Blue Creek Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Creek Observing Site clear sky chart from 2005-11-06 to 2024-01-16.

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	14% 8% 4 6% 6% 3 5% 5% 5% 7% 37%	75016
	Night	14% 8% 4 6% 5% 3 4 4 5% 7% 38%	74795
Transparency	Day	46% 6% 11% 21% 14% 2	74729
	Night	45% 6% 11% 21% 14% 2	74602
Seeing	Day	18% 14% 27% 23% 13% 6%	14278
	Night	18% 11% 28% 27% 14% 2	24164

By Month

Percentage Cloud Cover SE (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	16% 13% 5% 8% 6% 4 4 5% 4 6% 30%	1517
	11th to 20th	Day	19% 13% 6% 11% 6% 3 4 3 4 5% 26%	1513
	21st to end	Day	15% 15% 6% 8% 8% 3 4 4 5% 5% 26%	1782
February	1st to 10th	Day	19% 14% 6% 7% 6% 2 4 4 4 6% 28%	1750
	11th to 20th	Day	15% 11% 5% 8% 7% 3 4 4 5% 6% 30%	1748
	21st to end	Day	19% 12% 6% 9% 4 2 4 3 4 5% 31%	1480
March	1st to 10th	Day	18% 9% 4 7% 5% 4 4 5% 5% 6% 34%	1800
	11th to 20th	Day	19% 11% 4 6% 5% 3 4 5% 6% 5% 33%	1962
	21st to end	Day	22% 10% 4 7% 6% 3 5% 4 5% 6% 28%	2186
April	1st to 10th	Day	14% 9% 4 6% 6% 3 5% 5% 6% 7% 35%	2160
	11th to 20th	Day	16% 8% 3 6% 5% 3 4 4 5% 6% 39%	2155
	21st to end	Day	19% 8% 3 6% 6% 3 4 4 6% 7% 34%	2340
May	1st to 10th	Day	20% 8% 3 6% 5% 4 5% 5% 4 6% 33%	2340
	11th to 20th	Day	20% 8% 2 7% 5% 3 5% 5% 4 7% 35%	2301
	21st to end	Day	9% 5% 3 7% 7% 4 6% 7% 8% 9% 35%	2531
June	1st to 10th	Day	12% 5% 3 6% 5% 3 6% 5% 5% 8% 42%	2419
	11th to 20th	Day	9% 5% 3 5% 5% 3 6% 5% 6% 8% 45%	2520
	21st to end	Day	9% 5% 3 6% 6% 3 5% 6% 7% 10% 40%	2520
July	1st to 10th	Day	11% 6% 3 5% 6% 4 6% 6% 7% 9% 37%	2520
	11th to 20th	Day	8% 5% 2 5% 6% 3 6% 6% 7% 10% 42%	2510
	21st to end	Day	9% 4 2 5% 6% 4 6% 6% 7% 9% 41%	2547
August	1st to 10th	Day	8% 4 3 6% 6% 3 5% 5% 7% 11% 43%	2327
	11th to 20th	Day	9% 6% 3 7% 6% 4 5% 5% 6% 10% 39%	2277
	21st to end	Day	7% 5% 3 5% 5% 4 5% 6% 7% 8% 47%	2376
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 5% 6% 6% 44%	2078
	11th to 20th	Day	10% 5% 2 5% 6% 3 4 5% 5% 7% 48%	1980
	21st to end	Day	12% 6% 2 4 4 3 4 4 4 6% 50%	1980
October	1st to 10th	Day	10% 5% 2 6% 5% 4 4 5% 4 7% 47%	1980
	11th to 20th	Day	12% 6% 3 5% 4 3 4 4 6% 6% 46%	1857
	21st to end	Day	20% 10% 3 5% 5% 3 3 3 5% 7% 37%	1800
November	1st to 10th	Day	8% 6% 3 5% 5% 2 4 3 4 5% 54%	1672
	11th to 20th	Day	14% 9% 5% 7% 5% 3 4 4 6% 6% 38%	1710
	21st to end	Day	18% 9% 4 6% 6% 3 3 3 4 7% 37%	1710
December	1st to 10th	Day	20% 12% 5% 8% 6% 4 4 4 5% 5% 27%	1710
	11th to 20th	Day	15% 13% 6% 9% 7% 3 4 4 4 5% 30%	2153
	21st to end	Day	21% 22% 6% 8% 5% 3 4 3 4 5% 19%	2805
January	1st to 10th	Night	17% 10% 6% 10% 8% 3 5% 4 5% 5% 27%	2648
	11th to 20th	Night	19% 14% 6% 9% 7% 3 4 4 4 6% 24%	2571
	21st to end	Night	16% 14% 6% 9% 6% 3 4 4 4 4 29%	2574
February	1st to 10th	Night	21% 12% 5% 8% 6% 3 4 5% 4 5% 27%	2209
	11th to 20th	Night	15% 10% 4 7% 5% 4 5% 5% 5% 7% 32%	2098
