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 2024-05-15.

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% 11% 4 9% 5% 5% 4 4 5% 6% 31%	83211
	Night	15% 12% 4 9% 5% 4 4 3 5% 6% 30%	86339
Transparency	Day	52% 6% 12% 20% 9% 1	82910
	Night	54% 6% 12% 19% 9% 1	86176
Seeing	Day	21% 13% 31% 23% 10% 3	14527
	Night	22% 10% 30% 28% 9% 1	23857

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	16% 19% 7% 11% 5% 5% 5% 2 4 5% 23%	2208
	11th to 20th	Day	10% 13% 7% 17% 9% 5% 6% 5% 6% 5% 18%	2370
	21st to end	Day	16% 19% 9% 13% 6% 6% 4 3 4 4 16%	3005
February	1st to 10th	Day	10% 18% 4 11% 7% 5% 5% 4 5% 7% 24%	1710
	11th to 20th	Day	11% 16% 5% 11% 7% 5% 4 3 4 5% 29%	1663
	21st to end	Day	12% 17% 5% 12% 5% 4 4 5% 4 6% 25%	1489
March	1st to 10th	Day	18% 14% 4 8% 4 4 4 3 5% 7% 29%	3309
	11th to 20th	Day	10% 14% 5% 12% 6% 4 5% 3 4 7% 28%	3480
	21st to end	Day	12% 13% 5% 10% 5% 5% 3 3 4 5% 34%	3815
April	1st to 10th	Day	19% 12% 4 7% 4 4 5% 4 6% 6% 30%	2232
	11th to 20th	Day	20% 10% 3 6% 5% 3 5% 4 5% 6% 34%	2442
	21st to end	Day	22% 10% 3 7% 4 4 3 3 4 5% 36%	2470
May	1st to 10th	Day	17% 9% 2 7% 5% 4 4 4 6% 7% 34%	2475
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 4 5% 7% 36%	2534
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 40%	2792
June	1st to 10th	Day	16% 8% 3 8% 5% 4 4 5% 6% 7% 34%	2700
	11th to 20th	Day	12% 6% 2 7% 6% 5% 5% 4 6% 8% 39%	2700
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 7% 7% 33%	2700
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	2700
	11th to 20th	Day	12% 7% 2 7% 5% 6% 5% 5% 5% 8% 38%	2700
	21st to end	Day	12% 7% 2 7% 5% 5% 6% 4 7% 8% 37%	2704
August	1st to 10th	Day	14% 7% 3 7% 4 5% 6% 4 6% 9% 34%	2327
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 5% 8% 40%	2303
	21st to end	Day	14% 6% 3 7% 5% 5% 5% 5% 6% 9% 36%	2574
September	1st to 10th	Day	14% 8% 2 8% 5% 4 4 3 5% 7% 39%	2160
	11th to 20th	Day	14% 5% 1 7% 5% 5% 4 3 6% 8% 42%	1986
	21st to end	Day	22% 8% 2 7% 4 4 4 4 5% 6% 35%	1980
October	1st to 10th	Day	18% 10% 2 8% 5% 4 5% 4 4 6% 34%	1810
	11th to 20th	Day	25% 11% 4 9% 6% 5% 4 4 5% 6% 23%	1800
	21st to end	Day	26% 14% 4 8% 5% 5% 4 3 5% 5% 21%	1876
November	1st to 10th	Day	17% 13% 3 9% 5% 5% 4 3 6% 7% 28%	1558
	11th to 20th	Day	16% 15% 5% 10% 7% 5% 5% 3 4 6% 24%	1520
	21st to end	Day	23% 19% 5% 12% 5% 5% 4 3 3 3 19%	1520
December	1st to 10th	Day	13% 21% 5% 10% 7% 4 5% 4 5% 6% 19%	1497
	11th to 20th	Day	16% 20% 5% 12% 4 5% 4 3 5% 4 23%	1671
	21st to end	Day	24% 21% 6% 14% 5% 4 4 2 3 4 14%	2431
January	1st to 10th	Night	14% 16% 6% 13% 7% 5% 4 3 5% 6% 21%	4748
	11th to 20th	Night	12% 14% 6% 10% 8% 6% 6% 4 6% 7% 21%	4596
	21st to end	Night	14% 19% 8% 12% 6% 5% 4 3 5% 4 18%	4652
February	1st to 10th	Night	11% 16% 5% 11% 8% 5% 5% 4 5% 6% 24%	2470
	11th to 20th	Night	12% 14% 5% 11% 7% 5% 5% 3 5% 5% 28%	2403
