Collins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Collins Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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% 7% 3 5% 5% 3 4 4 5% 7% 38%	78044
	Night	18% 8% 3 6% 5% 3 4 4 5% 6% 37%	81236
Transparency	Day	46% 6% 12% 21% 13% 2	77749
	Night	47% 6% 12% 21% 13% 2	81066
Seeing	Day	24% 15% 31% 20% 8% 2	14416
	Night	23% 12% 33% 26% 7%	23152

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% 7% 3 6% 6% 3 4 5% 5% 6% 39%	2517
	11th to 20th	Day	17% 7% 3 6% 6% 3 4 3 4 7% 40%	2520
	21st to end	Day	17% 10% 5% 7% 5% 3 4 5% 5% 5% 35%	2772
February	1st to 10th	Day	18% 10% 3 5% 5% 3 5% 4 6% 7% 35%	1682
	11th to 20th	Day	13% 10% 4 7% 6% 4 5% 5% 5% 6% 35%	1810
	21st to end	Day	15% 10% 3 7% 5% 4 4 4 5% 7% 34%	1520
March	1st to 10th	Day	17% 9% 3 6% 5% 4 5% 4 5% 5% 36%	1797
	11th to 20th	Day	17% 7% 3 5% 6% 3 5% 4 4 6% 41%	1956
	21st to end	Day	18% 7% 3 6% 4 3 4 3 5% 6% 40%	2208
April	1st to 10th	Day	21% 10% 4 6% 6% 2 3 3 4 6% 34%	2160
	11th to 20th	Day	18% 6% 3 5% 5% 3 4 4 6% 7% 38%	2253
	21st to end	Day	24% 8% 3 5% 6% 3 4 4 5% 5% 32%	2340
May	1st to 10th	Day	23% 8% 3 5% 5% 2 5% 4 5% 6% 34%	2340
	11th to 20th	Day	20% 7% 3 6% 6% 3 5% 4 4 6% 35%	2331
	21st to end	Day	18% 6% 2 5% 5% 3 5% 5% 5% 8% 37%	2725
June	1st to 10th	Day	24% 7% 3 5% 5% 4 5% 5% 6% 8% 30%	2668
	11th to 20th	Day	18% 6% 3 5% 4 3 5% 5% 5% 7% 38%	2700
	21st to end	Day	24% 7% 3 5% 5% 3 5% 5% 5% 8% 30%	2655
July	1st to 10th	Day	15% 5% 3 5% 6% 3 5% 6% 7% 8% 39%	2520
	11th to 20th	Day	14% 5% 3 5% 6% 4 6% 7% 7% 8% 36%	2520
	21st to end	Day	14% 5% 3 6% 6% 4 6% 5% 6% 10% 36%	2744
August	1st to 10th	Day	13% 5% 2 6% 5% 4 5% 5% 6% 10% 39%	2340
	11th to 20th	Day	14% 6% 3 5% 5% 4 5% 5% 6% 8% 39%	2304
	21st to end	Day	13% 5% 2 4 4 3 5% 5% 6% 8% 45%	2512
September	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 5% 5% 7% 47%	2105
	11th to 20th	Day	12% 5% 2 4 4 3 4 3 4 6% 53%	1980
	21st to end	Day	17% 5% 2 4 4 3 4 5% 5% 8% 44%	1980
October	1st to 10th	Day	19% 6% 2 6% 5% 3 4 4 5% 5% 40%	1967
	11th to 20th	Day	20% 5% 2 5% 5% 3 5% 4 5% 7% 39%	1739
	21st to end	Day	24% 7% 3 7% 5% 3 3 4 4 5% 34%	1782
November	1st to 10th	Day	18% 6% 3 4 5% 3 4 5% 5% 7% 41%	1671
	11th to 20th	Day	21% 5% 2 4 4 3 4 4 5% 7% 40%	1710
	21st to end	Day	22% 8% 2 4 4 3 3 2 3 6% 43%	1577
December	1st to 10th	Day	23% 10% 3 6% 4 3 4 4 4 6% 33%	1354
	11th to 20th	Day	23% 8% 3 5% 4 2 3 3 4 6% 37%	1668
	21st to end	Day	26% 12% 2 5% 5% 3 4 4 4 5% 29%	2617
January	1st to 10th	Night	15% 9% 3 7% 5% 3 5% 4 5% 6% 39%	4429
	11th to 20th	Night	13% 9% 4 9% 7% 4 4 5% 6% 7% 32%	4440
	21st to end	Night	17% 11% 4 7% 6% 3 4 5% 5% 6% 32%	4884
February	1st to 10th	Night	16% 9% 4 7% 7% 3 5% 5% 5% 6% 32%	2496
	11th to 20th	Night	13% 10% 4 7% 6% 3 5% 4 5% 5% 38%	2252
	21st to end	Night	14% 10% 4 7% 6% 3 5% 4 4 5% 39%	1831
