Dorion
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dorion clear sky chart from 2005-11-06 to 2025-02-25.

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	13% 9% 4 8% 5% 5% 5% 4 6% 7% 34%	108343
	Night	12% 11% 4 9% 6% 5% 5% 4 6% 7% 32%	110529
Transparency	Day	47% 8% 14% 21% 10% 1	111795
	Night	50% 7% 13% 20% 9% 1	115239
Seeing	Day	19% 16% 35% 21% 8% 2	16215
	Night	18% 11% 31% 29% 10% 1	26055

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	8% 9% 5% 11% 6% 6% 6% 5% 6% 9% 29%	3617
	11th to 20th	Day	7% 10% 5% 13% 7% 6% 6% 6% 10% 9% 22%	3589
	21st to end	Day	8% 15% 10% 12% 5% 6% 6% 5% 6% 7% 20%	3982
February	1st to 10th	Day	7% 11% 4 11% 6% 5% 6% 5% 5% 7% 33%	1647
	11th to 20th	Day	7% 11% 5% 10% 7% 6% 5% 5% 5% 7% 33%	1793
	21st to end	Day	6% 12% 5% 10% 7% 7% 5% 4 6% 7% 32%	1610
March	1st to 10th	Day	10% 9% 3 7% 5% 5% 4 4 5% 6% 40%	3118
	11th to 20th	Day	7% 10% 6% 12% 6% 6% 6% 4 5% 6% 32%	3287
	21st to end	Day	16% 13% 4 10% 6% 5% 4 3 4 5% 31%	4165
April	1st to 10th	Day	13% 13% 5% 8% 5% 4 5% 4 5% 7% 32%	2280
	11th to 20th	Day	17% 11% 4 9% 6% 5% 4 3 6% 7% 30%	2286
	21st to end	Day	16% 12% 3 7% 6% 4 4 4 5% 5% 34%	2521
May	1st to 10th	Day	17% 10% 3 7% 4 4 4 3 5% 7% 37%	4428
	11th to 20th	Day	13% 8% 3 7% 5% 4 4 4 6% 7% 38%	4399
	21st to end	Day	15% 8% 2 6% 5% 4 4 4 6% 6% 39%	4837
June	1st to 10th	Day	18% 7% 2 6% 5% 4 4 4 6% 7% 37%	2840
	11th to 20th	Day	13% 7% 2 6% 5% 5% 6% 3 6% 8% 38%	2850
	21st to end	Day	17% 6% 2 6% 5% 5% 5% 4 6% 8% 36%	2850
July	1st to 10th	Day	10% 4 2 6% 5% 5% 6% 5% 8% 11% 39%	4453
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 8% 9% 44%	4440
	21st to end	Day	12% 6% 2 6% 4 4 4 4 7% 10% 42%	4814
August	1st to 10th	Day	10% 5% 3 8% 6% 6% 6% 5% 8% 9% 34%	4423
	11th to 20th	Day	12% 4 1 4 4 4 4 4 7% 8% 47%	4302
	21st to end	Day	11% 6% 3 7% 4 5% 5% 5% 6% 9% 40%	4194
September	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 3 6% 7% 41%	2280
	11th to 20th	Day	16% 6% 3 7% 4 4 4 3 6% 9% 36%	2207
	21st to end	Day	17% 8% 3 8% 5% 4 4 4 5% 8% 34%	2090
October	1st to 10th	Day	19% 11% 3 7% 5% 4 4 3 5% 6% 31%	1938
	11th to 20th	Day	18% 12% 4 9% 6% 5% 5% 4 6% 7% 23%	1805
	21st to end	Day	20% 13% 4 9% 6% 6% 5% 4 4 5% 25%	1881
November	1st to 10th	Day	14% 12% 4 10% 8% 6% 5% 4 6% 7% 25%	1753
	11th to 20th	Day	18% 17% 5% 11% 5% 5% 5% 3 5% 5% 22%	1600
	21st to end	Day	15% 12% 5% 12% 7% 6% 6% 4 6% 6% 20%	1489
December	1st to 10th	Day	12% 12% 5% 13% 8% 6% 6% 4 6% 6% 23%	2280
	11th to 20th	Day	8% 14% 6% 11% 7% 6% 5% 4 6% 6% 27%	2621
	21st to end	Day	17% 20% 5% 10% 6% 4 3 3 4 6% 21%	3674
January	1st to 10th	Night	7% 12% 5% 11% 6% 6% 6% 5% 6% 7% 29%	6348
	11th to 20th	Night	5% 11% 5% 15% 10% 8% 6% 5% 7% 7% 20%	6316
	21st to end	Night	7% 11% 7% 14% 7% 6% 6% 5% 7% 7% 24%	6996
February	1st to 10th	Night	8% 9% 5% 10% 6% 5% 7% 6% 7% 7% 32%	2750