	21st to end	Night	14% 16% 5% 9% 5% 6% 4 3 6% 5% 27%	1964
March	1st to 10th	Night	16% 13% 3 8% 4 4 4 3 5% 6% 34%	3517
	11th to 20th	Night	11% 13% 6% 11% 5% 4 4 3 5% 5% 33%	3479
	21st to end	Night	13% 8% 5% 9% 6% 5% 4 3 6% 6% 35%	3817
April	1st to 10th	Night	18% 12% 4 8% 5% 4 4 3 4 6% 31%	1962
	11th to 20th	Night	20% 9% 3 6% 5% 4 4 3 5% 6% 36%	1687
	21st to end	Night	19% 10% 3 7% 5% 5% 4 5% 4 5% 33%	1710
May	1st to 10th	Night	14% 8% 4 7% 5% 5% 5% 4 6% 6% 36%	1704
	11th to 20th	Night	14% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	1447
	21st to end	Night	12% 6% 3 6% 5% 4 4 4 5% 8% 44%	1478
June	1st to 10th	Night	12% 6% 3 7% 6% 5% 5% 5% 7% 8% 37%	1260
	11th to 20th	Night	13% 6% 2 5% 4 4 5% 4 7% 8% 42%	1260
	21st to end	Night	13% 5% 3 7% 3 5% 3 4 7% 8% 43%	1260
July	1st to 10th	Night	8% 5% 2 4 5% 3 5% 4 7% 9% 48%	1260
	11th to 20th	Night	10% 6% 2 4 5% 5% 4 4 5% 8% 47%	1260
	21st to end	Night	8% 7% 2 6% 4 4 5% 5% 6% 8% 45%	1611
August	1st to 10th	Night	10% 6% 3 7% 5% 4 4 4 6% 6% 45%	1611
	11th to 20th	Night	12% 6% 3 6% 4 4 4 3 6% 7% 47%	1598
	21st to end	Night	14% 6% 2 7% 4 4 6% 4 5% 7% 42%	1782
September	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 6% 7% 46%	1781
	11th to 20th	Night	13% 6% 2 5% 3 5% 4 3 5% 7% 47%	1970
	21st to end	Night	18% 7% 3 6% 4 4 5% 2 4 8% 40%	1980
October	1st to 10th	Night	20% 8% 3 6% 4 4 4 4 5% 5% 37%	2149
	11th to 20th	Night	24% 10% 3 7% 5% 4 4 3 5% 6% 29%	2160
	21st to end	Night	25% 13% 4 8% 4 5% 5% 3 4 5% 24%	2479
November	1st to 10th	Night	17% 12% 3 7% 5% 4 4 4 5% 7% 31%	2520
	11th to 20th	Night	18% 14% 3 9% 7% 6% 4 3 6% 5% 26%	2660
	21st to end	Night	21% 17% 4 8% 6% 5% 5% 4 5% 5% 22%	2660
December	1st to 10th	Night	17% 19% 5% 10% 6% 4 3 3 4 6% 21%	2682
	11th to 20th	Night	18% 16% 4 10% 6% 4 4 3 4 5% 25%	3526
	21st to end	Night	22% 22% 5% 14% 5% 3 4 2 3 4 15%	5236

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	66% 5% 13% 13% 3	2209
	11th to 20th	Day	67% 4 12% 11% 4 1	2370
	21st to end	Day	72% 5% 8% 10% 5%	3005
February	1st to 10th	Day	59% 5% 12% 17% 6% 1	1710
	11th to 20th	Day	57% 5% 10% 15% 11% 1	1672
	21st to end	Day	59% 5% 8% 17% 9% 1	1489
March	1st to 10th	Day	55% 4 14% 16% 10% 1	3312
	11th to 20th	Day	55% 5% 11% 20% 7% 1	3480
	21st to end	Day	52% 4 11% 19% 11% 2	3815
April	1st to 10th	Day	53% 5% 13% 19% 9% 1	2232
	11th to 20th	Day	51% 4 8% 24% 12% 1	2442
	21st to end	Day	52% 5% 8% 19% 14% 3	2470
May	1st to 10th	Day	48% 5% 11% 21% 13% 3	2475
	11th to 20th	Day	47% 6% 9% 21% 16% 2	2538
	21st to end	Day	41% 5% 15% 23% 13% 2	2792
June	1st to 10th	Day	47% 7% 11% 22% 12% 1	2700
	11th to 20th	Day	43% 6% 11% 22% 17% 1	2700
	21st to end	Day	48% 7% 11% 22% 10% 2	2700
July	1st to 10th	Day	36% 8% 13% 26% 16% 1	2626
	11th to 20th	Day	41% 7% 11% 27% 11% 1	2619
	21st to end	Day	42% 8% 13% 26% 11%	2554
