ObservÉtoiles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ObservÉtoiles clear sky chart from 2005-11-06 to 2025-01-07.

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	18% 13% 4 9% 5% 5% 4 3 5% 6% 26%	105101
	Night	18% 15% 5% 11% 6% 4 4 3 4 5% 25%	103940
Transparency	Day	56% 7% 12% 17% 8% 1	108587
	Night	59% 7% 10% 16% 8% 1	108543
Seeing	Day	26% 18% 28% 17% 8% 2	14829
	Night	24% 16% 27% 23% 8% 1	23829

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	14% 22% 9% 17% 7% 5% 4 2 4 4 13%	2565
	11th to 20th	Day	10% 18% 8% 21% 11% 6% 5% 3 4 4 11%	2520
	21st to end	Day	12% 23% 9% 14% 5% 8% 5% 3 4 5% 13%	2772
February	1st to 10th	Day	14% 17% 8% 15% 10% 6% 4 3 4 4 17%	1623
	11th to 20th	Day	12% 19% 9% 15% 8% 5% 5% 3 4 4 16%	1829
	21st to end	Day	12% 18% 6% 12% 9% 6% 4 4 5% 5% 19%	1570
March	1st to 10th	Day	20% 17% 4 10% 7% 5% 3 3 4 5% 20%	3116
	11th to 20th	Day	13% 22% 7% 13% 6% 4 4 2 4 4 21%	3242
	21st to end	Day	17% 14% 5% 12% 5% 3 3 2 3 4 32%	4166
April	1st to 10th	Day	20% 20% 4 9% 5% 3 3 3 4 4 25%	2280
	11th to 20th	Day	20% 15% 3 8% 4 4 4 3 4 5% 31%	2259
	21st to end	Day	24% 13% 3 8% 5% 4 4 3 4 5% 27%	2470
May	1st to 10th	Day	25% 12% 3 6% 5% 5% 4 4 5% 6% 26%	4219
	11th to 20th	Day	20% 9% 3 9% 6% 6% 5% 4 6% 6% 26%	4398
	21st to end	Day	15% 8% 3 7% 5% 4 5% 4 7% 9% 32%	4837
June	1st to 10th	Day	28% 8% 2 6% 5% 4 5% 4 6% 7% 26%	2660
	11th to 20th	Day	19% 7% 3 6% 5% 5% 4 4 6% 8% 33%	2660
	21st to end	Day	25% 8% 3 7% 5% 5% 5% 4 5% 7% 27%	2660
July	1st to 10th	Day	16% 6% 3 7% 5% 6% 5% 5% 7% 9% 32%	4440
	11th to 20th	Day	17% 7% 2 7% 5% 6% 6% 5% 7% 8% 30%	4440
	21st to end	Day	16% 7% 2 6% 5% 5% 5% 4 6% 8% 37%	4814
August	1st to 10th	Day	20% 8% 3 8% 4 5% 5% 4 6% 7% 28%	4423
	11th to 20th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 7% 29%	3987
	21st to end	Day	20% 7% 2 5% 3 4 4 3 6% 8% 37%	4180
September	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 5% 6% 40%	2280
	11th to 20th	Day	19% 6% 2 5% 3 3 3 2 5% 7% 44%	2152
	21st to end	Day	19% 7% 3 6% 4 3 4 4 7% 9% 35%	2090
October	1st to 10th	Day	24% 12% 2 7% 4 5% 4 4 5% 6% 28%	2036
	11th to 20th	Day	22% 13% 3 7% 5% 5% 4 4 5% 5% 28%	1815
	21st to end	Day	25% 20% 4 7% 3 4 4 2 3 4 23%	1881
November	1st to 10th	Day	19% 18% 4 8% 4 3 4 3 3 6% 27%	1761
	11th to 20th	Day	17% 20% 6% 10% 5% 4 3 3 4 5% 22%	1800
	21st to end	Day	18% 20% 7% 12% 5% 5% 3 3 5% 4 18%	1610
December	1st to 10th	Day	14% 24% 7% 21% 9% 4 4 2 3 3 10%	2600
	11th to 20th	Day	17% 20% 8% 16% 4 4 3 2 3 4 19%	2986
	21st to end	Day	17% 19% 6% 14% 6% 5% 6% 3 3 4 18%	3960
January	1st to 10th	Night	13% 21% 7% 20% 8% 6% 4 3 4 4 11%	4514
	11th to 20th	Night	10% 17% 8% 20% 11% 6% 5% 3 3 4 12%	4440
	21st to end	Night	14% 21% 8% 13% 8% 6% 4 2 3 4 17%	4884
February	1st to 10th	Night	13% 20% 6% 17% 8% 6% 4 3 3 4 16%	2556
