Tinyblue Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tinyblue Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	15% 9% 3 8% 5% 5% 5% 4 5% 6% 34%	131272
	Night	18% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	128134
Transparency	Day	30% 40% 11% 12% 5% 1	130984
	Night	34% 40% 10% 11% 4 1	127999
Seeing	Day	21% 9% 19% 26% 20% 6%	16574
	Night	25% 9% 17% 25% 20% 4	24876

By Month

Percentage Cloud Cover NW (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	22% 18% 6% 14% 7% 5% 5% 3 4 4 11%	3179
	11th to 20th	Day	15% 11% 4 11% 7% 6% 5% 4 7% 9% 21%	3431
	21st to end	Day	19% 13% 4 10% 7% 6% 6% 4 5% 6% 20%	4118
February	1st to 10th	Day	21% 14% 5% 11% 6% 5% 6% 5% 6% 6% 17%	1800
	11th to 20th	Day	20% 14% 4 9% 6% 5% 5% 5% 5% 6% 21%	1755
	21st to end	Day	19% 13% 4 9% 6% 6% 5% 5% 5% 5% 22%	1533
March	1st to 10th	Day	15% 12% 4 12% 7% 6% 7% 5% 7% 7% 18%	4647
	11th to 20th	Day	15% 14% 4 9% 5% 5% 5% 4 5% 6% 27%	5000
	21st to end	Day	15% 12% 5% 13% 8% 7% 7% 5% 6% 6% 16%	5651
April	1st to 10th	Day	19% 12% 4 9% 5% 6% 6% 5% 6% 7% 22%	2400
	11th to 20th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 6% 29%	2423
	21st to end	Day	18% 9% 3 8% 6% 5% 5% 4 6% 6% 29%	2645
May	1st to 10th	Day	14% 8% 3 6% 4 4 4 3 5% 6% 43%	6320
	11th to 20th	Day	16% 9% 3 9% 5% 5% 5% 4 6% 7% 31%	6277
	21st to end	Day	21% 8% 3 7% 5% 4 4 4 5% 9% 29%	6903
June	1st to 10th	Day	19% 8% 3 7% 4 4 4 4 5% 6% 36%	2925
	11th to 20th	Day	16% 9% 3 8% 6% 5% 6% 4 6% 7% 29%	3000
	21st to end	Day	12% 7% 2 6% 5% 4 4 4 7% 7% 41%	3000
July	1st to 10th	Day	8% 4 1 4 2 3 3 2 4 5% 64%	6780
	11th to 20th	Day	5% 3 1 3 3 2 3 2 3 5% 69%	6646
	21st to end	Day	9% 5% 2 5% 4 3 3 3 4 6% 57%	6874
August	1st to 10th	Day	5% 3 1 4 2 3 3 3 5% 7% 61%	4461
	11th to 20th	Day	10% 8% 3 7% 4 4 4 3 5% 7% 45%	4156
	21st to end	Day	13% 8% 3 7% 4 4 3 3 4 6% 45%	4506
September	1st to 10th	Day	10% 6% 2 5% 3 4 4 2 4 8% 52%	2400
	11th to 20th	Day	12% 7% 2 8% 5% 4 5% 4 4 6% 43%	2400
	21st to end	Day	14% 8% 3 8% 4 4 5% 4 5% 7% 39%	2108
October	1st to 10th	Day	13% 7% 3 8% 6% 6% 6% 5% 7% 9% 31%	3280
	11th to 20th	Day	21% 12% 4 10% 5% 5% 4 3 5% 6% 25%	3261
	21st to end	Day	18% 12% 5% 13% 7% 6% 6% 4 5% 6% 20%	3365
November	1st to 10th	Day	24% 11% 5% 9% 6% 5% 6% 4 5% 6% 18%	1663
	11th to 20th	Day	24% 11% 4 9% 6% 5% 4 3 4 6% 22%	1680
	21st to end	Day	25% 11% 3 10% 6% 5% 5% 3 6% 7% 18%	1680
December	1st to 10th	Day	14% 11% 3 14% 9% 7% 8% 5% 5% 7% 16%	2636
	11th to 20th	Day	21% 15% 6% 14% 6% 6% 5% 4 6% 5% 12%	2871
	21st to end	Day	22% 18% 5% 12% 7% 6% 5% 4 5% 5% 11%	3498
January	1st to 10th	Night	21% 17% 6% 12% 8% 6% 6% 3 4 5% 12%	6766
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 4 7% 9% 28%	6451
	21st to end	Night	20% 11% 3 9% 6% 6% 6% 4 5% 6% 24%	6842
February	1st to 10th	Night	22% 14% 4 9% 5% 5% 5% 4 5% 5% 21%	2600
