Bayt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bayt Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	7% 4 1 3 2 2 2 2 4 5% 67%	104994
	Night	9% 4 1 3 2 2 2 2 4 5% 65%	102769
Transparency	Day	21% 4 6% 18% 26% 25%	104743
	Night	23% 3 7% 20% 24% 22%	102610
Seeing	Day	11% 8% 29% 35% 13% 4	15253
	Night	11% 2 14% 36% 30% 7%	24728

By Month

Percentage Cloud Cover SW (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% 9% 2 5% 3 3 3 4 5% 7% 45%	2539
	11th to 20th	Day	16% 8% 4 7% 5% 5% 5% 4 4 6% 37%	2714
	21st to end	Day	18% 8% 2 6% 4 4 4 3 6% 8% 36%	3146
February	1st to 10th	Day	17% 7% 2 4 2 3 3 4 4 5% 49%	1790
	11th to 20th	Day	13% 9% 2 4 3 3 3 4 5% 5% 48%	1850
	21st to end	Day	17% 10% 3 6% 4 3 3 2 4 4 43%	1570
March	1st to 10th	Day	14% 13% 2 7% 5% 5% 4 4 6% 7% 32%	3138
	11th to 20th	Day	10% 6% 1 4 3 2 3 3 4 6% 57%	3422
	21st to end	Day	18% 11% 3 6% 4 4 4 4 5% 6% 36%	4089
April	1st to 10th	Day	12% 6% 2 4 3 3 3 3 5% 6% 52%	2280
	11th to 20th	Day	10% 7% 1 5% 4 3 3 3 5% 7% 52%	2249
	21st to end	Day	9% 6% 1 3 3 3 4 3 4 6% 57%	2280
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 3 4 72%	3910
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 5% 77%	4115
	21st to end	Day	4 2 1 3 2 2 2 2 3 4 76%	4813
June	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 6% 68%	2660
	11th to 20th	Day	1 1 1 2 2 2 2 1 2 4 83%	2660
	21st to end	Day	2 2 2 1 1 2 1 2 3 84%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 2 92%	4420
	11th to 20th	Day	1 1 1 1 1 2 2 3 3 84%	4420
	21st to end	Day	1 1 1 1 1 2 2 2 3 86%	4554
August	1st to 10th	Day	2 2 2 1 1 1 1 2 3 85%	3909
	11th to 20th	Day	1 1 1 1 1 1 2 2 91%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	4180
September	1st to 10th	Day	2 1 1 1 2 2 2 1 2 87%	2280
	11th to 20th	Day	2 2 2 2 2 2 2 3 4 80%	2231
	21st to end	Day	3 2 2 1 1 2 2 4 4 80%	2090
October	1st to 10th	Day	3 3 1 2 2 2 2 3 4 4 76%	3289
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 3 78%	3161
	21st to end	Day	4 2 1 4 3 3 4 3 4 7% 65%	3417
November	1st to 10th	Day	9% 5% 2 3 3 3 3 3 5% 6% 58%	1710
	11th to 20th	Day	8% 4 1 3 3 2 3 3 5% 6% 63%	1710
	21st to end	Day	10% 5% 2 4 3 4 3 3 3 6% 60%	1710
December	1st to 10th	Day	14% 5% 1 3 2 3 4 3 5% 7% 53%	1582
	11th to 20th	Day	24% 8% 2 7% 4 3 3 3 3 5% 36%	1928
	21st to end	Day	13% 8% 3 6% 4 5% 4 4 5% 7% 40%	2794
January	1st to 10th	Night	14% 7% 2 6% 2 3 4 3 6% 6% 48%	4406
	11th to 20th	Night	19% 8% 3 5% 3 4 4 4 6% 7% 39%	4234
	21st to end	Night	19% 7% 2 5% 4 4 3 4 5% 8% 39%	4521
February	1st to 10th	Night	16% 7% 2 4 3 2 3 3 4 6% 50%	2387
	11th to 20th	Night	13% 5% 1 4 3 2 3 3 4 7% 54%	2218
	21st to end	Night	15% 8% 2 4 3 2 2 2 4 5% 51%	1884
March	1st to 10th	Night	19% 8% 2 5% 3 4 4 4 6% 6% 40%	3736