	11th to 20th	Night	9% 9% 3 9% 7% 7% 7% 5% 7% 8% 30%	2493
	21st to end	Night	7% 12% 5% 11% 6% 7% 5% 4 5% 6% 30%	1939
March	1st to 10th	Night	11% 10% 4 9% 5% 5% 5% 4 6% 7% 34%	3722
	11th to 20th	Night	9% 15% 4 9% 6% 5% 5% 3 5% 7% 33%	3699
	21st to end	Night	16% 12% 4 7% 5% 6% 4 3 5% 5% 33%	3466
April	1st to 10th	Night	14% 12% 3 8% 5% 6% 5% 4 5% 7% 31%	1900
	11th to 20th	Night	17% 10% 3 8% 5% 4 4 4 6% 8% 32%	1843
	21st to end	Night	16% 11% 3 7% 6% 4 3 3 5% 7% 36%	1657
May	1st to 10th	Night	17% 9% 2 4 3 4 3 4 5% 6% 42%	2523
	11th to 20th	Night	14% 7% 2 7% 5% 5% 5% 4 6% 6% 38%	2506
	21st to end	Night	21% 7% 2 5% 3 4 4 3 5% 5% 42%	2750
June	1st to 10th	Night	16% 9% 2 7% 5% 5% 4 4 5% 7% 36%	1339
	11th to 20th	Night	13% 8% 2 7% 5% 4 5% 3 6% 7% 40%	1330
	21st to end	Night	17% 5% 2 5% 5% 4 6% 3 7% 9% 36%	1330
July	1st to 10th	Night	8% 6% 2 7% 5% 5% 4 4 6% 9% 43%	2512
	11th to 20th	Night	7% 4 2 6% 4 4 5% 5% 5% 9% 49%	2530
	21st to end	Night	10% 5% 2 5% 3 4 5% 4 6% 10% 46%	2750
August	1st to 10th	Night	12% 6% 2 5% 5% 4 5% 4 6% 10% 41%	2517
	11th to 20th	Night	12% 4 2 4 4 4 4 4 6% 9% 48%	2534
	21st to end	Night	15% 6% 3 6% 4 3 4 3 5% 6% 45%	3461
September	1st to 10th	Night	13% 5% 2 6% 3 4 5% 4 6% 7% 47%	1900
	11th to 20th	Night	15% 6% 2 5% 4 4 4 4 7% 8% 42%	1953
	21st to end	Night	18% 7% 3 7% 4 4 3 3 6% 7% 38%	2090
October	1st to 10th	Night	18% 7% 2 7% 4 4 5% 4 5% 6% 35%	2238
	11th to 20th	Night	16% 11% 3 8% 5% 5% 5% 4 6% 7% 31%	2375
	21st to end	Night	19% 12% 3 9% 5% 5% 5% 4 5% 6% 27%	2717
November	1st to 10th	Night	14% 12% 3 9% 6% 5% 5% 5% 6% 6% 28%	2541
	11th to 20th	Night	17% 14% 5% 12% 6% 5% 5% 4 5% 5% 23%	2800
	21st to end	Night	15% 14% 5% 12% 7% 5% 5% 5% 5% 6% 22%	2908
December	1st to 10th	Night	11% 16% 5% 12% 7% 5% 6% 4 6% 7% 20%	4910
	11th to 20th	Night	8% 13% 6% 14% 6% 5% 4 4 5% 6% 29%	5586
	21st to end	Night	19% 20% 6% 10% 6% 4 4 3 4 5% 20%	7300

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	49% 9% 15% 19% 7% 1	3618
	11th to 20th	Day	52% 7% 11% 24% 5%	3620
	21st to end	Day	61% 7% 10% 12% 9%	3982
February	1st to 10th	Day	47% 8% 14% 20% 11% 1	1647
	11th to 20th	Day	48% 6% 15% 21% 10% 1	1802
	21st to end	Day	50% 5% 12% 17% 14% 1	1610
March	1st to 10th	Day	44% 6% 13% 21% 15% 1	3120
	11th to 20th	Day	50% 7% 13% 20% 9% 1	3287
	21st to end	Day	54% 6% 10% 13% 15% 2	4165
April	1st to 10th	Day	51% 6% 11% 19% 11% 2	2280
	11th to 20th	Day	52% 7% 12% 21% 8% 1	2286
	21st to end	Day	49% 6% 10% 17% 15% 2	2521
May	1st to 10th	Day	45% 7% 11% 20% 16% 1	4440
	11th to 20th	Day	41% 8% 13% 24% 12% 2	4403
	21st to end	Day	42% 9% 15% 16% 16% 2	4837
June	1st to 10th	Day	45% 8% 14% 19% 13% 1	2840
	11th to 20th	Day	41% 8% 13% 25% 11% 1	2850
	21st to end	Day	45% 8% 15% 23% 9% 1	2850
July	1st to 10th	Day	35% 11% 20% 23% 11% 1	4384
	11th to 20th	Day	32% 8% 19% 32% 7%	4365
	21st to end	Day	37% 8% 15% 32% 8% 1	4662