	11th to 20th	Night	19% 13% 3 10% 7% 5% 5% 3 4 5% 25%	2532
	21st to end	Night	19% 15% 3 8% 6% 4 4 4 5% 6% 24%	2096
March	1st to 10th	Night	13% 12% 4 11% 7% 7% 7% 5% 6% 6% 20%	5244
	11th to 20th	Night	14% 12% 4 8% 6% 5% 4 4 6% 6% 31%	4999
	21st to end	Night	21% 14% 5% 11% 6% 5% 5% 4 6% 5% 17%	5324
April	1st to 10th	Night	21% 12% 3 9% 7% 4 5% 5% 6% 6% 21%	2000
	11th to 20th	Night	19% 11% 3 7% 5% 5% 5% 4 4 6% 32%	1925
	21st to end	Night	21% 10% 2 7% 5% 6% 4 4 7% 7% 28%	1755
May	1st to 10th	Night	15% 8% 3 7% 4 4 3 3 3 5% 45%	3640
	11th to 20th	Night	19% 7% 3 7% 5% 5% 4 4 5% 6% 34%	3616
	21st to end	Night	21% 8% 3 7% 4 5% 5% 4 7% 7% 29%	3972
June	1st to 10th	Night	20% 7% 3 7% 5% 4 4 4 6% 6% 33%	1470
	11th to 20th	Night	22% 9% 4 9% 5% 5% 4 3 6% 6% 28%	1400
	21st to end	Night	13% 6% 2 6% 5% 5% 5% 3 6% 6% 44%	1400
July	1st to 10th	Night	14% 5% 1 4 2 2 2 1 3 4 62%	3180
	11th to 20th	Night	5% 2 1 3 2 2 2 2 3 5% 75%	3313
	21st to end	Night	10% 4 2 4 3 3 4 3 6% 7% 55%	3969
August	1st to 10th	Night	6% 3 1 3 3 3 4 3 4 6% 66%	2690
	11th to 20th	Night	13% 6% 3 7% 4 3 4 2 4 5% 49%	2902
	21st to end	Night	15% 6% 2 6% 3 3 4 3 4 6% 50%	3407
September	1st to 10th	Night	9% 4 3 5% 4 3 3 3 4 6% 55%	2000
	11th to 20th	Night	14% 8% 3 6% 4 4 4 4 5% 7% 42%	2000
	21st to end	Night	14% 7% 3 7% 3 4 4 3 5% 6% 44%	2290
October	1st to 10th	Night	13% 6% 3 8% 5% 4 4 4 6% 7% 39%	3920
	11th to 20th	Night	20% 10% 3 7% 4 4 4 4 5% 6% 32%	3897
	21st to end	Night	20% 11% 4 10% 6% 5% 5% 4 5% 6% 26%	4439
November	1st to 10th	Night	25% 11% 3 8% 5% 5% 5% 4 5% 6% 24%	2853
	11th to 20th	Night	23% 13% 5% 9% 6% 5% 4 4 4 5% 23%	2940
	21st to end	Night	21% 14% 4 9% 6% 6% 5% 3 5% 6% 21%	2940
December	1st to 10th	Night	16% 10% 3 11% 8% 6% 6% 4 5% 6% 23%	4588
	11th to 20th	Night	24% 14% 5% 12% 6% 6% 5% 4 5% 6% 14%	5316
	21st to end	Night	21% 16% 5% 12% 7% 6% 5% 4 5% 5% 14%	7458

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% 31% 6% 6% 1	3179
	11th to 20th	Day	38% 32% 11% 15% 4	3455
	21st to end	Day	44% 33% 6% 13% 3	4118
February	1st to 10th	Day	26% 64% 4 3 3 1	1800
	11th to 20th	Day	29% 62% 3 3 3	1764
	21st to end	Day	25% 63% 5% 5% 2	1533
March	1st to 10th	Day	43% 35% 10% 9% 3	4647
	11th to 20th	Day	39% 32% 11% 16% 1	5000
	21st to end	Day	47% 35% 11% 7% 1	5651
April	1st to 10th	Day	23% 64% 7% 6%	2400
	11th to 20th	Day	19% 63% 7% 8% 3 1	2423
	21st to end	Day	20% 63% 6% 8% 2	2645
May	1st to 10th	Day	28% 31% 13% 19% 9% 1	6320
	11th to 20th	Day	37% 34% 16% 11% 2	6280
	21st to end	Day	34% 35% 18% 12% 2	6904
June	1st to 10th	Day	14% 64% 9% 9% 3	2925
	11th to 20th	Day	19% 63% 8% 8% 3	3000
	21st to end	Day	17% 61% 10% 9% 2	3000
July	1st to 10th	Day	15% 27% 16% 16% 18% 8%	6706
	11th to 20th	Day	10% 27% 12% 23% 25% 2	6565
	21st to end	Day	22% 28% 23% 21% 6%	6681
August	1st to 10th	Day	12% 37% 8% 29% 13% 1	4444
