 6% 3 5% 4 5% 6% 31%	2603
	21st to end	Night	14% 13% 5% 9% 7% 4 5% 5% 6% 6% 27%	2660
December	1st to 10th	Night	16% 15% 6% 10% 8% 4 6% 5% 5% 7% 19%	2660
	11th to 20th	Night	18% 15% 6% 11% 7% 3 5% 4 4 5% 22%	3288
	21st to end	Night	24% 18% 6% 8% 6% 3 5% 4 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	59% 4 11% 18% 7% 1	2002
	11th to 20th	Day	57% 4 8% 17% 11% 2	2090
	21st to end	Day	58% 4 7% 17% 11% 2	2309
April	1st to 10th	Day	54% 5% 9% 21% 10% 2	2160
	11th to 20th	Day	50% 7% 10% 18% 14% 1	2134
	21st to end	Day	51% 5% 9% 20% 13% 3	2214
May	1st to 10th	Day	52% 6% 9% 19% 12% 2	2407
	11th to 20th	Day	50% 7% 10% 19% 12% 2	2482
	21st to end	Day	44% 8% 12% 22% 12% 1	2725
June	1st to 10th	Day	47% 7% 12% 19% 14% 1	2520
	11th to 20th	Day	45% 7% 14% 22% 11% 1	2520
	21st to end	Day	42% 10% 13% 22% 13% 1	2520
July	1st to 10th	Day	43% 9% 11% 23% 12% 2	2450
	11th to 20th	Day	38% 10% 14% 24% 13% 1	2445
	21st to end	Day	43% 10% 13% 20% 13% 1	2515
August	1st to 10th	Day	41% 9% 14% 24% 11% 1	2314
	11th to 20th	Day	46% 11% 13% 22% 8% 1	2267
	21st to end	Day	45% 9% 11% 22% 12% 1	2376
September	1st to 10th	Day	44% 7% 10% 25% 12% 2	2160
	11th to 20th	Day	41% 7% 10% 24% 17% 1	2160
	21st to end	Day	48% 6% 10% 22% 13% 1	1899
October	1st to 10th	Day	47% 9% 12% 19% 13% 1	1800
	11th to 20th	Day	53% 7% 11% 20% 9%	1800
	21st to end	Day	59% 6% 12% 16% 7% 2	1968
November	1st to 10th	Day	48% 6% 12% 20% 12% 2	1671
	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	58% 5% 10% 18% 7% 1	2176
	11th to 20th	Night	56% 6% 9% 17% 10% 1	2090
	21st to end	Night	54% 5% 9% 17% 12% 3	2201
April	1st to 10th	Night	55% 5% 10% 18% 11% 1	1800
	11th to 20th	Night	50% 6% 11% 18% 15% 1	1775
	21st to end	Night	48% 7% 10% 19% 14% 2	1762
May	1st to 10th	Night	54% 5% 8% 19% 11% 3	1551
	11th to 20th	Night	46% 8% 11% 19% 15% 1	1416
	21st to end	Night	39% 9% 14% 22% 14% 2	1551
June	1st to 10th	Night	42% 7% 10% 26% 15%	1440
	11th to 20th	Night	41% 8% 14% 23% 12% 2	1440
	21st to end	Night	35% 9% 14% 27% 14% 1	1440
July	1st to 10th	Night	37% 7% 14% 26% 14% 1	1400
	11th to 20th	Night	31% 8% 15% 28% 16% 1	1392
	21st to end	Night	37% 9% 14% 21% 17% 2	1559
August	1st to 10th	Night	33% 9% 16% 29% 12% 1	1600
	11th to 20th	Night	40% 8% 16% 27% 10%	1615
	21st to end	Night	36% 10% 14% 25% 14% 2	1980
September	1st to 10th	Night	39% 6% 13% 28% 12% 2	1800
	11th to 20th	Night	35% 7% 13% 26% 18% 2	1800
	21st to end	Night	43% 7% 13% 23% 14% 1	2059
October	1st to 10th	Night	45% 8% 13% 21% 12% 1	2160
	11th to 20th	Night	47% 8% 13% 20% 11% 1	2160
	21st to end	Night	57% 6% 11% 16% 9% 1	2386
November	1st to 10th	Night	45% 6% 12% 22% 13% 2	2407
	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	34% 22% 27% 12% 4 1	490
April	1st to 10th	Day	30% 22% 28% 14% 4 2	420
	11th to 20th	Day	28% 22% 28% 12% 6% 4	412
	21st to end	Day	31% 21% 28% 13% 6% 1	420
May	1st to 10th	Day	28% 19% 27% 15% 6% 4	423
	11th to 20th	Day	22% 17% 31% 17% 11% 3	447
	21st to end	Day	19% 12% 28% 24% 11% 6%	485
June	1st to 10th	Day	16% 17% 32% 22% 10% 3	450
	11th to 20th	Day	18% 12% 31% 25% 12% 2	450
	21st to end	Day	13% 15% 36% 24% 9% 4	450
July	1st to 10th	Day	11% 8% 29% 31% 14% 6%	445
	11th to 20th	Day	9% 10% 26% 33% 17% 5%	444
	21st to end	Day	11% 8% 37% 29% 10% 4	488
August	1st to 10th	Day	10% 7% 28% 28% 18% 8%	446
	11th to 20th	Day	15% 6% 27% 28% 15% 9%	439
	21st to end	Day	15% 11% 26% 24% 14% 10%	495
September	1st to 10th	Day	20% 9% 21% 26% 16% 8%	450
	11th to 20th	Day	17% 7% 26% 22% 18% 10%	450
	21st to end	Day	22% 13% 27% 20% 14% 4	450
October	1st to 10th	Day	26% 11% 24% 20% 14% 6%	450
	11th to 20th	Day	25% 18% 29% 20% 7% 1	450
	21st to end	Day	31% 18% 28% 19% 4	489
November	1st to 10th	Day	27% 17% 28% 17% 10% 2	365
	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% 30% 16% 5% 1	821
April	1st to 10th	Night	26% 18% 32% 21% 3	720
	11th to 20th	Night	25% 19% 29% 19% 8% 1	711
	21st to end	Night	24% 12% 30% 26% 7%	682
May	1st to 10th	Night	26% 11% 23% 28% 11% 1	471
	11th to 20th	Night	17% 9% 34% 28% 11%	354
	21st to end	Night	13% 8% 25% 38% 16%	389
June	1st to 10th	Night	16% 8% 25% 35% 15% 1	360
	11th to 20th	Night	16% 8% 24% 36% 15%	360
	21st to end	Night	11% 7% 29% 38% 14%	360
July	1st to 10th	Night	9% 5% 22% 42% 21% 2	350
	11th to 20th	Night	10% 4 25% 37% 22% 2	348
	21st to end	Night	12% 4 20% 42% 20% 2	471
August	1st to 10th	Night	8% 4 22% 39% 25% 2	537
	11th to 20th	Night	13% 4 18% 34% 27% 3	551
	21st to end	Night	12% 6% 18% 37% 24% 3	792
September	1st to 10th	Night	19% 6% 22% 31% 19% 3	720
	11th to 20th	Night	16% 7% 21% 27% 25% 5%	720
	21st to end	Night	17% 10% 26% 26% 18% 3	720
October	1st to 10th	Night	22% 9% 26% 23% 18% 2	720
	11th to 20th	Night	24% 14% 29% 26% 7% 1	720
	21st to end	Night	31% 15% 28% 21% 5%	792
November	1st to 10th	Night	20% 15% 32% 23% 9%	740
	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 2024-03-24 23:37:00. Version ac07

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