	11th to 20th	Night	12% 17% 7% 15% 8% 6% 4 4 3 3 19%	2237
	21st to end	Night	12% 19% 6% 12% 7% 6% 4 3 4 4 23%	1884
March	1st to 10th	Night	20% 15% 5% 9% 5% 5% 3 3 4 5% 26%	3724
	11th to 20th	Night	14% 22% 6% 12% 6% 5% 3 2 3 3 24%	3698
	21st to end	Night	14% 20% 6% 11% 5% 4 3 2 3 3 29%	3466
April	1st to 10th	Night	21% 15% 4 10% 5% 3 4 2 4 5% 26%	1900
	11th to 20th	Night	22% 13% 3 6% 5% 4 4 2 4 6% 30%	1868
	21st to end	Night	22% 13% 3 7% 4 4 3 3 4 5% 31%	1710
May	1st to 10th	Night	24% 13% 4 8% 4 4 4 3 5% 6% 25%	2751
	11th to 20th	Night	20% 10% 4 8% 5% 4 4 3 5% 6% 32%	2495
	21st to end	Night	17% 6% 3 7% 4 5% 5% 3 6% 8% 36%	2738
June	1st to 10th	Night	28% 7% 3 6% 4 4 4 4 5% 7% 29%	1520
	11th to 20th	Night	20% 7% 3 5% 4 4 4 4 5% 8% 36%	1520
	21st to end	Night	21% 8% 2 6% 5% 5% 5% 5% 8% 7% 29%	1520
July	1st to 10th	Night	17% 8% 3 7% 6% 5% 5% 4 6% 8% 32%	2520
	11th to 20th	Night	20% 8% 3 6% 4 4 4 4 6% 7% 35%	2520
	21st to end	Night	18% 7% 3 6% 5% 5% 5% 3 5% 6% 37%	2739
August	1st to 10th	Night	24% 8% 3 7% 3 4 4 3 5% 6% 32%	2506
	11th to 20th	Night	19% 8% 3 8% 6% 5% 5% 5% 5% 5% 32%	2806
	21st to end	Night	17% 7% 2 6% 4 4 5% 3 5% 6% 41%	3476
September	1st to 10th	Night	18% 7% 2 7% 4 4 4 3 5% 5% 41%	1900
	11th to 20th	Night	17% 6% 2 4 3 4 3 4 6% 7% 46%	2028
	21st to end	Night	21% 6% 2 5% 4 3 4 3 5% 6% 41%	2090
October	1st to 10th	Night	21% 9% 3 7% 4 4 4 4 5% 6% 33%	2142
	11th to 20th	Night	22% 13% 3 8% 4 4 4 4 4 4 29%	2365
	21st to end	Night	26% 15% 3 7% 4 4 4 2 5% 6% 24%	2717
November	1st to 10th	Night	20% 16% 4 8% 4 4 4 4 4 5% 26%	2537
	11th to 20th	Night	18% 17% 6% 10% 5% 4 3 3 3 5% 26%	2600
	21st to end	Night	18% 19% 7% 13% 6% 4 4 3 4 4 20%	2785
December	1st to 10th	Night	15% 24% 11% 20% 6% 4 3 2 3 3 9%	4580
	11th to 20th	Night	19% 23% 7% 16% 6% 4 4 2 2 2 15%	5208
	21st to end	Night	17% 20% 6% 14% 6% 5% 5% 2 3 3 20%	6996

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	74% 7% 10% 8% 1	2566
	11th to 20th	Day	74% 7% 8% 8% 3	2520
	21st to end	Day	73% 6% 9% 8% 4 1	2772
February	1st to 10th	Day	72% 5% 6% 11% 4 1	1623
	11th to 20th	Day	71% 6% 9% 10% 4 1	1838
	21st to end	Day	66% 5% 10% 11% 7% 2	1570
March	1st to 10th	Day	65% 7% 8% 11% 7% 1	3119
	11th to 20th	Day	65% 6% 10% 10% 8% 1	3242
	21st to end	Day	57% 4 7% 16% 15% 1	4166
April	1st to 10th	Day	63% 5% 8% 16% 8% 1	2280
	11th to 20th	Day	55% 5% 8% 19% 12% 1	2259
	21st to end	Day	60% 6% 8% 16% 9% 3	2470
May	1st to 10th	Day	58% 6% 10% 18% 7% 1	4233
	11th to 20th	Day	53% 11% 14% 14% 7% 1	4402
	21st to end	Day	44% 10% 13% 23% 10% 1	4837
June	1st to 10th	Day	55% 7% 12% 17% 8% 1	2660
	11th to 20th	Day	47% 7% 13% 21% 11% 2	2660
	21st to end	Day	55% 8% 11% 17% 8% 1	2660
July	1st to 10th	Day	44% 7% 16% 24% 9%	4370
	11th to 20th	Day	45% 9% 15% 27% 4	4365
	21st to end	Day	44% 8% 12% 29% 7%	4661
August	1st to 10th	Day	52% 8% 15% 18% 6%	4409