	21st to end	Night	21% 13% 5% 7% 6% 3 4 3 5% 5% 30%	1776
March	1st to 10th	Night	17% 8% 4 6% 5% 3 5% 5% 5% 6% 37%	2160
	11th to 20th	Night	16% 9% 4 7% 5% 3 4 4 5% 7% 36%	1996
	21st to end	Night	20% 9% 4 6% 5% 3 4 4 5% 7% 34%	2178
April	1st to 10th	Night	13% 8% 4 5% 6% 3 5% 5% 6% 7% 39%	1806
	11th to 20th	Night	15% 7% 3 5% 4 2 5% 5% 6% 7% 40%	1753
	21st to end	Night	17% 7% 3 6% 5% 3 5% 5% 6% 7% 37%	1620
May	1st to 10th	Night	18% 8% 3 7% 6% 3 5% 5% 6% 8% 33%	1620
	11th to 20th	Night	15% 7% 3 7% 6% 2 4 4 6% 7% 38%	1592
	21st to end	Night	7% 5% 2 5% 5% 4 4 7% 7% 11% 43%	1744
June	1st to 10th	Night	9% 4 2 5% 5% 3 5% 5% 4 8% 48%	1536
	11th to 20th	Night	8% 4 2 4 5% 3 5% 4 8% 10% 47%	1440
	21st to end	Night	6% 3 2 4 5% 3 4 6% 8% 9% 49%	1440
July	1st to 10th	Night	9% 6% 2 4 5% 2 5% 6% 7% 9% 45%	1440
	11th to 20th	Night	7% 4 1 4 4 3 5% 5% 6% 9% 51%	1448
	21st to end	Night	10% 5% 2 4 4 3 5% 4 6% 9% 49%	1771
August	1st to 10th	Night	7% 4 1 5% 6% 4 4 6% 8% 11% 46%	1611
	11th to 20th	Night	6% 5% 2 4 5% 3 5% 5% 7% 9% 50%	1620
	21st to end	Night	5% 4 2 4 5% 3 4 4 6% 10% 52%	1980
September	1st to 10th	Night	12% 5% 3 4 4 3 3 4 5% 6% 50%	1862
	11th to 20th	Night	8% 4 2 4 4 3 4 5% 5% 8% 54%	1980
	21st to end	Night	12% 5% 3 5% 4 2 3 4 4 7% 51%	1980
October	1st to 10th	Night	10% 4 2 5% 4 3 3 3 4 7% 54%	1980
	11th to 20th	Night	14% 7% 2 5% 6% 3 4 5% 4 7% 44%	2101
	21st to end	Night	17% 7% 3 6% 6% 3 4 3 4 5% 40%	2551
November	1st to 10th	Night	12% 7% 2 5% 5% 2 4 4 4 6% 49%	2406
	11th to 20th	Night	15% 7% 3 7% 5% 4 3 4 5% 7% 41%	2470
	21st to end	Night	17% 11% 4 6% 6% 2 4 4 5% 7% 33%	2470
December	1st to 10th	Night	20% 11% 5% 7% 5% 3 4 4 5% 6% 30%	2470
	11th to 20th	Night	18% 12% 6% 7% 6% 3 4 5% 4 5% 30%	3042
	21st to end	Night	22% 17% 6% 9% 7% 3 5% 4 4 5% 18%	4852

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	56% 4 9% 17% 12% 1	1518
	11th to 20th	Day	61% 5% 10% 16% 8%	1513
	21st to end	Day	59% 5% 12% 19% 5% 1	1782
February	1st to 10th	Day	57% 5% 11% 17% 9% 1	1750
	11th to 20th	Day	55% 5% 10% 20% 9% 1	1758
	21st to end	Day	56% 3 9% 21% 8% 2	1480
March	1st to 10th	Day	50% 5% 9% 23% 12% 1	1800
	11th to 20th	Day	51% 6% 12% 18% 11% 2	1962
	21st to end	Day	57% 5% 11% 19% 7% 1	2186
April	1st to 10th	Day	45% 7% 12% 22% 13% 1	2160
	11th to 20th	Day	47% 5% 10% 20% 15% 3	2155
	21st to end	Day	48% 6% 10% 22% 12% 2	2340
May	1st to 10th	Day	51% 7% 11% 20% 10% 1	2340
	11th to 20th	Day	50% 7% 11% 18% 13% 1	2305
	21st to end	Day	42% 9% 16% 19% 12% 2	2531
June	1st to 10th	Day	39% 7% 10% 24% 16% 3	2419
	11th to 20th	Day	36% 8% 14% 21% 18% 2	2520
	21st to end	Day	38% 8% 16% 20% 15% 4	2520
July	1st to 10th	Day	40% 9% 13% 19% 17% 2	2451
	11th to 20th	Day	35% 8% 13% 25% 17% 2	2435
	21st to end	Day	37% 9% 15% 21% 14% 3	2405
August	1st to 10th	Day	34% 9% 15% 27% 14% 2	2314