	11th to 20th	Night	12% 5% 1 4 2 2 2 2 3 6% 60%	3555
	21st to end	Night	20% 7% 2 4 3 3 3 3 4 6% 45%	3562
April	1st to 10th	Night	12% 5% 1 3 2 2 2 3 5% 6% 58%	1900
	11th to 20th	Night	8% 4 1 3 3 3 4 3 4 4 62%	1879
	21st to end	Night	7% 4 1 2 3 2 2 2 5% 5% 66%	1900
May	1st to 10th	Night	4 2 1 2 2 1 2 2 3 5% 77%	3064
	11th to 20th	Night	3 1 1 2 1 1 2 2 3 6% 78%	2783
	21st to end	Night	3 2 1 2 1 1 1 2 2 4 82%	2762
June	1st to 10th	Night	4 3 1 2 2 2 2 3 4 6% 72%	1520
	11th to 20th	Night	1 1 1 1 1 2 2 2 4 86%	1520
	21st to end	Night	1 1 1 1 1 2 1 1 4 87%	1520
July	1st to 10th	Night	1 1 1 1 1 94%	2540
	11th to 20th	Night	2 3 1 2 1 2 2 2 3 3 79%	2540
	21st to end	Night	1 1 1 2 1 2 1 1 3 3 84%	3002
August	1st to 10th	Night	1 2 2 1 2 2 2 3 3 82%	3034
	11th to 20th	Night	1 1 1 1 1 1 2 91%	3126
	21st to end	Night	2 1 1 1 1 1 1 1 2 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	2 2 1 2 1 2 1 2 3 3 82%	1947
	21st to end	Night	2 1 1 1 1 1 2 1 3 3 83%	2090
October	1st to 10th	Night	2 1 1 1 1 2 1 2 3 3 82%	3675
	11th to 20th	Night	2 1 1 2 1 2 2 2 3 4 81%	3777
	21st to end	Night	5% 2 1 3 1 2 2 2 3 5% 75%	4149
November	1st to 10th	Night	9% 4 1 3 2 2 2 2 4 6% 63%	2469
	11th to 20th	Night	7% 3 1 4 2 2 3 3 5% 6% 65%	2470
	21st to end	Night	9% 4 1 3 3 2 3 3 5% 6% 59%	2470
December	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 4 7% 54%	2593
	11th to 20th	Night	25% 7% 2 4 4 2 3 4 6% 6% 39%	3276
	21st to end	Night	14% 6% 2 7% 5% 5% 5% 4 6% 7% 39%	4862

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	37% 4 14% 27% 14% 3	2539
	11th to 20th	Day	47% 6% 9% 16% 12% 9%	2714
	21st to end	Day	44% 5% 12% 22% 11% 5%	3146
February	1st to 10th	Day	37% 4 7% 25% 18% 9%	1790
	11th to 20th	Day	37% 4 8% 22% 22% 8%	1860
	21st to end	Day	45% 5% 7% 23% 19% 2	1570
March	1st to 10th	Day	48% 8% 10% 16% 13% 5%	3138
	11th to 20th	Day	27% 7% 11% 34% 11% 11%	3422
	21st to end	Day	45% 9% 11% 20% 12% 2	4089
April	1st to 10th	Day	33% 4 12% 26% 20% 5%	2280
	11th to 20th	Day	31% 4 10% 27% 22% 6%	2252
	21st to end	Day	28% 5% 9% 32% 23% 3	2280
May	1st to 10th	Day	17% 4 7% 31% 31% 11%	3910
	11th to 20th	Day	13% 2 5% 31% 41% 8%	4118
	21st to end	Day	14% 3 4 33% 40% 5%	4814
June	1st to 10th	Day	18% 5% 9% 22% 33% 14%	2660
	11th to 20th	Day	8% 3 4 19% 43% 24%	2660
	21st to end	Day	9% 2 2 9% 36% 42%	2660
July	1st to 10th	Day	4 1 4 16% 75%	4351
	11th to 20th	Day	8% 1 2 6% 18% 64%	4345
	21st to end	Day	7% 2 2 7% 29% 54%	4410
August	1st to 10th	Day	8% 1 1 7% 32% 51%	3894
	11th to 20th	Day	4 1 1 6% 26% 62%	3760
	21st to end	Day	6% 1 2 7% 31% 53%	4179
September	1st to 10th	Day	7% 2 2 8% 31% 51%	2280
	11th to 20th	Day	11% 3 2 21% 38% 26%	2231
	21st to end	Day	10% 2 2 18% 38% 30%	2090
