BarC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BarC Observatory clear sky chart from 2005-11-06 to 2024-11-02.

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	10% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	100256
	Night	10% 6% 2 6% 4 4 4 4 6% 9% 45%	98652
Transparency	Day	19% 50% 9% 11% 8% 3	100085
	Night	19% 50% 8% 11% 9% 3	98492
Seeing	Day	12% 5% 18% 26% 26% 13%	14214
	Night	12% 4 18% 29% 27% 9%	24110

By Month

Percentage Cloud Cover SE (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	15% 5% 2 5% 3 4 5% 3 6% 7% 44%	3156
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 5% 5% 55%	3160
	21st to end	Day	23% 9% 2 6% 3 3 4 3 4 6% 37%	3476
February	1st to 10th	Day	17% 7% 3 6% 5% 4 3 3 4 6% 42%	1900
	11th to 20th	Day	16% 7% 2 5% 4 4 4 3 5% 7% 43%	1848
	21st to end	Day	16% 8% 2 7% 6% 5% 4 4 6% 7% 36%	1570
March	1st to 10th	Day	21% 10% 3 8% 4 3 4 4 5% 7% 31%	3120
	11th to 20th	Day	13% 7% 2 7% 4 5% 5% 5% 6% 9% 38%	3443
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 7% 43%	4101
April	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 4 6% 10% 44%	2280
	11th to 20th	Day	13% 6% 3 6% 4 4 5% 4 6% 8% 41%	2254
	21st to end	Day	8% 6% 2 5% 4 4 5% 4 6% 7% 51%	2280
May	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 5% 9% 11% 38%	3765
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 4 6% 8% 43%	3764
	21st to end	Day	5% 4 1 5% 5% 4 4 4 9% 12% 47%	4310
June	1st to 10th	Day	8% 5% 2 7% 5% 5% 7% 6% 8% 11% 35%	2470
	11th to 20th	Day	4 4 1 5% 5% 6% 6% 6% 9% 12% 41%	2470
	21st to end	Day	8% 6% 2 6% 5% 6% 7% 7% 10% 13% 30%	2407
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 8% 7% 10% 14% 27%	3800
	11th to 20th	Day	5% 5% 2 7% 5% 6% 6% 6% 9% 11% 40%	3800
	21st to end	Day	4 4 2 8% 7% 6% 8% 7% 11% 15% 26%	4118
August	1st to 10th	Day	3 2 1 4 4 4 5% 5% 8% 13% 49%	3785
	11th to 20th	Day	5% 4 2 4 4 5% 6% 6% 8% 12% 43%	3729
	21st to end	Day	6% 6% 3 9% 6% 6% 7% 5% 8% 9% 35%	4180
September	1st to 10th	Day	10% 4 2 6% 5% 6% 6% 5% 8% 10% 37%	2280
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 5% 7% 9% 42%	2161
	21st to end	Day	8% 3 2 4 4 5% 5% 4 7% 8% 51%	2090
October	1st to 10th	Day	6% 2 1 4 3 3 4 5% 6% 8% 58%	2090
	11th to 20th	Day	5% 3 2 5% 3 4 4 3 5% 7% 59%	1938
	21st to end	Day	9% 4 2 4 3 3 4 3 4 6% 58%	2090
November	1st to 10th	Day	9% 4 1 4 3 2 3 4 5% 8% 58%	1845
	11th to 20th	Day	12% 4 2 4 4 3 3 3 5% 5% 58%	1710
	21st to end	Day	12% 5% 2 6% 4 4 5% 3 5% 7% 47%	1710
December	1st to 10th	Day	13% 8% 3 6% 4 3 5% 2 5% 6% 44%	1710
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 3 4 7% 41%	2153
	21st to end	Day	17% 7% 3 7% 5% 4 4 3 5% 6% 39%	3293
January	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 7% 47%	3789
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 6% 52%	3800
	21st to end	Night	24% 8% 2 4 3 3 3 3 4 6% 41%	4180