	11th to 20th	Day	39% 7% 13% 25% 16% 1	2277
	21st to end	Day	33% 8% 15% 22% 16% 6%	2376
September	1st to 10th	Day	38% 6% 11% 22% 17% 5%	2078
	11th to 20th	Day	33% 6% 13% 25% 20% 3	1980
	21st to end	Day	34% 5% 11% 25% 22% 2	1980
October	1st to 10th	Day	37% 6% 10% 23% 22% 3	1980
	11th to 20th	Day	37% 5% 10% 26% 20% 3	1857
	21st to end	Day	48% 4 10% 21% 15% 2	1800
November	1st to 10th	Day	34% 4 9% 30% 20% 3	1672
	11th to 20th	Day	47% 4 9% 21% 16% 3	1710
	21st to end	Day	49% 4 9% 23% 14% 1	1710
December	1st to 10th	Day	60% 4 11% 18% 7% 1	1710
	11th to 20th	Day	57% 5% 8% 18% 11%	2150
	21st to end	Day	69% 5% 9% 11% 7%	2805
January	1st to 10th	Night	59% 6% 8% 18% 10%	2648
	11th to 20th	Night	63% 5% 11% 16% 6%	2571
	21st to end	Night	59% 5% 9% 19% 8% 1	2574
February	1st to 10th	Night	60% 5% 11% 18% 6% 1	2209
	11th to 20th	Night	52% 5% 12% 19% 11% 1	2099
	21st to end	Night	58% 5% 10% 17% 9% 2	1776
March	1st to 10th	Night	47% 5% 8% 24% 15% 1	2160
	11th to 20th	Night	47% 6% 12% 21% 11% 2	1996
	21st to end	Night	51% 5% 10% 24% 8% 2	2178
April	1st to 10th	Night	43% 6% 11% 26% 13% 2	1806
	11th to 20th	Night	41% 7% 11% 18% 18% 5%	1753
	21st to end	Night	45% 7% 11% 20% 14% 3	1620
May	1st to 10th	Night	49% 7% 11% 21% 9% 3	1620
	11th to 20th	Night	45% 6% 13% 20% 13% 2	1593
	21st to end	Night	34% 10% 16% 23% 14% 4	1745
June	1st to 10th	Night	32% 8% 11% 26% 21% 2	1536
	11th to 20th	Night	30% 7% 15% 23% 20% 4	1440
	21st to end	Night	29% 10% 16% 22% 18% 5%	1440
July	1st to 10th	Night	35% 9% 15% 20% 17% 6%	1400
	11th to 20th	Night	28% 8% 14% 28% 19% 3	1400
	21st to end	Night	31% 8% 13% 29% 17% 2	1672
August	1st to 10th	Night	31% 8% 16% 27% 15% 2	1600
	11th to 20th	Night	30% 8% 14% 30% 17% 1	1620
	21st to end	Night	26% 9% 15% 25% 21% 5%	1980
September	1st to 10th	Night	33% 6% 13% 23% 17% 8%	1862
	11th to 20th	Night	28% 6% 11% 27% 25% 3	1980
	21st to end	Night	35% 5% 9% 23% 25% 3	1980
October	1st to 10th	Night	30% 5% 11% 27% 24% 4	1980
	11th to 20th	Night	40% 4 8% 24% 21% 2	2101
	21st to end	Night	46% 5% 9% 21% 16% 3	2551
November	1st to 10th	Night	37% 4 11% 22% 23% 3	2406
	11th to 20th	Night	43% 4 10% 22% 18% 4	2470
	21st to end	Night	52% 4 10% 21% 12% 1	2470
December	1st to 10th	Night	56% 4 11% 19% 9% 1	2470
	11th to 20th	Night	55% 5% 10% 19% 11% 1	3044
	21st to end	Night	68% 5% 10% 12% 5%	4852

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	24% 24% 34% 14% 3 1	270
	11th to 20th	Day	29% 25% 32% 12% 2	257
	21st to end	Day	27% 27% 31% 14% 3	264
February	1st to 10th	Day	28% 31% 30% 10% 1	240
	11th to 20th	Day	27% 30% 33% 10%	240
	21st to end	Day	29% 24% 39% 7% 1	198
March	1st to 10th	Day	24% 33% 32% 10% 1	240
	11th to 20th	Day	25% 22% 34% 14% 3 1	402
	21st to end	Day	31% 19% 29% 13% 6% 1	462
April	1st to 10th	Day	20% 21% 31% 18% 8% 2	420
	11th to 20th	Day	21% 21% 24% 21% 10% 3	412