March	1st to 10th	Night	14% 12% 3 7% 6% 3 5% 5% 4 5% 36%	2150
	11th to 20th	Night	18% 8% 4 5% 6% 3 5% 4 5% 6% 36%	2002
	21st to end	Night	18% 8% 3 5% 5% 3 3 4 5% 5% 40%	2146
April	1st to 10th	Night	22% 11% 3 6% 5% 2 4 4 4 4 35%	1800
	11th to 20th	Night	19% 5% 2 5% 6% 3 5% 5% 5% 7% 37%	1656
	21st to end	Night	26% 8% 3 6% 5% 3 4 3 5% 6% 32%	1620
May	1st to 10th	Night	22% 9% 3 5% 5% 3 5% 5% 5% 7% 30%	1620
	11th to 20th	Night	21% 7% 4 5% 6% 3 5% 6% 5% 7% 32%	1576
	21st to end	Night	18% 8% 3 5% 5% 4 4 5% 5% 8% 35%	1550
June	1st to 10th	Night	22% 8% 3 5% 5% 4 4 4 6% 8% 32%	1288
	11th to 20th	Night	16% 6% 3 6% 6% 3 5% 4 5% 10% 38%	1260
	21st to end	Night	23% 8% 2 5% 5% 3 4 5% 5% 8% 32%	1299
July	1st to 10th	Night	15% 6% 3 5% 6% 3 5% 4 6% 10% 36%	1440
	11th to 20th	Night	14% 6% 3 6% 5% 3 5% 3 6% 9% 40%	1440
	21st to end	Night	16% 4 3 5% 5% 3 5% 5% 7% 9% 39%	1574
August	1st to 10th	Night	12% 6% 2 5% 4 4 6% 6% 6% 8% 41%	1580
	11th to 20th	Night	14% 8% 2 5% 5% 4 5% 5% 6% 8% 38%	1596
	21st to end	Night	14% 4 2 4 5% 3 4 4 5% 7% 49%	1840
September	1st to 10th	Night	14% 5% 1 4 4 2 4 5% 5% 7% 48%	1854
	11th to 20th	Night	16% 5% 2 4 3 2 3 3 4 6% 51%	1980
	21st to end	Night	16% 6% 2 4 4 3 4 3 5% 6% 45%	1980
October	1st to 10th	Night	19% 6% 3 5% 5% 4 4 4 5% 6% 39%	1993
	11th to 20th	Night	18% 6% 2 4 5% 3 5% 5% 5% 7% 39%	2218
	21st to end	Night	26% 8% 2 5% 4 2 4 4 4 6% 34%	2574
November	1st to 10th	Night	19% 6% 2 5% 5% 2 4 4 5% 6% 42%	2407
	11th to 20th	Night	18% 5% 2 6% 5% 3 4 5% 4 7% 42%	2470
	21st to end	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 39%	2599
December	1st to 10th	Night	22% 9% 3 6% 6% 3 4 4 4 6% 32%	2821
	11th to 20th	Night	23% 7% 3 5% 4 3 4 4 4 6% 37%	3531
	21st to end	Night	25% 10% 3 5% 4 3 4 3 5% 6% 33%	5040

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	44% 6% 15% 24% 11% 1	2518
	11th to 20th	Day	44% 5% 12% 25% 11% 2	2520
	21st to end	Day	50% 5% 14% 17% 13% 1	2772
February	1st to 10th	Day	49% 6% 10% 19% 14% 2	1682
	11th to 20th	Day	49% 6% 11% 18% 13% 3	1819
	21st to end	Day	50% 5% 10% 18% 11% 5%	1520
March	1st to 10th	Day	50% 4 13% 16% 14% 4	1800
	11th to 20th	Day	44% 4 10% 20% 17% 4	1956
	21st to end	Day	46% 5% 9% 18% 17% 5%	2208
April	1st to 10th	Day	52% 5% 8% 16% 14% 5%	2160
	11th to 20th	Day	44% 7% 9% 23% 15% 2	2253
	21st to end	Day	53% 5% 8% 18% 12% 4	2340
May	1st to 10th	Day	50% 5% 8% 21% 13% 3	2340
	11th to 20th	Day	49% 6% 8% 19% 15% 2	2335
	21st to end	Day	44% 7% 14% 21% 12% 2	2725
June	1st to 10th	Day	52% 7% 12% 20% 9% 1	2668
	11th to 20th	Day	45% 7% 11% 20% 15% 2	2700
	21st to end	Day	51% 8% 13% 17% 10% 1	2655
July	1st to 10th	Day	41% 9% 13% 24% 13% 1	2450
	11th to 20th	Day	42% 8% 14% 24% 10% 1	2445
	21st to end	Day	43% 8% 15% 23% 11% 1	2591
August	1st to 10th	Day	40% 8% 15% 27% 9%	2326
	11th to 20th	Day	41% 8% 15% 24% 11%	2304