February	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 5% 6% 43%	2280
	11th to 20th	Night	17% 7% 3 6% 4 3 3 3 5% 6% 42%	2218
	21st to end	Night	16% 9% 2 7% 5% 4 5% 3 5% 7% 38%	1884
March	1st to 10th	Night	22% 11% 4 7% 4 4 4 4 5% 6% 31%	3733
	11th to 20th	Night	11% 7% 2 6% 5% 5% 5% 5% 6% 8% 39%	3544
	21st to end	Night	13% 5% 2 5% 4 4 5% 4 5% 6% 46%	3515
April	1st to 10th	Night	12% 7% 3 6% 4 3 5% 3 5% 6% 46%	1900
	11th to 20th	Night	12% 5% 2 6% 4 5% 5% 4 6% 6% 45%	1875
	21st to end	Night	7% 4 1 4 3 3 3 4 6% 8% 57%	1900
May	1st to 10th	Night	11% 5% 2 6% 5% 4 4 4 5% 8% 45%	3130
	11th to 20th	Night	11% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	3129
	21st to end	Night	4 4 1 5% 4 4 5% 5% 8% 14% 47%	3236
June	1st to 10th	Night	7% 5% 2 5% 5% 5% 7% 5% 8% 10% 40%	1710
	11th to 20th	Night	3 4 1 5% 4 5% 6% 6% 9% 13% 44%	1710
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 6% 10% 12% 38%	1764
July	1st to 10th	Night	4 5% 2 5% 5% 6% 6% 6% 9% 15% 36%	3160
	11th to 20th	Night	2 3 1 6% 5% 5% 5% 5% 8% 13% 47%	3160
	21st to end	Night	4 5% 2 6% 6% 6% 7% 7% 11% 16% 29%	3435
August	1st to 10th	Night	2 2 1 3 3 3 4 5% 7% 13% 57%	3144
	11th to 20th	Night	3 3 2 5% 4 5% 6% 6% 9% 15% 43%	3100
	21st to end	Night	5% 4 2 7% 6% 6% 7% 5% 8% 10% 39%	3476
September	1st to 10th	Night	8% 5% 2 7% 5% 6% 6% 6% 8% 9% 38%	1900
	11th to 20th	Night	9% 6% 3 6% 4 3 5% 5% 7% 10% 43%	1998
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 9% 52%	2090
October	1st to 10th	Night	6% 3 2 6% 3 3 4 4 4 6% 58%	2090
	11th to 20th	Night	6% 3 1 4 3 3 3 2 5% 7% 61%	2236
	21st to end	Night	10% 5% 2 5% 3 4 3 3 5% 6% 56%	2508
November	1st to 10th	Night	9% 4 2 5% 3 4 3 3 5% 6% 56%	2263
	11th to 20th	Night	10% 5% 2 4 3 4 3 3 5% 6% 55%	2470
	21st to end	Night	10% 6% 2 7% 4 5% 4 3 5% 7% 48%	2470
December	1st to 10th	Night	14% 7% 3 6% 4 4 3 3 5% 7% 45%	2470
	11th to 20th	Night	13% 7% 2 5% 4 4 4 4 5% 6% 44%	3042
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 5% 5% 38%	4343

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	25% 38% 5% 9% 14% 9%	3157
	11th to 20th	Day	19% 38% 3 13% 22% 6%	3160
	21st to end	Day	30% 40% 2 11% 13% 4	3476
February	1st to 10th	Day	18% 65% 3 4 6% 4	1900
	11th to 20th	Day	15% 65% 3 8% 7% 2	1858
	21st to end	Day	15% 66% 4 4 7% 3	1570
March	1st to 10th	Day	37% 42% 6% 9% 4 1	3120
	11th to 20th	Day	26% 43% 7% 13% 11% 1	3443
	21st to end	Day	24% 42% 8% 9% 11% 6%	4101
April	1st to 10th	Day	14% 66% 3 6% 8% 3	2280
	11th to 20th	Day	15% 66% 4 6% 6% 3	2254
	21st to end	Day	12% 65% 3 8% 7% 4	2280
May	1st to 10th	Day	21% 44% 13% 12% 7% 3	3800
	11th to 20th	Day	24% 42% 8% 12% 13% 1	3768
	21st to end	Day	15% 42% 14% 24% 5% 1	4323
June	1st to 10th	Day	14% 67% 7% 7% 5%	2470
	11th to 20th	Day	14% 69% 7% 7% 4	2470
	21st to end	Day	15% 67% 8% 6% 3	2407
July	1st to 10th	Day	23% 45% 20% 11% 1	3740
