Doran Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Doran Planetarium 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	16% 10% 4 9% 5% 4 4 3 5% 7% 33%	127206
	Night	15% 12% 4 9% 5% 4 4 3 5% 6% 32%	123581
Transparency	Day	50% 6% 11% 22% 10% 1	130703
	Night	52% 6% 11% 20% 10% 1	128172
Seeing	Day	21% 13% 31% 23% 10% 3	16644
	Night	22% 10% 30% 28% 9% 1	25770

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	12% 17% 6% 11% 5% 5% 5% 3 5% 7% 24%	3158
	11th to 20th	Day	9% 15% 8% 17% 8% 4 5% 4 5% 5% 19%	3374
	21st to end	Day	13% 17% 8% 14% 7% 6% 4 3 4 4 20%	4308
February	1st to 10th	Day	10% 18% 4 11% 7% 6% 5% 4 5% 7% 24%	1800
	11th to 20th	Day	11% 16% 5% 11% 7% 5% 4 3 4 5% 29%	1753
	21st to end	Day	12% 17% 5% 12% 5% 4 4 5% 4 5% 24%	1565
March	1st to 10th	Day	19% 14% 4 9% 4 4 3 3 4 6% 30%	4753
	11th to 20th	Day	14% 13% 5% 11% 6% 4 5% 3 4 8% 27%	4980
	21st to end	Day	15% 13% 5% 10% 5% 4 3 3 4 4 32%	5465
April	1st to 10th	Day	18% 12% 4 7% 4 4 5% 4 6% 6% 30%	2350
	11th to 20th	Day	20% 10% 3 6% 4 3 4 4 5% 6% 35%	2572
	21st to end	Day	22% 10% 3 7% 4 4 3 3 4 5% 36%	2600
May	1st to 10th	Day	19% 8% 3 6% 4 4 4 3 4 6% 38%	5906
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 5% 7% 41%	6318
	21st to end	Day	20% 8% 3 7% 4 4 4 3 5% 6% 37%	7128
June	1st to 10th	Day	15% 8% 3 7% 5% 5% 4 5% 6% 7% 34%	3000
	11th to 20th	Day	12% 7% 2 7% 6% 4 5% 4 6% 8% 39%	3000
	21st to end	Day	18% 8% 3 7% 5% 5% 4 4 7% 7% 32%	3000
July	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	6819
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 5% 8% 45%	6820
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 9% 47%	6805
August	1st to 10th	Day	11% 7% 3 7% 4 5% 6% 4 7% 9% 38%	4234
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 6% 8% 40%	4174
	21st to end	Day	12% 6% 2 7% 4 5% 5% 4 5% 7% 42%	4675
September	1st to 10th	Day	13% 8% 3 8% 5% 5% 5% 3 6% 7% 39%	2400
	11th to 20th	Day	13% 5% 1 7% 4 5% 3 3 6% 8% 45%	2206
	21st to end	Day	21% 8% 2 7% 4 4 4 4 5% 6% 36%	2200
October	1st to 10th	Day	17% 9% 2 7% 4 4 5% 4 4 6% 37%	2010
	11th to 20th	Day	24% 11% 3 8% 5% 4 4 4 5% 6% 25%	2000
	21st to end	Day	26% 14% 4 8% 5% 5% 4 3 4 6% 23%	2084
November	1st to 10th	Day	18% 13% 3 9% 5% 5% 4 3 6% 7% 28%	1725
	11th to 20th	Day	17% 15% 5% 10% 7% 5% 5% 3 4 6% 24%	1680
	21st to end	Day	22% 19% 5% 12% 5% 5% 4 3 3 4 19%	1680
December	1st to 10th	Day	13% 21% 6% 12% 7% 5% 6% 4 5% 6% 16%	2567
	11th to 20th	Day	15% 17% 5% 14% 6% 4 4 3 4 5% 23%	2621
	21st to end	Day	24% 20% 7% 13% 5% 4 3 2 3 4 15%	3476
January	1st to 10th	Night	11% 13% 5% 12% 7% 6% 5% 4 6% 7% 23%	6808
	11th to 20th	Night	12% 16% 6% 11% 7% 5% 5% 3 5% 6% 23%	6527
	21st to end	Night	11% 18% 8% 13% 6% 5% 5% 4 5% 5% 21%	6652
February	1st to 10th	Night	11% 16% 5% 12% 8% 5% 5% 4 5% 6% 24%	2600
	11th to 20th	Night	12% 14% 6% 11% 7% 5% 5% 3 5% 6% 27%	2533
