Old Hastings Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Hastings Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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% 4 4 3 5% 6% 31%	109141
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 30%	110172
Transparency	Day	52% 5% 11% 21% 10% 1	112730
	Night	53% 5% 11% 20% 10% 1	114746
Seeing	Day	23% 17% 31% 18% 8% 2	14805
	Night	22% 14% 31% 23% 9% 1	24194

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	12% 16% 5% 12% 5% 5% 5% 3 6% 6% 25%	3597
	11th to 20th	Day	10% 19% 11% 16% 7% 6% 4 3 4 4 15%	3570
	21st to end	Day	14% 19% 8% 16% 6% 5% 3 3 4 4 18%	4439
February	1st to 10th	Day	13% 17% 6% 15% 7% 5% 4 3 5% 5% 21%	1800
	11th to 20th	Day	13% 16% 6% 13% 6% 5% 4 4 3 5% 26%	1753
	21st to end	Day	17% 17% 4 11% 6% 5% 4 3 4 5% 24%	1570
March	1st to 10th	Day	19% 15% 4 10% 5% 4 4 2 4 6% 27%	3414
	11th to 20th	Day	11% 18% 6% 10% 6% 4 4 3 5% 7% 27%	3590
	21st to end	Day	13% 10% 3 6% 4 3 3 3 4 5% 44%	3962
April	1st to 10th	Day	21% 14% 4 8% 5% 3 4 3 4 6% 29%	2280
	11th to 20th	Day	22% 11% 3 5% 4 4 3 2 5% 5% 36%	2254
	21st to end	Day	22% 12% 4 7% 5% 4 4 2 3 5% 33%	2465
May	1st to 10th	Day	21% 8% 3 7% 4 5% 4 3 5% 6% 34%	4325
	11th to 20th	Day	18% 9% 3 8% 4 5% 5% 4 5% 8% 31%	4398
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 41%	4837
June	1st to 10th	Day	21% 8% 3 7% 6% 5% 5% 4 5% 8% 29%	2660
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 8% 36%	2660
	21st to end	Day	18% 7% 2 6% 5% 5% 4 4 6% 7% 34%	2660
July	1st to 10th	Day	13% 6% 3 8% 5% 6% 5% 4 7% 9% 35%	4440
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 4 6% 9% 41%	4440
	21st to end	Day	9% 6% 2 7% 4 5% 5% 5% 7% 8% 42%	4814
August	1st to 10th	Day	16% 8% 3 8% 5% 4 5% 4 6% 7% 34%	4106
	11th to 20th	Day	17% 6% 3 8% 4 5% 5% 4 7% 9% 32%	4044
	21st to end	Day	11% 6% 3 6% 5% 4 5% 5% 7% 9% 39%	4257
September	1st to 10th	Day	16% 7% 2 6% 5% 4 4 3 6% 7% 40%	2280
	11th to 20th	Day	12% 6% 1 7% 5% 3 4 3 5% 7% 47%	2280
	21st to end	Day	18% 6% 2 7% 4 5% 5% 3 6% 7% 36%	1984
October	1st to 10th	Day	21% 9% 3 8% 4 3 4 3 4 6% 34%	1900
	11th to 20th	Day	24% 12% 3 8% 5% 4 4 3 5% 6% 26%	1900
	21st to end	Day	28% 12% 3 8% 6% 4 3 2 4 6% 23%	2043
November	1st to 10th	Day	17% 10% 4 8% 5% 4 5% 4 6% 6% 32%	1673
	11th to 20th	Day	16% 15% 5% 9% 6% 4 4 3 5% 6% 27%	1600
	21st to end	Day	21% 19% 6% 10% 6% 4 5% 3 3 5% 19%	1600
December	1st to 10th	Day	18% 25% 6% 11% 7% 5% 3 3 4 5% 15%	2600
	11th to 20th	Day	17% 21% 7% 13% 5% 3 3 2 3 4 22%	2986
	21st to end	Day	22% 21% 6% 12% 4 3 3 2 3 5% 20%	3960
January	1st to 10th	Night	15% 16% 7% 12% 6% 5% 4 3 4 6% 21%	6349
	11th to 20th	Night	11% 16% 7% 17% 10% 6% 5% 4 5% 5% 14%	6315
	21st to end	Night	14% 16% 8% 15% 7% 5% 5% 3 4 4 19%	6513
February	1st to 10th	Night	12% 17% 5% 11% 6% 5% 5% 4 5% 7% 21%	2600
	11th to 20th	Night	13% 15% 6% 10% 6% 5% 5% 4 4 5% 28%	2533