	11th to 20th	Day	19% 44% 32% 4	3737
	21st to end	Day	26% 46% 17% 9% 2	3987
August	1st to 10th	Day	9% 39% 11% 22% 18% 1	3773
	11th to 20th	Day	12% 43% 15% 22% 7%	3765
	21st to end	Day	27% 43% 9% 11% 9% 1	4179
September	1st to 10th	Day	16% 63% 8% 6% 4 2	2280
	11th to 20th	Day	9% 62% 10% 9% 8% 2	2161
	21st to end	Day	11% 63% 6% 13% 6% 2	2090
October	1st to 10th	Day	9% 62% 8% 10% 7% 4	2090
	11th to 20th	Day	8% 60% 3 10% 13% 6%	1938
	21st to end	Day	12% 59% 2 5% 12% 8%	2090
November	1st to 10th	Day	15% 59% 5% 11% 7% 4	1845
	11th to 20th	Day	16% 60% 3 8% 10% 4	1710
	21st to end	Day	16% 60% 4 8% 7% 5%	1710
December	1st to 10th	Day	19% 60% 3 7% 8% 2	1710
	11th to 20th	Day	21% 48% 4 6% 15% 7%	2150
	21st to end	Day	30% 38% 6% 10% 13% 3	3293
January	1st to 10th	Night	26% 38% 4 8% 16% 8%	3789
	11th to 20th	Night	21% 38% 3 12% 22% 4	3800
	21st to end	Night	28% 40% 3 11% 14% 4	4180
February	1st to 10th	Night	15% 65% 3 6% 7% 3	2280
	11th to 20th	Night	15% 65% 2 8% 8% 3	2219
	21st to end	Night	17% 65% 4 5% 6% 4	1884
March	1st to 10th	Night	40% 42% 7% 7% 4 1	3733
	11th to 20th	Night	26% 44% 6% 12% 8% 4	3544
	21st to end	Night	25% 42% 5% 13% 10% 5%	3515
April	1st to 10th	Night	14% 65% 4 4 8% 5%	1900
	11th to 20th	Night	15% 67% 3 7% 6% 3	1875
	21st to end	Night	11% 65% 3 9% 9% 3	1900
May	1st to 10th	Night	23% 42% 11% 11% 10% 3	3160
	11th to 20th	Night	27% 42% 6% 14% 10% 1	3130
	21st to end	Night	14% 42% 17% 20% 7% 1	3244
June	1st to 10th	Night	13% 67% 6% 10% 4	1710
	11th to 20th	Night	12% 67% 7% 9% 4	1710
	21st to end	Night	13% 67% 10% 5% 4	1764
July	1st to 10th	Night	18% 43% 26% 11% 2	3110
	11th to 20th	Night	15% 42% 36% 5% 1	3100
	21st to end	Night	24% 48% 14% 12% 2	3325
August	1st to 10th	Night	9% 39% 12% 22% 16% 1	3132
	11th to 20th	Night	13% 42% 18% 22% 5%	3130
	21st to end	Night	22% 44% 11% 14% 7% 2	3476
September	1st to 10th	Night	15% 62% 9% 7% 5% 2	1900
	11th to 20th	Night	11% 61% 9% 8% 8% 3	1998
	21st to end	Night	12% 61% 7% 13% 5% 2	2090
October	1st to 10th	Night	9% 61% 7% 10% 8% 4	2090
	11th to 20th	Night	8% 60% 4 12% 10% 7%	2236
	21st to end	Night	13% 59% 3 5% 13% 7%	2508
November	1st to 10th	Night	17% 58% 5% 9% 8% 3	2263
	11th to 20th	Night	16% 59% 3 7% 11% 4	2470
	21st to end	Night	14% 60% 3 9% 9% 5%	2470
December	1st to 10th	Night	19% 60% 4 8% 7% 2	2470
	11th to 20th	Night	19% 49% 2 7% 11% 11%	3044
	21st to end	Night	33% 38% 5% 10% 11% 3	4343

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	17% 10% 34% 28% 9% 1	270
	11th to 20th	Day	17% 10% 33% 26% 13% 1	270
	21st to end	Day	20% 10% 39% 25% 6%	297
February	1st to 10th	Day	21% 13% 29% 26% 9% 1	270
	11th to 20th	Day	25% 8% 36% 20% 11% 1	270
	21st to end	Day	14% 14% 49% 20% 3	225
March	1st to 10th	Day	16% 11% 36% 32% 6%	270
	11th to 20th	Day	14% 12% 40% 23% 10% 1	270
	21st to end	Day	16% 9% 31% 30% 13% 2	468
April	1st to 10th	Day	14% 8% 32% 32% 13% 2	460