	21st to end	Night	13% 16% 5% 10% 6% 6% 5% 3 6% 5% 27%	2064
March	1st to 10th	Night	18% 13% 4 8% 4 4 4 3 5% 5% 33%	5073
	11th to 20th	Night	14% 11% 5% 11% 6% 5% 5% 4 5% 7% 28%	4979
	21st to end	Night	17% 10% 6% 10% 5% 4 4 3 5% 5% 31%	5467
April	1st to 10th	Night	18% 11% 4 8% 5% 4 4 3 4 6% 31%	2065
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 3 5% 6% 36%	1777
	21st to end	Night	19% 9% 3 7% 5% 5% 4 5% 4 5% 33%	1800
May	1st to 10th	Night	16% 8% 3 7% 5% 5% 4 4 5% 6% 37%	4020
	11th to 20th	Night	13% 7% 2 7% 5% 4 5% 3 5% 8% 42%	3575
	21st to end	Night	16% 8% 3 7% 4 3 4 4 6% 7% 38%	3750
June	1st to 10th	Night	13% 6% 3 7% 6% 5% 5% 5% 6% 9% 36%	1400
	11th to 20th	Night	13% 6% 2 5% 4 4 5% 4 7% 9% 42%	1400
	21st to end	Night	13% 5% 3 7% 4 5% 3 4 6% 7% 43%	1400
July	1st to 10th	Night	15% 6% 3 6% 4 4 4 3 6% 7% 42%	3160
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 8% 48%	3160
	21st to end	Night	9% 6% 2 5% 4 3 4 4 6% 8% 50%	4026
August	1st to 10th	Night	11% 6% 2 6% 4 4 5% 4 6% 7% 43%	2915
	11th to 20th	Night	15% 5% 2 6% 4 4 4 3 5% 7% 45%	2875
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 5% 7% 48%	3223
September	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 6% 7% 46%	1979
	11th to 20th	Night	12% 6% 2 5% 3 5% 4 3 4 7% 50%	2190
	21st to end	Night	18% 6% 3 6% 4 4 4 2 4 7% 41%	2200
October	1st to 10th	Night	19% 8% 3 6% 4 4 3 4 5% 6% 39%	2389
	11th to 20th	Night	23% 11% 3 7% 5% 4 4 3 5% 6% 30%	2400
	21st to end	Night	24% 13% 4 8% 4 5% 4 3 4 6% 26%	2755
November	1st to 10th	Night	18% 12% 3 7% 5% 4 5% 4 5% 7% 31%	2792
	11th to 20th	Night	18% 14% 3 9% 7% 5% 4 3 6% 5% 26%	2940
	21st to end	Night	21% 17% 4 8% 5% 4 4 4 5% 5% 22%	2940
December	1st to 10th	Night	16% 19% 5% 11% 5% 4 5% 4 5% 6% 19%	4659
	11th to 20th	Night	15% 16% 4 12% 7% 4 4 3 4 6% 24%	5586
	21st to end	Night	22% 22% 5% 13% 4 3 3 2 3 4 17%	7502

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	60% 5% 11% 19% 5%	3159
	11th to 20th	Day	66% 4 10% 12% 8% 1	3404
	21st to end	Day	67% 5% 6% 12% 9%	4308
February	1st to 10th	Day	59% 5% 11% 17% 6% 1	1800
	11th to 20th	Day	58% 5% 10% 15% 10% 1	1762
	21st to end	Day	60% 5% 8% 18% 9% 1	1565
March	1st to 10th	Day	56% 4 10% 12% 15% 3	4756
	11th to 20th	Day	56% 5% 12% 16% 9% 1	4980
	21st to end	Day	54% 4 11% 18% 10% 2	5465
April	1st to 10th	Day	53% 5% 12% 19% 9% 1	2350
	11th to 20th	Day	50% 4 8% 24% 13% 1	2572
	21st to end	Day	51% 5% 8% 19% 14% 3	2600
May	1st to 10th	Day	47% 5% 8% 23% 14% 4	5915
	11th to 20th	Day	41% 7% 11% 23% 16% 3	6322
	21st to end	Day	48% 4 10% 19% 16% 3	7128
June	1st to 10th	Day	46% 7% 11% 22% 13% 1	3000
	11th to 20th	Day	42% 6% 11% 23% 17% 1	3000
	21st to end	Day	49% 7% 11% 21% 10% 1	3000
July	1st to 10th	Day	45% 8% 13% 24% 9%	6746
	11th to 20th	Day	35% 7% 14% 36% 7%	6739
	21st to end	Day	33% 7% 14% 34% 12%	6612
August	1st to 10th	Day	40% 8% 13% 30% 9% 1	4221