August	1st to 10th	Day	44% 8% 16% 22% 9% 1	2314
	11th to 20th	Day	40% 8% 17% 26% 8% 1	2303
	21st to end	Day	43% 7% 15% 27% 8% 1	2574
September	1st to 10th	Day	44% 6% 15% 25% 10% 1	2160
	11th to 20th	Day	40% 7% 16% 27% 9%	1986
	21st to end	Day	50% 6% 15% 23% 6%	1980
October	1st to 10th	Day	50% 6% 12% 20% 11%	1810
	11th to 20th	Day	61% 7% 10% 15% 7%	1800
	21st to end	Day	64% 6% 12% 11% 7%	1876
November	1st to 10th	Day	55% 6% 14% 18% 7%	1558
	11th to 20th	Day	61% 5% 12% 16% 6%	1520
	21st to end	Day	71% 5% 9% 10% 5%	1520
December	1st to 10th	Day	66% 6% 9% 14% 5%	1497
	11th to 20th	Day	63% 7% 7% 18% 4 1	1671
	21st to end	Day	76% 4 6% 12% 3	2431
January	1st to 10th	Night	62% 7% 13% 12% 6%	4748
	11th to 20th	Night	60% 7% 14% 13% 5%	4596
	21st to end	Night	69% 5% 8% 13% 5%	4652
February	1st to 10th	Night	60% 5% 10% 18% 6% 1	2470
	11th to 20th	Night	58% 6% 11% 18% 7% 1	2405
	21st to end	Night	59% 5% 11% 15% 10% 1	1964
March	1st to 10th	Night	50% 6% 11% 19% 13% 1	3527
	11th to 20th	Night	51% 6% 13% 18% 10% 2	3480
	21st to end	Night	48% 7% 10% 19% 13% 3	3817
April	1st to 10th	Night	54% 4 9% 19% 13% 1	1962
	11th to 20th	Night	48% 5% 6% 23% 16% 2	1687
	21st to end	Night	52% 5% 10% 16% 16% 2	1710
May	1st to 10th	Night	47% 6% 11% 21% 12% 3	1704
	11th to 20th	Night	44% 6% 8% 24% 15% 2	1448
	21st to end	Night	38% 6% 13% 23% 17% 3	1479
June	1st to 10th	Night	41% 8% 12% 23% 13% 2	1260
	11th to 20th	Night	38% 6% 13% 21% 19% 2	1260
	21st to end	Night	38% 8% 15% 22% 14% 2	1260
July	1st to 10th	Night	28% 8% 15% 30% 16% 3	1225
	11th to 20th	Night	33% 6% 15% 30% 14% 1	1218
	21st to end	Night	35% 7% 18% 27% 12%	1522
August	1st to 10th	Night	38% 7% 19% 23% 12% 1	1600
	11th to 20th	Night	36% 8% 17% 27% 11% 2	1598
	21st to end	Night	41% 7% 15% 25% 11% 1	1782
September	1st to 10th	Night	37% 7% 14% 30% 11% 3	1781
	11th to 20th	Night	36% 7% 15% 33% 9%	1970
	21st to end	Night	44% 6% 15% 27% 7%	1980
October	1st to 10th	Night	49% 5% 13% 21% 12%	2149
	11th to 20th	Night	56% 6% 11% 22% 6%	2160
	21st to end	Night	62% 5% 10% 15% 7% 1	2479
November	1st to 10th	Night	50% 7% 14% 21% 8%	2520
	11th to 20th	Night	59% 6% 12% 16% 7% 1	2660
	21st to end	Night	63% 7% 12% 13% 6%	2660
December	1st to 10th	Night	65% 6% 10% 16% 4	2682
	11th to 20th	Night	61% 6% 10% 15% 8% 1	3525
	21st to end	Night	74% 4 8% 10% 4	5236

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	26% 19% 27% 21% 6% 1	180
	11th to 20th	Day	17% 10% 36% 28% 7% 2	225
	21st to end	Day	26% 7% 31% 23% 10% 3	297
February	1st to 10th	Day	23% 20% 33% 18% 5%	270
	11th to 20th	Day	21% 15% 38% 21% 4	270
	21st to end	Day	23% 25% 37% 14% 1	225
March	1st to 10th	Day	21% 15% 37% 21% 6%	384
	11th to 20th	Day	19% 15% 32% 25% 8% 1	460
	21st to end	Day	23% 13% 34% 23% 6% 1	506
April	1st to 10th	Day	24% 11% 30% 24% 10% 1	460
	11th to 20th	Day	31% 16% 23% 16% 10% 4	452