	11th to 20th	Day	17% 12% 29% 26% 14% 3	452
	21st to end	Day	9% 7% 22% 36% 22% 4	460
May	1st to 10th	Day	11% 5% 25% 34% 20% 5%	463
	11th to 20th	Day	11% 4 14% 36% 25% 11%	487
	21st to end	Day	7% 2 12% 32% 35% 12%	529
June	1st to 10th	Day	9% 2 9% 30% 32% 18%	490
	11th to 20th	Day	8% 2 10% 27% 36% 19%	490
	21st to end	Day	9% 2 6% 27% 34% 23%	490
July	1st to 10th	Day	9% 2 3 19% 36% 32%	485
	11th to 20th	Day	10% 2 4 12% 37% 36%	484
	21st to end	Day	7% 1 4 19% 41% 29%	532
August	1st to 10th	Day	5% 1 2 14% 43% 34%	486
	11th to 20th	Day	8% 1 3 16% 35% 37%	479
	21st to end	Day	8% 3 6% 18% 38% 27%	539
September	1st to 10th	Day	11% 2 4 18% 40% 25%	490
	11th to 20th	Day	12% 3 8% 21% 38% 19%	490
	21st to end	Day	8% 2 9% 30% 34% 17%	490
October	1st to 10th	Day	6% 2 14% 36% 31% 10%	490
	11th to 20th	Day	6% 2 18% 33% 28% 12%	338
	21st to end	Day	14% 5% 18% 34% 22% 7%	330
November	1st to 10th	Day	13% 2 18% 33% 29% 5%	273
	11th to 20th	Day	17% 5% 24% 32% 17% 5%	270
	21st to end	Day	16% 4 31% 28% 18% 3	270
December	1st to 10th	Day	16% 6% 27% 34% 14% 3	270
	11th to 20th	Day	22% 8% 30% 30% 10%	270
	21st to end	Day	22% 9% 28% 24% 14% 2	297
January	1st to 10th	Night	19% 8% 30% 33% 10%	760
	11th to 20th	Night	14% 7% 31% 35% 12% 1	760
	21st to end	Night	22% 7% 32% 32% 6%	836
February	1st to 10th	Night	18% 7% 32% 34% 9%	760
	11th to 20th	Night	22% 8% 30% 28% 11%	743
	21st to end	Night	18% 10% 33% 32% 7%	628
March	1st to 10th	Night	16% 8% 30% 34% 12% 1	760
	11th to 20th	Night	14% 5% 28% 33% 18% 1	608
	21st to end	Night	14% 5% 25% 36% 19% 1	627
April	1st to 10th	Night	13% 7% 27% 35% 17% 1	570
	11th to 20th	Night	14% 6% 29% 29% 21% 2	563
	21st to end	Night	9% 4 21% 39% 25% 2	570
May	1st to 10th	Night	11% 3 16% 37% 29% 4	570
	11th to 20th	Night	7% 2 14% 37% 33% 7%	561
	21st to end	Night	5% 3 7% 33% 41% 12%	617
June	1st to 10th	Night	9% 1 4 26% 45% 14%	570
	11th to 20th	Night	3 2 5% 22% 51% 18%	570
	21st to end	Night	7% 2 4 20% 44% 24%	570
July	1st to 10th	Night	7% 2 2 12% 49% 28%	555
	11th to 20th	Night	3 1 2 7% 46% 41%	552
	21st to end	Night	5% 1 2 13% 49% 29%	624
August	1st to 10th	Night	3 1 2 10% 47% 37%	567
	11th to 20th	Night	5% 3 9% 46% 36%	564
	21st to end	Night	9% 2 4 15% 44% 27%	627
September	1st to 10th	Night	10% 1 5% 16% 42% 26%	570
	11th to 20th	Night	10% 2 6% 23% 45% 13%	668
	21st to end	Night	9% 1 8% 30% 42% 10%	760
October	1st to 10th	Night	8% 2 12% 37% 35% 6%	760
	11th to 20th	Night	8% 5% 13% 36% 31% 8%	760
	21st to end	Night	12% 3 16% 36% 29% 4	836
November	1st to 10th	Night	11% 4 14% 39% 26% 5%	748
	11th to 20th	Night	13% 5% 24% 35% 21% 2	760
	21st to end	Night	12% 6% 25% 35% 20% 2	760
December	1st to 10th	Night	15% 7% 27% 30% 19% 2	760
	11th to 20th	Night	16% 7% 26% 34% 15% 2	760
	21st to end	Night	25% 7% 27% 29% 11%	836

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