	11th to 20th	Day	40% 7% 15% 28% 9%	4213
	21st to end	Day	39% 7% 13% 31% 9% 1	4674
September	1st to 10th	Day	44% 6% 15% 25% 10% 1	2400
	11th to 20th	Day	37% 6% 16% 29% 11%	2206
	21st to end	Day	49% 6% 14% 23% 8%	2200
October	1st to 10th	Day	40% 6% 10% 31% 13%	3270
	11th to 20th	Day	48% 7% 12% 23% 10%	3240
	21st to end	Day	63% 5% 13% 13% 6%	3347
November	1st to 10th	Day	56% 6% 13% 19% 7%	1725
	11th to 20th	Day	61% 5% 12% 16% 6%	1680
	21st to end	Day	71% 5% 9% 10% 5%	1680
December	1st to 10th	Day	69% 6% 12% 10% 3	2567
	11th to 20th	Day	63% 5% 6% 18% 6%	2621
	21st to end	Day	74% 4 5% 12% 5%	3476
January	1st to 10th	Night	57% 7% 11% 17% 7%	6808
	11th to 20th	Night	61% 6% 12% 12% 9%	6572
	21st to end	Night	63% 5% 7% 13% 10% 1	6652
February	1st to 10th	Night	60% 5% 10% 18% 7% 1	2600
	11th to 20th	Night	58% 5% 11% 17% 7% 1	2535
	21st to end	Night	59% 5% 11% 16% 9% 1	2064
March	1st to 10th	Night	52% 5% 9% 16% 14% 4	5083
	11th to 20th	Night	55% 6% 13% 16% 8% 1	4980
	21st to end	Night	54% 5% 8% 17% 13% 3	5467
April	1st to 10th	Night	54% 4 8% 19% 13% 1	2065
	11th to 20th	Night	48% 5% 6% 23% 17% 2	1777
	21st to end	Night	52% 5% 10% 15% 16% 2	1800
May	1st to 10th	Night	45% 7% 8% 21% 16% 3	4027
	11th to 20th	Night	40% 6% 10% 23% 17% 4	3576
	21st to end	Night	45% 6% 9% 23% 13% 4	3751
June	1st to 10th	Night	42% 8% 12% 23% 13% 3	1400
	11th to 20th	Night	38% 6% 13% 22% 19% 2	1400
	21st to end	Night	39% 8% 15% 23% 14% 2	1400
July	1st to 10th	Night	40% 7% 12% 29% 11% 1	3125
	11th to 20th	Night	33% 5% 16% 36% 10% 1	3118
	21st to end	Night	31% 6% 14% 32% 16%	3906
August	1st to 10th	Night	39% 6% 15% 26% 13% 1	2904
	11th to 20th	Night	37% 7% 18% 27% 9% 1	2902
	21st to end	Night	35% 7% 15% 31% 12% 1	3223
September	1st to 10th	Night	37% 7% 14% 30% 10% 2	1979
	11th to 20th	Night	34% 7% 14% 33% 11%	2190
	21st to end	Night	44% 6% 14% 26% 10%	2200
October	1st to 10th	Night	40% 5% 12% 31% 13%	3909
	11th to 20th	Night	45% 6% 12% 29% 7%	3899
	21st to end	Night	55% 6% 17% 15% 6% 1	4442
November	1st to 10th	Night	51% 7% 13% 21% 8%	2792
	11th to 20th	Night	59% 5% 12% 16% 7% 1	2940
	21st to end	Night	64% 6% 11% 13% 6%	2940
December	1st to 10th	Night	65% 6% 13% 14% 3	4659
	11th to 20th	Night	61% 6% 8% 13% 11% 1	5585
	21st to end	Night	72% 4 7% 12% 6%	7502

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% 18% 29% 22% 6% 1	200
	11th to 20th	Day	17% 12% 37% 27% 6% 2	251
	21st to end	Day	25% 9% 32% 22% 10% 2	330
February	1st to 10th	Day	23% 22% 33% 17% 5%	300
	11th to 20th	Day	21% 16% 39% 20% 4	300
	21st to end	Day	23% 24% 37% 15% 1	249
March	1st to 10th	Day	21% 16% 37% 20% 6%	420
	11th to 20th	Day	21% 15% 32% 24% 7% 1	500
	21st to end	Day	24% 15% 33% 22% 5% 1	550
April	1st to 10th	Day	24% 11% 31% 23% 9% 2	500
	11th to 20th	Day	30% 16% 23% 16% 11% 4	492
	21st to end	Day	32% 14% 24% 19% 9% 2	500