	11th to 20th	Day	28% 39% 14% 14% 4 1	4195
	21st to end	Day	29% 37% 8% 18% 8% 1	4505
September	1st to 10th	Day	18% 59% 10% 7% 5% 1	2400
	11th to 20th	Day	20% 59% 7% 7% 7%	2400
	21st to end	Day	20% 58% 9% 9% 4	2108
October	1st to 10th	Day	30% 40% 15% 14% 2	3280
	11th to 20th	Day	42% 36% 8% 9% 4	3261
	21st to end	Day	43% 38% 8% 7% 4	3365
November	1st to 10th	Day	31% 55% 8% 4 1	1663
	11th to 20th	Day	28% 56% 7% 6% 3 1	1680
	21st to end	Day	29% 56% 5% 6% 4	1680
December	1st to 10th	Day	41% 39% 6% 9% 4	2636
	11th to 20th	Day	52% 35% 7% 5% 1	2873
	21st to end	Day	57% 31% 9% 2 1	3498
January	1st to 10th	Night	55% 31% 7% 5% 2	6766
	11th to 20th	Night	34% 31% 13% 19% 3	6489
	21st to end	Night	42% 33% 8% 12% 4	6842
February	1st to 10th	Night	25% 63% 4 3 4 1	2600
	11th to 20th	Night	28% 61% 4 5% 2 1	2534
	21st to end	Night	23% 63% 5% 6% 2	2096
March	1st to 10th	Night	43% 35% 10% 9% 4	5244
	11th to 20th	Night	35% 33% 12% 18% 1	5000
	21st to end	Night	49% 33% 10% 8% 1	5324
April	1st to 10th	Night	24% 63% 7% 6% 1	2000
	11th to 20th	Night	21% 63% 6% 7% 3 1	1925
	21st to end	Night	22% 63% 5% 7% 2	1755
May	1st to 10th	Night	28% 31% 8% 25% 8%	3637
	11th to 20th	Night	34% 33% 17% 12% 3	3616
	21st to end	Night	36% 38% 18% 8% 1	3972
June	1st to 10th	Night	17% 63% 10% 8% 3	1470
	11th to 20th	Night	20% 63% 6% 8% 2	1400
	21st to end	Night	18% 62% 10% 9% 2	1400
July	1st to 10th	Night	21% 26% 13% 18% 15% 8%	3145
	11th to 20th	Night	9% 26% 12% 23% 29% 1	3271
	21st to end	Night	21% 28% 20% 22% 8%	3857
August	1st to 10th	Night	10% 38% 9% 29% 14% 1	2682
	11th to 20th	Night	29% 38% 15% 13% 4 1	2929
	21st to end	Night	27% 37% 9% 19% 8% 1	3407
September	1st to 10th	Night	17% 59% 9% 8% 6% 1	2000
	11th to 20th	Night	21% 59% 6% 8% 6% 1	2000
	21st to end	Night	20% 59% 6% 13% 3	2290
October	1st to 10th	Night	27% 39% 16% 17% 2	3920
	11th to 20th	Night	36% 38% 9% 13% 3	3897
	21st to end	Night	42% 38% 9% 8% 4	4439
November	1st to 10th	Night	30% 55% 8% 7% 1	2853
	11th to 20th	Night	31% 55% 4 5% 3 2	2940
	21st to end	Night	29% 56% 5% 5% 5% 1	2940
December	1st to 10th	Night	39% 39% 7% 12% 3	4588
	11th to 20th	Night	50% 36% 7% 5% 1	5313
	21st to end	Night	52% 34% 7% 6% 1	7458

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% 14% 24% 16% 10% 3	200
	11th to 20th	Day	25% 17% 26% 17% 15% 2	200
	21st to end	Day	38% 10% 19% 20% 11% 3	269
February	1st to 10th	Day	34% 15% 21% 19% 11% 1	400
	11th to 20th	Day	36% 10% 22% 21% 10% 1	395
	21st to end	Day	29% 17% 21% 20% 11% 2	357
March	1st to 10th	Day	27% 13% 23% 23% 12% 2	500
	11th to 20th	Day	26% 15% 16% 22% 14% 7%	500
	21st to end	Day	29% 11% 26% 19% 12% 3	550
April	1st to 10th	Day	27% 15% 19% 22% 13% 4	500
	11th to 20th	Day	21% 13% 26% 24% 10% 7%	492
	21st to end	Day	21% 11% 20% 27% 16% 4	500
May	1st to 10th	Day	14% 9% 21% 32% 19% 4	503