	21st to end	Day	35% 8% 14% 24% 18% 1	2512
September	1st to 10th	Day	36% 7% 15% 26% 16% 1	2105
	11th to 20th	Day	33% 5% 9% 29% 21% 2	1980
	21st to end	Day	38% 6% 12% 28% 16% 1	1980
October	1st to 10th	Day	44% 6% 11% 23% 14% 1	1967
	11th to 20th	Day	44% 7% 14% 26% 9% 1	1739
	21st to end	Day	52% 5% 10% 19% 13% 1	1782
November	1st to 10th	Day	42% 5% 14% 21% 15% 2	1671
	11th to 20th	Day	42% 6% 11% 20% 17% 3	1710
	21st to end	Day	45% 4 16% 23% 11% 1	1577
December	1st to 10th	Day	53% 5% 10% 20% 10% 1	1354
	11th to 20th	Day	49% 5% 10% 19% 15% 3	1668
	21st to end	Day	58% 5% 9% 18% 8% 2	2617
January	1st to 10th	Night	46% 5% 13% 25% 9% 1	4429
	11th to 20th	Night	49% 7% 14% 18% 10% 3	4440
	21st to end	Night	52% 6% 9% 19% 12% 1	4884
February	1st to 10th	Night	51% 6% 12% 18% 11% 3	2496
	11th to 20th	Night	47% 5% 10% 20% 15% 3	2254
	21st to end	Night	48% 5% 9% 19% 15% 4	1831
March	1st to 10th	Night	50% 5% 8% 20% 14% 3	2160
	11th to 20th	Night	49% 5% 12% 15% 15% 4	2003
	21st to end	Night	47% 4 9% 18% 18% 4	2146
April	1st to 10th	Night	53% 5% 8% 16% 15% 4	1800
	11th to 20th	Night	46% 5% 9% 22% 16% 1	1656
	21st to end	Night	53% 5% 11% 15% 12% 3	1620
May	1st to 10th	Night	52% 6% 9% 18% 14% 2	1620
	11th to 20th	Night	50% 7% 10% 18% 12% 3	1577
	21st to end	Night	48% 7% 13% 20% 12% 1	1551
June	1st to 10th	Night	53% 5% 12% 21% 9% 1	1288
	11th to 20th	Night	43% 8% 14% 22% 12% 2	1260
	21st to end	Night	49% 8% 14% 18% 9% 2	1299
July	1st to 10th	Night	42% 9% 14% 23% 11% 1	1400
	11th to 20th	Night	41% 8% 16% 26% 10%	1392
	21st to end	Night	40% 9% 18% 23% 9% 1	1486
August	1st to 10th	Night	39% 9% 17% 26% 9%	1570
	11th to 20th	Night	42% 9% 17% 20% 13%	1596
	21st to end	Night	36% 7% 13% 27% 17% 1	1840
September	1st to 10th	Night	34% 7% 15% 29% 15%	1854
	11th to 20th	Night	34% 6% 10% 30% 19% 2	1980
	21st to end	Night	39% 6% 9% 30% 14% 2	1980
October	1st to 10th	Night	44% 6% 12% 23% 13% 1	1993
	11th to 20th	Night	43% 6% 14% 25% 11% 1	2218
	21st to end	Night	51% 5% 11% 19% 13% 1	2574
November	1st to 10th	Night	43% 5% 12% 24% 14% 1	2407
	11th to 20th	Night	43% 5% 11% 24% 15% 2	2470
	21st to end	Night	45% 5% 15% 20% 14% 1	2599
December	1st to 10th	Night	53% 5% 10% 20% 11% 1	2821
	11th to 20th	Night	49% 5% 10% 19% 17% 1	3532
	21st to end	Night	51% 6% 12% 18% 11% 1	5040

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	29% 18% 29% 18% 6%	180
	11th to 20th	Day	32% 22% 27% 15% 3 2	180
	21st to end	Day	18% 25% 34% 21% 2	198
February	1st to 10th	Day	30% 18% 29% 19% 4	342
	11th to 20th	Day	27% 24% 34% 14% 2	438
	21st to end	Day	22% 22% 35% 17% 4	362
March	1st to 10th	Day	29% 18% 34% 15% 4	420
	11th to 20th	Day	27% 21% 34% 14% 3 1	420
	21st to end	Day	27% 22% 28% 15% 6% 2	462
April	1st to 10th	Day	35% 19% 28% 11% 5% 1	420
	11th to 20th	Day	28% 24% 28% 13% 6% 1	412