	11th to 20th	Day	50% 8% 18% 19% 5%	4026
	21st to end	Day	44% 7% 15% 19% 13% 1	4180
September	1st to 10th	Day	41% 7% 13% 27% 11% 1	2280
	11th to 20th	Day	40% 4 13% 30% 12% 1	2152
	21st to end	Day	44% 7% 15% 22% 9% 1	2090
October	1st to 10th	Day	48% 6% 15% 23% 8%	3387
	11th to 20th	Day	52% 5% 15% 20% 8%	2992
	21st to end	Day	53% 4 15% 20% 7% 1	3081
November	1st to 10th	Day	57% 7% 11% 17% 7%	1761
	11th to 20th	Day	64% 6% 10% 11% 7% 2	1800
	21st to end	Day	67% 7% 13% 10% 3	1610
December	1st to 10th	Day	81% 4 8% 6% 1	2600
	11th to 20th	Day	69% 3 7% 14% 6%	2986
	21st to end	Day	70% 5% 7% 10% 8% 1	3960
January	1st to 10th	Night	74% 8% 11% 5% 2	4514
	11th to 20th	Night	74% 8% 9% 7% 2	4440
	21st to end	Night	72% 6% 10% 8% 4 1	4884
February	1st to 10th	Night	71% 6% 7% 11% 5%	2556
	11th to 20th	Night	68% 6% 8% 12% 6%	2239
	21st to end	Night	63% 4 10% 13% 8% 1	1884
March	1st to 10th	Night	59% 7% 10% 15% 7% 2	3734
	11th to 20th	Night	63% 7% 8% 13% 9% 1	3699
	21st to end	Night	60% 5% 6% 16% 11% 2	3466
April	1st to 10th	Night	60% 6% 8% 15% 10% 1	1900
	11th to 20th	Night	55% 5% 9% 18% 13%	1868
	21st to end	Night	55% 6% 9% 17% 12% 2	1710
May	1st to 10th	Night	57% 7% 10% 16% 8% 1	2757
	11th to 20th	Night	50% 10% 13% 17% 9% 1	2496
	21st to end	Night	44% 9% 13% 19% 15% 1	2739
June	1st to 10th	Night	53% 6% 11% 21% 9% 1	1520
	11th to 20th	Night	45% 8% 13% 22% 10% 2	1520
	21st to end	Night	51% 10% 13% 17% 9% 1	1520
July	1st to 10th	Night	48% 8% 12% 21% 11%	2480
	11th to 20th	Night	43% 9% 14% 27% 7%	2472
	21st to end	Night	45% 8% 14% 23% 10%	2651
August	1st to 10th	Night	50% 7% 15% 23% 5%	2496
	11th to 20th	Night	48% 9% 18% 18% 6%	2833
	21st to end	Night	42% 7% 13% 24% 13% 1	3476
September	1st to 10th	Night	42% 8% 12% 26% 11% 1	1900
	11th to 20th	Night	37% 7% 16% 28% 11% 1	2028
	21st to end	Night	43% 7% 13% 24% 12% 1	2090
October	1st to 10th	Night	52% 6% 11% 23% 8%	3571
	11th to 20th	Night	50% 5% 12% 22% 11%	3927
	21st to end	Night	53% 5% 14% 18% 9% 1	4467
November	1st to 10th	Night	58% 6% 12% 17% 6% 1	2537
	11th to 20th	Night	60% 6% 9% 14% 9% 2	2600
	21st to end	Night	67% 6% 11% 12% 3	2785
December	1st to 10th	Night	80% 6% 7% 5% 1	4580
	11th to 20th	Night	76% 4 5% 9% 6%	5208
	21st to end	Night	70% 5% 6% 11% 8% 1	6996

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	28% 22% 31% 15% 4	192
	11th to 20th	Day	27% 31% 28% 9% 4 1	180
	21st to end	Day	25% 23% 34% 12% 6% 1	198
February	1st to 10th	Day	38% 29% 23% 8% 1	229
	11th to 20th	Day	30% 29% 32% 8% 2	437
	21st to end	Day	31% 23% 29% 15% 2	382
March	1st to 10th	Day	31% 21% 30% 15% 3	460
	11th to 20th	Day	28% 26% 27% 14% 4	460
	21st to end	Day	34% 23% 23% 13% 6% 1	506
April	1st to 10th	Day	35% 18% 27% 15% 5%	460
	11th to 20th	Day	34% 23% 23% 13% 6% 2	452
	21st to end	Day	35% 18% 29% 11% 5% 2	460