	11th to 20th	Day	19% 7% 17% 28% 18% 10%	528
	21st to end	Day	20% 6% 15% 30% 20% 10%	573
June	1st to 10th	Day	18% 7% 22% 30% 19% 4	655
	11th to 20th	Day	17% 8% 23% 29% 18% 4	730
	21st to end	Day	13% 5% 17% 33% 24% 8%	730
July	1st to 10th	Day	10% 4 17% 32% 28% 9%	721
	11th to 20th	Day	8% 5% 15% 31% 31% 10%	662
	21st to end	Day	7% 3 9% 31% 35% 14%	575
August	1st to 10th	Day	9% 2 10% 28% 36% 14%	527
	11th to 20th	Day	7% 3 11% 34% 31% 15%	517
	21st to end	Day	12% 5% 15% 34% 25% 9%	583
September	1st to 10th	Day	15% 3 14% 34% 29% 6%	530
	11th to 20th	Day	15% 6% 17% 26% 29% 6%	530
	21st to end	Day	20% 7% 21% 25% 22% 5%	530
October	1st to 10th	Day	21% 8% 21% 21% 21% 8%	530
	11th to 20th	Day	27% 10% 21% 19% 16% 7%	530
	21st to end	Day	30% 13% 22% 18% 12% 4	465
November	1st to 10th	Day	40% 12% 19% 18% 11%	228
	11th to 20th	Day	36% 11% 21% 13% 13% 6%	220
	21st to end	Day	35% 15% 20% 14% 8% 8%	220
December	1st to 10th	Day	30% 11% 21% 13% 15% 9%	204
	11th to 20th	Day	37% 17% 26% 14% 6% 1	200
	21st to end	Day	34% 16% 25% 17% 6% 1	220
January	1st to 10th	Night	34% 13% 22% 18% 11% 2	900
	11th to 20th	Night	31% 13% 14% 22% 18% 2	838
	21st to end	Night	31% 9% 18% 22% 17% 4	880
February	1st to 10th	Night	31% 13% 19% 22% 12% 3	800
	11th to 20th	Night	28% 10% 20% 24% 16% 1	784
	21st to end	Night	29% 13% 19% 23% 13% 3	660
March	1st to 10th	Night	26% 14% 21% 23% 15% 2	800
	11th to 20th	Night	28% 15% 18% 22% 14% 3	800
	21st to end	Night	29% 12% 21% 25% 11% 2	880
April	1st to 10th	Night	28% 9% 19% 26% 17% 2	800
	11th to 20th	Night	24% 9% 22% 27% 15% 4	737
	21st to end	Night	25% 7% 21% 29% 15% 3	555
May	1st to 10th	Night	19% 6% 19% 32% 21% 4	400
	11th to 20th	Night	19% 4 16% 27% 30% 3	394
	21st to end	Night	23% 6% 14% 28% 24% 4	433
June	1st to 10th	Night	22% 5% 19% 34% 19% 2	400
	11th to 20th	Night	23% 7% 21% 28% 19% 3	400
	21st to end	Night	17% 4 12% 36% 29% 2	400
July	1st to 10th	Night	13% 3 12% 35% 34% 4	390
	11th to 20th	Night	8% 3 15% 33% 38% 3	388
	21st to end	Night	7% 3 11% 34% 41% 4	438
August	1st to 10th	Night	12% 2 7% 26% 45% 8%	440
	11th to 20th	Night	10% 1 7% 27% 42% 12%	594
	21st to end	Night	13% 4 12% 31% 36% 4	757
September	1st to 10th	Night	12% 3 9% 35% 34% 7%	800
	11th to 20th	Night	17% 5% 13% 23% 31% 12%	800
	21st to end	Night	18% 6% 18% 27% 26% 6%	800
October	1st to 10th	Night	18% 8% 16% 26% 25% 7%	800
	11th to 20th	Night	24% 10% 17% 22% 21% 6%	800
	21st to end	Night	27% 9% 19% 26% 15% 4	880
November	1st to 10th	Night	33% 11% 20% 24% 13%	820
	11th to 20th	Night	35% 10% 20% 20% 12% 3	840
	21st to end	Night	31% 10% 19% 20% 14% 5%	840
December	1st to 10th	Night	28% 10% 15% 24% 18% 4	808
	11th to 20th	Night	35% 12% 21% 19% 11% 1	830
	21st to end	Night	37% 13% 22% 18% 9% 1	990

Page generated at 2025-11-29 22:00:57. Version ac07

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