	21st to end	Day	31% 19% 31% 13% 5% 1	420
May	1st to 10th	Day	35% 17% 28% 17% 3 1	423
	11th to 20th	Day	23% 19% 35% 17% 5% 2	447
	21st to end	Day	21% 18% 38% 17% 7%	485
June	1st to 10th	Day	26% 14% 31% 19% 9% 1	598
	11th to 20th	Day	22% 13% 30% 21% 13% 1	630
	21st to end	Day	25% 11% 27% 28% 7% 2	585
July	1st to 10th	Day	20% 11% 35% 26% 8% 1	445
	11th to 20th	Day	16% 7% 39% 22% 12% 4	444
	21st to end	Day	14% 8% 39% 27% 10% 3	488
August	1st to 10th	Day	12% 9% 33% 30% 14% 2	446
	11th to 20th	Day	16% 8% 29% 29% 15% 4	439
	21st to end	Day	15% 12% 31% 27% 12% 4	495
September	1st to 10th	Day	14% 10% 32% 26% 14% 4	450
	11th to 20th	Day	20% 11% 29% 21% 12% 7%	450
	21st to end	Day	17% 14% 30% 23% 11% 5%	450
October	1st to 10th	Day	23% 14% 26% 23% 11% 3	444
	11th to 20th	Day	26% 14% 28% 19% 11% 2	360
	21st to end	Day	34% 11% 29% 19% 7% 1	396
November	1st to 10th	Day	24% 14% 31% 21% 8% 1	365
	11th to 20th	Day	31% 14% 28% 21% 5% 1	370
	21st to end	Day	29% 17% 26% 21% 6% 1	370
December	1st to 10th	Day	28% 17% 33% 16% 6%	204
	11th to 20th	Day	35% 21% 33% 8% 3	180
	21st to end	Day	36% 17% 26% 14% 7% 1	198
January	1st to 10th	Night	24% 13% 36% 24% 2	760
	11th to 20th	Night	25% 16% 34% 20% 4	760
	21st to end	Night	19% 17% 40% 20% 4	836
February	1st to 10th	Night	23% 16% 32% 25% 3	760
	11th to 20th	Night	21% 19% 38% 20% 2	743
	21st to end	Night	20% 16% 37% 22% 4	611
March	1st to 10th	Night	25% 15% 38% 21% 2	720
	11th to 20th	Night	23% 16% 33% 22% 6%	720
	21st to end	Night	23% 17% 32% 22% 5%	792
April	1st to 10th	Night	29% 18% 30% 20% 3	720
	11th to 20th	Night	23% 15% 36% 21% 4	592
	21st to end	Night	31% 13% 31% 21% 4	540
May	1st to 10th	Night	26% 13% 31% 26% 5%	540
	11th to 20th	Night	23% 13% 38% 23% 4	515
	21st to end	Night	21% 7% 34% 32% 5%	389
June	1st to 10th	Night	26% 8% 31% 28% 6%	360
	11th to 20th	Night	17% 8% 29% 37% 10%	360
	21st to end	Night	25% 3 31% 36% 6%	360
July	1st to 10th	Night	18% 3 31% 38% 9%	350
	11th to 20th	Night	16% 3 29% 40% 11%	348
	21st to end	Night	16% 3 27% 39% 14% 1	394
August	1st to 10th	Night	13% 4 29% 41% 12%	506
	11th to 20th	Night	16% 6% 28% 37% 13% 1	532
	21st to end	Night	15% 8% 26% 37% 13% 1	652
September	1st to 10th	Night	16% 4 34% 33% 13% 1	720
	11th to 20th	Night	18% 7% 28% 28% 16% 3	720
	21st to end	Night	20% 8% 30% 28% 12% 2	720
October	1st to 10th	Night	22% 8% 26% 29% 14%	720
	11th to 20th	Night	21% 10% 32% 27% 9% 1	720
	21st to end	Night	31% 13% 29% 22% 5%	792
November	1st to 10th	Night	23% 10% 32% 25% 8% 1	740
	11th to 20th	Night	22% 13% 32% 24% 7% 1	760
	21st to end	Night	24% 16% 32% 22% 6%	821
December	1st to 10th	Night	29% 13% 35% 19% 5%	850
	11th to 20th	Night	26% 15% 36% 20% 3	850
	21st to end	Night	29% 13% 35% 20% 3	879

Page generated at 2024-02-22 06:05:43. Version ac07

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