August	1st to 10th	Day	40% 10% 16% 26% 7% 1	4408
	11th to 20th	Day	33% 8% 19% 29% 11%	4344
	21st to end	Day	39% 8% 14% 28% 10%	4193
September	1st to 10th	Day	40% 8% 16% 26% 10%	2280
	11th to 20th	Day	44% 9% 20% 22% 4	2207
	21st to end	Day	48% 8% 18% 19% 6% 1	2090
October	1st to 10th	Day	44% 6% 16% 20% 13% 1	3224
	11th to 20th	Day	52% 6% 14% 24% 3	2982
	21st to end	Day	57% 7% 16% 17% 3	3081
November	1st to 10th	Day	57% 8% 14% 15% 5% 1	1753
	11th to 20th	Day	64% 6% 13% 13% 4	1600
	21st to end	Day	64% 9% 13% 9% 5%	1489
December	1st to 10th	Day	60% 10% 16% 11% 3	2280
	11th to 20th	Day	56% 6% 13% 14% 11%	2621
	21st to end	Day	64% 6% 9% 17% 4	3674
January	1st to 10th	Night	52% 8% 14% 18% 7% 1	6348
	11th to 20th	Night	62% 6% 9% 18% 5%	6360
	21st to end	Night	57% 8% 10% 17% 8%	6996
February	1st to 10th	Night	48% 8% 12% 19% 11% 1	2750
	11th to 20th	Night	50% 6% 14% 19% 10% 1	2494
	21st to end	Night	53% 6% 11% 20% 9% 1	1939
March	1st to 10th	Night	48% 7% 13% 18% 13% 2	3733
	11th to 20th	Night	50% 7% 13% 20% 9% 1	3700
	21st to end	Night	52% 6% 9% 17% 14% 2	3466
April	1st to 10th	Night	51% 6% 10% 21% 12%	1900
	11th to 20th	Night	49% 6% 11% 21% 12% 2	1843
	21st to end	Night	46% 5% 12% 21% 13% 3	1657
May	1st to 10th	Night	41% 7% 14% 24% 13% 1	2530
	11th to 20th	Night	43% 8% 12% 25% 9% 3	2507
	21st to end	Night	44% 7% 10% 21% 17% 1	2751
June	1st to 10th	Night	46% 7% 12% 20% 13% 2	1339
	11th to 20th	Night	42% 8% 11% 22% 15% 1	1330
	21st to end	Night	43% 11% 15% 21% 9% 1	1330
July	1st to 10th	Night	38% 9% 18% 22% 13% 1	2472
	11th to 20th	Night	28% 11% 20% 32% 8% 1	2482
	21st to end	Night	32% 8% 14% 33% 11%	2662
August	1st to 10th	Night	38% 8% 17% 30% 7% 1	2508
	11th to 20th	Night	32% 7% 20% 32% 9%	2558
	21st to end	Night	41% 8% 16% 25% 11%	3461
September	1st to 10th	Night	36% 8% 16% 28% 12%	1900
	11th to 20th	Night	40% 10% 20% 26% 4	1953
	21st to end	Night	47% 7% 17% 23% 6%	2090
October	1st to 10th	Night	37% 6% 16% 26% 15%	3732
	11th to 20th	Night	47% 7% 13% 24% 8%	3937
	21st to end	Night	57% 8% 16% 14% 5%	4467
November	1st to 10th	Night	56% 8% 13% 18% 4 1	2541
	11th to 20th	Night	62% 7% 14% 13% 4	2800
	21st to end	Night	63% 8% 14% 11% 4	2908
December	1st to 10th	Night	61% 7% 17% 12% 3	4910
	11th to 20th	Night	55% 6% 12% 19% 8%	5585
	21st to end	Night	67% 7% 8% 15% 4	7300

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	16% 12% 45% 20% 6%	300
	11th to 20th	Day	15% 14% 38% 25% 7% 1	300
	21st to end	Day	18% 13% 44% 18% 5% 2	330
February	1st to 10th	Day	15% 17% 46% 15% 6% 1	300
	11th to 20th	Day	12% 20% 44% 20% 3	300
	21st to end	Day	14% 31% 39% 14% 3	237
March	1st to 10th	Day	16% 21% 39% 20% 3 1	270
	11th to 20th	Day	15% 26% 40% 12% 6% 1	274
	21st to end	Day	18% 15% 39% 21% 6% 2	506
April	1st to 10th	Day	20% 21% 30% 22% 6%	460
	11th to 20th	Day	26% 20% 25% 19% 9% 2	452