May	1st to 10th	Day	34% 20% 23% 14% 7% 2	463
	11th to 20th	Day	22% 18% 32% 18% 8% 2	487
	21st to end	Day	21% 20% 32% 21% 6% 1	529
June	1st to 10th	Day	31% 13% 32% 16% 7% 1	490
	11th to 20th	Day	21% 14% 33% 21% 9% 2	490
	21st to end	Day	26% 16% 33% 17% 7% 1	490
July	1st to 10th	Day	18% 10% 34% 26% 11% 1	485
	11th to 20th	Day	17% 12% 37% 25% 7% 2	484
	21st to end	Day	17% 12% 35% 26% 8% 1	532
August	1st to 10th	Day	17% 14% 26% 28% 13% 1	486
	11th to 20th	Day	21% 9% 29% 24% 14% 4	479
	21st to end	Day	19% 15% 26% 23% 11% 6%	539
September	1st to 10th	Day	20% 13% 29% 21% 13% 5%	490
	11th to 20th	Day	18% 11% 23% 21% 20% 7%	490
	21st to end	Day	19% 14% 27% 24% 12% 5%	490
October	1st to 10th	Day	25% 19% 25% 15% 13% 3	490
	11th to 20th	Day	29% 21% 22% 18% 9% 1	445
	21st to end	Day	36% 20% 25% 14% 6%	429
November	1st to 10th	Day	34% 19% 27% 15% 5% 1	395
	11th to 20th	Day	30% 24% 25% 15% 5% 2	400
	21st to end	Day	30% 25% 30% 15%	210
December	1st to 10th	Day	28% 26% 29% 12% 6%	200
	11th to 20th	Day	34% 26% 31% 8% 1 1	200
	21st to end	Day	32% 24% 28% 12% 3	220
January	1st to 10th	Night	25% 25% 29% 18% 2	785
	11th to 20th	Night	26% 24% 29% 16% 4	760
	21st to end	Night	19% 21% 33% 22% 5% 1	836
February	1st to 10th	Night	22% 24% 32% 18% 4	760
	11th to 20th	Night	21% 24% 33% 18% 3	743
	21st to end	Night	25% 21% 31% 21% 2	628
March	1st to 10th	Night	26% 19% 30% 21% 3	760
	11th to 20th	Night	28% 20% 26% 21% 6%	760
	21st to end	Night	26% 19% 26% 20% 7% 1	836
April	1st to 10th	Night	30% 17% 27% 22% 4	760
	11th to 20th	Night	29% 19% 24% 20% 8%	744
	21st to end	Night	30% 15% 30% 19% 6%	570
May	1st to 10th	Night	26% 13% 29% 22% 8% 1	526
	11th to 20th	Night	21% 14% 30% 26% 8% 1	374
	21st to end	Night	19% 13% 28% 31% 8% 1	411
June	1st to 10th	Night	29% 8% 26% 27% 9% 1	380
	11th to 20th	Night	20% 8% 26% 32% 13% 1	380
	21st to end	Night	20% 9% 25% 36% 9% 1	380
July	1st to 10th	Night	13% 5% 31% 37% 14%	370
	11th to 20th	Night	13% 4 26% 40% 16% 2	368
	21st to end	Night	13% 6% 25% 40% 16%	416
August	1st to 10th	Night	16% 6% 20% 37% 20% 1	378
	11th to 20th	Night	15% 8% 23% 34% 19% 1	572
	21st to end	Night	15% 7% 24% 33% 19% 2	836
September	1st to 10th	Night	18% 8% 27% 32% 14% 1	760
	11th to 20th	Night	19% 8% 19% 29% 23% 3	760
	21st to end	Night	21% 10% 27% 23% 16% 2	760
October	1st to 10th	Night	26% 13% 23% 24% 12% 1	760
	11th to 20th	Night	26% 15% 26% 22% 10% 1	760
	21st to end	Night	33% 16% 27% 20% 4	836
November	1st to 10th	Night	29% 17% 29% 19% 5% 1	780
	11th to 20th	Night	27% 20% 28% 16% 8% 1	800
	21st to end	Night	28% 24% 26% 16% 5% 1	800
December	1st to 10th	Night	31% 24% 27% 15% 4	800
	11th to 20th	Night	30% 22% 27% 17% 5% 1	800
	21st to end	Night	30% 22% 27% 18% 3	880

Page generated at 2025-01-04 03:23:58. Version ac07

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