October	1st to 10th	Day	13% 2 3 10% 21% 50%	3289
	11th to 20th	Day	13% 2 3 19% 40% 23%	3161
	21st to end	Day	19% 3 4 16% 39% 19%	3417
November	1st to 10th	Day	28% 4 7% 21% 23% 18%	1710
	11th to 20th	Day	23% 4 8% 23% 28% 15%	1710
	21st to end	Day	29% 2 7% 17% 28% 17%	1710
December	1st to 10th	Day	32% 4 8% 22% 21% 13%	1582
	11th to 20th	Day	51% 5% 7% 12% 15% 10%	1928
	21st to end	Day	42% 5% 9% 26% 12% 6%	2794
January	1st to 10th	Night	36% 4 14% 25% 17% 4	4406
	11th to 20th	Night	43% 5% 9% 22% 12% 8%	4234
	21st to end	Night	43% 6% 10% 22% 13% 6%	4521
February	1st to 10th	Night	36% 3 8% 23% 20% 10%	2387
	11th to 20th	Night	31% 4 10% 24% 23% 8%	2219
	21st to end	Night	38% 2 11% 26% 17% 5%	1884
March	1st to 10th	Night	43% 6% 11% 21% 15% 5%	3736
	11th to 20th	Night	27% 4 9% 35% 14% 10%	3555
	21st to end	Night	40% 5% 15% 24% 13% 3	3562
April	1st to 10th	Night	28% 3 17% 27% 19% 6%	1900
	11th to 20th	Night	25% 5% 12% 30% 20% 8%	1879
	21st to end	Night	20% 5% 11% 35% 25% 3	1900
May	1st to 10th	Night	13% 3 8% 36% 31% 9%	3064
	11th to 20th	Night	9% 3 5% 32% 41% 10%	2784
	21st to end	Night	9% 3 6% 37% 40% 5%	2762
June	1st to 10th	Night	15% 4 10% 24% 33% 14%	1520
	11th to 20th	Night	6% 2 4 24% 39% 25%	1520
	21st to end	Night	6% 1 2 12% 34% 45%	1520
July	1st to 10th	Night	2 1 1 3 18% 75%	2500
	11th to 20th	Night	12% 1 2 5% 18% 61%	2492
	21st to end	Night	8% 2 1 8% 23% 57%	2908
August	1st to 10th	Night	10% 2 1 9% 27% 52%	3025
	11th to 20th	Night	4 1 1 5% 20% 68%	3156
	21st to end	Night	7% 1 1 6% 32% 54%	3498
September	1st to 10th	Night	5% 2 1 8% 29% 55%	1900
	11th to 20th	Night	10% 2 3 16% 44% 25%	1947
	21st to end	Night	9% 2 2 19% 40% 28%	2090
October	1st to 10th	Night	10% 2 3 11% 27% 48%	3675
	11th to 20th	Night	10% 2 4 21% 41% 22%	3777
	21st to end	Night	14% 2 3 20% 36% 24%	4149
November	1st to 10th	Night	23% 3 9% 22% 25% 18%	2469
	11th to 20th	Night	21% 3 8% 23% 29% 16%	2470
	21st to end	Night	27% 2 7% 21% 29% 15%	2470
December	1st to 10th	Night	33% 3 9% 19% 22% 13%	2593
	11th to 20th	Night	46% 4 10% 16% 17% 8%	3276
	21st to end	Night	42% 6% 10% 22% 13% 6%	4862

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	33% 6% 23% 22% 11% 5%	180
	11th to 20th	Day	22% 6% 25% 27% 15% 6%	289
	21st to end	Day	22% 6% 27% 26% 14% 5%	407
February	1st to 10th	Day	25% 11% 26% 23% 13% 3	408
	11th to 20th	Day	21% 12% 32% 23% 10% 2	455
	21st to end	Day	20% 13% 34% 24% 8% 1	382
March	1st to 10th	Day	27% 9% 32% 23% 8% 1	460
	11th to 20th	Day	24% 13% 27% 24% 10% 2	460
	21st to end	Day	24% 14% 30% 26% 5% 1	506
April	1st to 10th	Day	19% 15% 32% 25% 9%	460
	11th to 20th	Day	19% 17% 31% 26% 7% 1	452
	21st to end	Day	11% 16% 31% 28% 12% 2	460
May	1st to 10th	Day	12% 16% 34% 27% 7% 3	463