	21st to end	Day	23% 19% 29% 18% 8% 3	511
May	1st to 10th	Day	21% 20% 35% 17% 7% 2	653
	11th to 20th	Day	18% 18% 37% 18% 6% 2	675
	21st to end	Day	22% 16% 35% 21% 6% 1	734
June	1st to 10th	Day	20% 14% 40% 19% 7% 2	680
	11th to 20th	Day	18% 16% 39% 19% 7% 2	680
	21st to end	Day	19% 14% 34% 23% 8% 1	680
July	1st to 10th	Day	10% 12% 38% 27% 11% 3	671
	11th to 20th	Day	12% 12% 39% 24% 10% 3	668
	21st to end	Day	10% 11% 35% 26% 13% 5%	739
August	1st to 10th	Day	12% 13% 37% 26% 10% 2	676
	11th to 20th	Day	16% 8% 31% 29% 12% 4	637
	21st to end	Day	18% 13% 33% 22% 11% 3	539
September	1st to 10th	Day	16% 12% 31% 27% 13% 3	490
	11th to 20th	Day	20% 12% 32% 22% 12% 2	490
	21st to end	Day	22% 13% 32% 19% 11% 2	490
October	1st to 10th	Day	30% 18% 27% 17% 7% 1	338
	11th to 20th	Day	22% 21% 34% 20% 3	300
	21st to end	Day	32% 23% 33% 10% 3	330
November	1st to 10th	Day	23% 21% 33% 17% 5%	300
	11th to 20th	Day	32% 21% 33% 10% 5%	300
	21st to end	Day	26% 19% 34% 15% 5%	244
December	1st to 10th	Day	23% 14% 35% 21% 6% 3	200
	11th to 20th	Day	23% 20% 34% 19% 5% 1	200
	21st to end	Day	25% 16% 31% 21% 5% 1	261
January	1st to 10th	Night	16% 12% 40% 26% 6% 1	1000
	11th to 20th	Night	16% 13% 36% 26% 8% 1	1000
	21st to end	Night	17% 8% 38% 27% 9% 1	1100
February	1st to 10th	Night	14% 12% 36% 29% 8% 1	1000
	11th to 20th	Night	15% 12% 38% 26% 8% 1	939
	21st to end	Night	13% 12% 39% 28% 7% 1	647
March	1st to 10th	Night	14% 12% 36% 28% 9% 1	760
	11th to 20th	Night	18% 17% 30% 28% 6% 1	717
	21st to end	Night	16% 9% 33% 28% 11% 2	627
April	1st to 10th	Night	19% 13% 27% 32% 7% 1	570
	11th to 20th	Night	22% 11% 27% 28% 11% 1	563
	21st to end	Night	21% 6% 28% 31% 13% 1	570
May	1st to 10th	Night	21% 10% 28% 31% 10% 1	570
	11th to 20th	Night	17% 9% 29% 34% 9% 1	561
	21st to end	Night	23% 9% 22% 34% 12%	617
June	1st to 10th	Night	17% 7% 31% 33% 11%	389
	11th to 20th	Night	16% 7% 30% 39% 8% 1	380
	21st to end	Night	21% 5% 29% 36% 10%	380
July	1st to 10th	Night	12% 4 30% 39% 14% 1	537
	11th to 20th	Night	11% 5% 28% 42% 13% 1	552
	21st to end	Night	9% 4 26% 41% 20% 1	624
August	1st to 10th	Night	12% 5% 28% 39% 16% 1	567
	11th to 20th	Night	14% 6% 21% 34% 23% 2	564
	21st to end	Night	18% 7% 28% 28% 17% 2	627
September	1st to 10th	Night	16% 6% 24% 35% 18% 2	570
	11th to 20th	Night	18% 9% 30% 30% 12% 1	623
	21st to end	Night	22% 9% 27% 29% 12% 1	760
October	1st to 10th	Night	24% 13% 30% 23% 10% 1	760
	11th to 20th	Night	22% 14% 31% 23% 9% 1	760
	21st to end	Night	27% 10% 33% 23% 7%	836
November	1st to 10th	Night	22% 14% 33% 24% 6% 1	785
	11th to 20th	Night	26% 16% 30% 21% 7% 1	1000
	21st to end	Night	24% 17% 31% 21% 6%	1000
December	1st to 10th	Night	18% 16% 34% 25% 7% 1	1000
	11th to 20th	Night	20% 13% 31% 28% 8% 1	1000
	21st to end	Night	23% 13% 30% 27% 6% 1	1100

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