May	1st to 10th	Day	22% 11% 31% 22% 11% 2	503
	11th to 20th	Day	20% 14% 30% 21% 11% 4	527
	21st to end	Day	21% 12% 30% 23% 10% 3	648
June	1st to 10th	Day	17% 16% 37% 24% 5% 1	730
	11th to 20th	Day	17% 16% 35% 22% 7% 2	730
	21st to end	Day	21% 12% 33% 23% 8% 2	730
July	1st to 10th	Day	12% 11% 34% 28% 12% 2	721
	11th to 20th	Day	13% 12% 33% 27% 11% 4	718
	21st to end	Day	13% 7% 30% 31% 13% 6%	673
August	1st to 10th	Day	16% 8% 25% 27% 18% 6%	526
	11th to 20th	Day	13% 7% 26% 25% 19% 10%	519
	21st to end	Day	16% 7% 29% 25% 15% 8%	583
September	1st to 10th	Day	17% 12% 27% 27% 12% 6%	530
	11th to 20th	Day	13% 5% 26% 31% 16% 8%	530
	21st to end	Day	24% 11% 23% 24% 13% 4	530
October	1st to 10th	Day	22% 14% 26% 23% 12% 3	530
	11th to 20th	Day	26% 16% 34% 18% 5% 1	530
	21st to end	Day	30% 16% 30% 19% 4 1	467
November	1st to 10th	Day	29% 14% 35% 17% 4	263
	11th to 20th	Day	22% 20% 35% 17% 5% 1	220
	21st to end	Day	28% 19% 31% 15% 6%	220
December	1st to 10th	Day	22% 13% 32% 24% 9%	204
	11th to 20th	Day	32% 15% 35% 14% 5%	200
	21st to end	Day	32% 11% 35% 15% 5%	220
January	1st to 10th	Night	23% 12% 33% 25% 7% 1	1000
	11th to 20th	Night	25% 13% 29% 25% 8%	1000
	21st to end	Night	21% 8% 29% 31% 9% 1	951
February	1st to 10th	Night	19% 15% 37% 26% 4	800
	11th to 20th	Night	20% 15% 35% 24% 6% 1	783
	21st to end	Night	21% 16% 35% 25% 4	660
March	1st to 10th	Night	20% 14% 36% 24% 6%	800
	11th to 20th	Night	22% 13% 33% 24% 7% 1	800
	21st to end	Night	21% 10% 36% 25% 7% 1	880
April	1st to 10th	Night	24% 13% 31% 28% 5%	785
	11th to 20th	Night	22% 13% 29% 27% 8% 1	593
	21st to end	Night	25% 11% 30% 27% 8%	600
May	1st to 10th	Night	17% 7% 32% 34% 9% 1	593
	11th to 20th	Night	15% 9% 32% 35% 8% 1	394
	21st to end	Night	15% 8% 37% 30% 9%	433
June	1st to 10th	Night	16% 6% 38% 32% 8%	400
	11th to 20th	Night	16% 10% 31% 36% 8%	400
	21st to end	Night	15% 7% 31% 34% 14% 1	400
July	1st to 10th	Night	9% 5% 34% 42% 8% 1	390
	11th to 20th	Night	10% 5% 32% 40% 13%	388
	21st to end	Night	10% 4 29% 40% 16% 1	577
August	1st to 10th	Night	11% 5% 25% 39% 19% 1	597
	11th to 20th	Night	14% 5% 24% 34% 21% 2	594
	21st to end	Night	17% 4 26% 34% 18% 1	660
September	1st to 10th	Night	17% 6% 28% 30% 19% 1	779
	11th to 20th	Night	14% 8% 25% 31% 19% 3	800
	21st to end	Night	21% 6% 26% 28% 16% 3	800
October	1st to 10th	Night	23% 9% 27% 29% 11% 1	800
	11th to 20th	Night	31% 10% 28% 24% 7% 1	800
	21st to end	Night	31% 12% 27% 25% 4 1	880
November	1st to 10th	Night	26% 12% 31% 24% 6% 1	820
	11th to 20th	Night	25% 12% 30% 25% 7%	840
	21st to end	Night	31% 12% 29% 22% 6%	840
December	1st to 10th	Night	28% 13% 30% 22% 6%	833
	11th to 20th	Night	26% 14% 31% 21% 7% 1	1000
	21st to end	Night	30% 13% 28% 22% 7%	1100

Page generated at 2025-11-30 01:00:51. Version ac07

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