	21st to end	Day	32% 14% 24% 19% 9% 3	460
May	1st to 10th	Day	21% 10% 31% 23% 12% 3	463
	11th to 20th	Day	22% 14% 29% 20% 11% 4	463
	21st to end	Day	21% 13% 31% 22% 11% 3	552
June	1st to 10th	Day	18% 15% 36% 24% 6% 1	630
	11th to 20th	Day	18% 15% 33% 23% 8% 2	630
	21st to end	Day	20% 11% 33% 25% 9% 2	630
July	1st to 10th	Day	11% 11% 33% 29% 13% 3	621
	11th to 20th	Day	14% 12% 32% 26% 12% 5%	618
	21st to end	Day	13% 7% 29% 31% 13% 6%	576
August	1st to 10th	Day	17% 7% 23% 26% 19% 7%	446
	11th to 20th	Day	13% 7% 22% 27% 20% 11%	439
	21st to end	Day	18% 6% 29% 24% 15% 8%	495
September	1st to 10th	Day	16% 11% 27% 26% 13% 6%	450
	11th to 20th	Day	15% 6% 28% 31% 14% 5%	450
	21st to end	Day	26% 12% 24% 23% 12% 4	450
October	1st to 10th	Day	23% 15% 24% 23% 12% 3	450
	11th to 20th	Day	27% 16% 32% 18% 5% 1	450
	21st to end	Day	30% 16% 31% 18% 3 1	391
November	1st to 10th	Day	29% 12% 38% 18% 4	216
	11th to 20th	Day	20% 19% 34% 19% 6% 1	180
	21st to end	Day	28% 18% 30% 16% 8% 1	180
December	1st to 10th	Day	24% 13% 31% 23% 9%	180
	11th to 20th	Day	34% 13% 34% 13% 6%	180
	21st to end	Day	32% 12% 37% 14% 6%	198
January	1st to 10th	Night	23% 12% 32% 25% 7% 1	950
	11th to 20th	Night	25% 13% 29% 25% 8%	950
	21st to end	Night	21% 8% 29% 30% 10% 1	904
February	1st to 10th	Night	19% 14% 37% 25% 4	760
	11th to 20th	Night	20% 15% 35% 24% 6% 1	743
	21st to end	Night	21% 16% 35% 24% 4	628
March	1st to 10th	Night	20% 14% 36% 24% 6%	760
	11th to 20th	Night	22% 13% 33% 24% 8% 1	760
	21st to end	Night	20% 10% 36% 26% 7% 1	836
April	1st to 10th	Night	24% 12% 31% 28% 5%	746
	11th to 20th	Night	23% 12% 29% 27% 8% 1	563
	21st to end	Night	24% 10% 29% 28% 9%	570
May	1st to 10th	Night	17% 8% 32% 34% 9% 1	564
	11th to 20th	Night	16% 9% 31% 35% 8% 1	362
	21st to end	Night	13% 8% 39% 30% 9% 1	389
June	1st to 10th	Night	15% 5% 39% 32% 9%	360
	11th to 20th	Night	17% 9% 29% 36% 9%	360
	21st to end	Night	15% 6% 29% 35% 15% 1	360
July	1st to 10th	Night	8% 5% 35% 43% 9% 1	350
	11th to 20th	Night	11% 4 32% 40% 14%	348
	21st to end	Night	11% 4 29% 40% 15% 1	519
August	1st to 10th	Night	11% 4 26% 38% 19% 1	537
	11th to 20th	Night	14% 4 23% 34% 22% 3	534
	21st to end	Night	17% 4 26% 32% 19% 1	594
September	1st to 10th	Night	16% 5% 28% 30% 20% 1	701
	11th to 20th	Night	16% 8% 27% 30% 17% 3	720
	21st to end	Night	21% 6% 26% 28% 16% 3	720
October	1st to 10th	Night	25% 9% 27% 28% 11% 1	720
	11th to 20th	Night	32% 10% 28% 22% 7% 1	720
	21st to end	Night	32% 12% 27% 24% 4 1	792
November	1st to 10th	Night	25% 12% 31% 25% 7% 1	740
	11th to 20th	Night	26% 13% 29% 25% 7%	760
	21st to end	Night	30% 12% 28% 24% 7%	760
December	1st to 10th	Night	29% 13% 31% 21% 6%	782
	11th to 20th	Night	27% 15% 31% 21% 7% 1	950
	21st to end	Night	30% 13% 28% 22% 7%	1045

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