	21st to end	Day	23% 20% 33% 14% 7% 3	420
May	1st to 10th	Day	25% 14% 34% 16% 7% 4	423
	11th to 20th	Day	22% 13% 26% 24% 10% 4	447
	21st to end	Day	13% 8% 28% 24% 18% 10%	485
June	1st to 10th	Day	13% 8% 28% 29% 17% 6%	529
	11th to 20th	Day	11% 7% 27% 33% 15% 7%	630
	21st to end	Day	13% 8% 26% 32% 15% 7%	630
July	1st to 10th	Day	12% 4 18% 29% 22% 16%	621
	11th to 20th	Day	7% 5% 24% 33% 21% 10%	608
	21st to end	Day	12% 4 19% 32% 23% 11%	488
August	1st to 10th	Day	9% 4 18% 29% 24% 15%	446
	11th to 20th	Day	12% 6% 21% 30% 20% 12%	439
	21st to end	Day	8% 6% 19% 26% 28% 13%	495
September	1st to 10th	Day	12% 5% 19% 29% 28% 8%	450
	11th to 20th	Day	11% 2 18% 28% 29% 12%	450
	21st to end	Day	16% 10% 21% 25% 21% 7%	450
October	1st to 10th	Day	11% 8% 28% 30% 18% 6%	450
	11th to 20th	Day	14% 16% 35% 22% 11% 1	450
	21st to end	Day	26% 16% 40% 16% 3	315
November	1st to 10th	Day	13% 20% 33% 23% 9% 2	270
	11th to 20th	Day	21% 24% 25% 22% 6% 2	270
	21st to end	Day	27% 16% 37% 17% 3	270
December	1st to 10th	Day	24% 24% 33% 14% 5%	270
	11th to 20th	Day	22% 23% 38% 11% 5%	270
	21st to end	Day	31% 24% 29% 14% 3	297
January	1st to 10th	Night	22% 20% 33% 20% 4	950
	11th to 20th	Night	26% 19% 36% 17% 2	898
	21st to end	Night	25% 18% 34% 21% 2	792
February	1st to 10th	Night	28% 14% 35% 21% 2	720
	11th to 20th	Night	23% 22% 35% 19% 2	703
	21st to end	Night	28% 17% 35% 19% 1	592
March	1st to 10th	Night	22% 18% 34% 23% 3	720
	11th to 20th	Night	21% 15% 38% 20% 6% 1	720
	21st to end	Night	25% 13% 30% 26% 6%	792
April	1st to 10th	Night	17% 14% 35% 27% 7% 1	546
	11th to 20th	Night	21% 13% 28% 27% 11% 1	533
	21st to end	Night	21% 10% 39% 21% 9% 1	540
May	1st to 10th	Night	20% 10% 36% 23% 10% 1	540
	11th to 20th	Night	18% 6% 28% 29% 16% 3	531
	21st to end	Night	10% 3 24% 31% 28% 3	584
June	1st to 10th	Night	10% 3 23% 43% 21% 1	540
	11th to 20th	Night	9% 4 23% 40% 22% 3	540
	21st to end	Night	7% 3 26% 38% 24% 2	540
July	1st to 10th	Night	11% 2 16% 30% 32% 9%	525
	11th to 20th	Night	8% 4 17% 37% 31% 4	522
	21st to end	Night	12% 3 14% 37% 31% 4	591
August	1st to 10th	Night	8% 2 13% 36% 36% 4	537
	11th to 20th	Night	7% 1 16% 38% 32% 5%	534
	21st to end	Night	6% 2 17% 34% 36% 6%	594
September	1st to 10th	Night	14% 3 15% 32% 31% 5%	602
	11th to 20th	Night	10% 3 17% 33% 32% 6%	720
	21st to end	Night	14% 6% 25% 29% 24% 3	720
October	1st to 10th	Night	12% 7% 23% 31% 22% 5%	720
	11th to 20th	Night	17% 11% 29% 28% 14% 1	720
	21st to end	Night	21% 14% 32% 23% 9%	792
November	1st to 10th	Night	15% 10% 33% 26% 15% 1	740
	11th to 20th	Night	20% 17% 29% 25% 9% 1	760
	21st to end	Night	24% 15% 25% 30% 6%	760
December	1st to 10th	Night	25% 17% 32% 21% 5%	760
	11th to 20th	Night	23% 15% 35% 21% 5% 1	760
	21st to end	Night	29% 19% 31% 18% 4	1026

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