	11th to 20th	Day	12% 11% 31% 32% 11% 3	488
	21st to end	Day	13% 9% 27% 36% 11% 4	529
June	1st to 10th	Day	6% 11% 28% 34% 16% 5%	490
	11th to 20th	Day	4 10% 34% 35% 13% 5%	490
	21st to end	Day	2 7% 31% 40% 13% 7%	490
July	1st to 10th	Day	2 2 25% 50% 13% 7%	486
	11th to 20th	Day	2 1 19% 52% 14% 11%	484
	21st to end	Day	1 2 23% 52% 14% 8%	531
August	1st to 10th	Day	4 3 33% 41% 13% 5%	487
	11th to 20th	Day	2 5% 26% 46% 15% 6%	477
	21st to end	Day	2 2 26% 48% 17% 5%	539
September	1st to 10th	Day	4 3 29% 46% 15% 3	490
	11th to 20th	Day	4 8% 33% 39% 15% 1	490
	21st to end	Day	4 7% 29% 45% 14% 1	490
October	1st to 10th	Day	4 9% 31% 38% 16% 2	490
	11th to 20th	Day	4 10% 32% 38% 14% 2	490
	21st to end	Day	7% 8% 27% 38% 17% 3	520
November	1st to 10th	Day	13% 5% 27% 30% 20% 5%	282
	11th to 20th	Day	7% 6% 31% 31% 18% 7%	270
	21st to end	Day	14% 7% 23% 31% 19% 6%	270
December	1st to 10th	Day	17% 3 28% 29% 19% 4	210
	11th to 20th	Day	17% 6% 26% 31% 14% 7%	180
	21st to end	Day	22% 6% 27% 24% 13% 8%	198
January	1st to 10th	Night	20% 3 18% 33% 18% 8%	760
	11th to 20th	Night	14% 3 21% 36% 20% 6%	760
	21st to end	Night	20% 4 17% 34% 19% 6%	836
February	1st to 10th	Night	21% 3 15% 36% 19% 5%	760
	11th to 20th	Night	18% 6% 16% 31% 24% 7%	744
	21st to end	Night	21% 4 16% 39% 17% 3	628
March	1st to 10th	Night	23% 4 15% 37% 16% 4	760
	11th to 20th	Night	18% 5% 13% 32% 28% 5%	760
	21st to end	Night	19% 3 16% 38% 21% 3	836
April	1st to 10th	Night	15% 4 16% 39% 22% 3	760
	11th to 20th	Night	11% 6% 18% 36% 25% 4	751
	21st to end	Night	8% 3 16% 39% 27% 7%	760
May	1st to 10th	Night	7% 3 18% 38% 27% 6%	696
	11th to 20th	Night	6% 2 16% 37% 36% 3	537
	21st to end	Night	7% 1 15% 36% 36% 6%	411
June	1st to 10th	Night	6% 2 9% 36% 41% 7%	380
	11th to 20th	Night	1 1 14% 39% 42% 4	380
	21st to end	Night	1 12% 31% 50% 5%	380
July	1st to 10th	Night	9% 35% 49% 7%	370
	11th to 20th	Night	4 1 6% 35% 45% 10%	368
	21st to end	Night	2 1 9% 26% 48% 15%	556
August	1st to 10th	Night	2 1 11% 33% 44% 8%	675
	11th to 20th	Night	1 10% 32% 48% 9%	752
	21st to end	Night	2 7% 28% 52% 11%	836
September	1st to 10th	Night	1 1 8% 36% 43% 11%	760
	11th to 20th	Night	3 1 10% 36% 41% 9%	760
	21st to end	Night	3 1 11% 38% 41% 7%	760
October	1st to 10th	Night	5% 1 12% 46% 29% 6%	760
	11th to 20th	Night	6% 2 13% 40% 31% 8%	760
	21st to end	Night	5% 2 16% 31% 31% 14%	836
November	1st to 10th	Night	12% 2 12% 36% 28% 12%	760
	11th to 20th	Night	9% 2 17% 37% 28% 7%	760
	21st to end	Night	11% 2 18% 38% 24% 8%	760
December	1st to 10th	Night	17% 3 15% 38% 22% 5%	760
	11th to 20th	Night	21% 2 19% 35% 18% 5%	760
	21st to end	Night	22% 4 19% 35% 17% 5%	836

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