	21st to end	Night	13% 16% 5% 9% 6% 4 4 4 6% 7% 27%	2060
March	1st to 10th	Night	18% 13% 4 10% 4 4 3 3 4 5% 30%	3631
	11th to 20th	Night	13% 17% 7% 8% 5% 4 4 4 5% 6% 27%	3589
	21st to end	Night	14% 9% 4 8% 5% 4 4 4 5% 6% 37%	3809
April	1st to 10th	Night	20% 12% 3 6% 5% 4 4 3 5% 5% 32%	1900
	11th to 20th	Night	18% 10% 3 8% 4 4 4 3 4 5% 38%	1875
	21st to end	Night	19% 9% 4 7% 5% 4 5% 4 5% 7% 32%	1729
May	1st to 10th	Night	20% 9% 3 6% 5% 3 4 4 5% 6% 35%	2615
	11th to 20th	Night	18% 8% 2 6% 5% 5% 4 3 6% 7% 36%	2495
	21st to end	Night	13% 7% 2 6% 5% 4 4 2 5% 8% 44%	2738
June	1st to 10th	Night	16% 8% 3 7% 5% 4 4 5% 7% 8% 34%	1520
	11th to 20th	Night	11% 6% 2 5% 5% 5% 5% 3 6% 8% 44%	1520
	21st to end	Night	16% 7% 3 7% 5% 4 5% 4 6% 7% 37%	1520
July	1st to 10th	Night	9% 5% 2 8% 6% 6% 6% 4 6% 8% 41%	2520
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 5% 6% 48%	2520
	21st to end	Night	10% 6% 2 5% 4 4 4 4 6% 8% 48%	2739
August	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 6% 7% 46%	2820
	11th to 20th	Night	15% 8% 2 5% 4 4 4 3 6% 7% 43%	2785
	21st to end	Night	12% 5% 2 5% 4 4 5% 4 6% 8% 45%	3366
September	1st to 10th	Night	12% 5% 2 6% 4 3 4 4 5% 8% 47%	1900
	11th to 20th	Night	13% 4 2 4 3 4 5% 4 5% 7% 51%	1900
	21st to end	Night	19% 8% 2 5% 4 4 4 3 5% 5% 41%	2190
October	1st to 10th	Night	18% 7% 3 6% 4 5% 4 3 5% 6% 38%	2280
	11th to 20th	Night	21% 12% 3 8% 5% 5% 4 3 4 5% 30%	2280
	21st to end	Night	22% 13% 3 8% 4 5% 4 4 6% 6% 25%	2555
November	1st to 10th	Night	18% 10% 3 7% 5% 5% 4 3 5% 5% 34%	2622
	11th to 20th	Night	17% 13% 4 9% 6% 3 4 4 5% 6% 29%	2800
	21st to end	Night	19% 16% 5% 9% 5% 5% 5% 4 4 6% 21%	2800
December	1st to 10th	Night	20% 20% 7% 10% 5% 5% 4 4 5% 6% 15%	4580
	11th to 20th	Night	19% 19% 7% 13% 5% 4 3 3 3 4 19%	5208
	21st to end	Night	24% 19% 5% 12% 5% 4 3 2 3 4 17%	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	59% 3 8% 20% 8% 1	3598
	11th to 20th	Day	72% 3 9% 9% 6% 1	3600
	21st to end	Day	71% 3 6% 10% 8% 1	4439
February	1st to 10th	Day	65% 5% 9% 14% 6% 1	1800
	11th to 20th	Day	62% 4 8% 14% 10% 1	1762
	21st to end	Day	63% 4 9% 13% 11% 1	1570
March	1st to 10th	Day	59% 3 12% 14% 11% 1	3417
	11th to 20th	Day	56% 4 12% 17% 9% 2	3590
	21st to end	Day	42% 3 8% 26% 17% 4	3962
April	1st to 10th	Day	57% 4 6% 19% 11% 3	2280
	11th to 20th	Day	51% 4 8% 22% 15% 1	2254
	21st to end	Day	56% 4 7% 17% 13% 3	2465
May	1st to 10th	Day	50% 5% 8% 24% 11% 2	4338
	11th to 20th	Day	50% 7% 11% 21% 10% 1	4402
	21st to end	Day	41% 6% 13% 22% 16% 3	4837
June	1st to 10th	Day	53% 5% 10% 19% 12% 1	2660
	11th to 20th	Day	43% 6% 12% 24% 13% 2	2660
	21st to end	Day	47% 7% 12% 21% 12% 1	2660
July	1st to 10th	Day	44% 5% 14% 23% 13% 1	4370
	11th to 20th	Day	38% 5% 14% 32% 10%	4365
	21st to end	Day	38% 7% 11% 32% 12%	4661
August	1st to 10th	Day	49% 5% 13% 22% 11%	4092
	11th to 20th	Day	47% 6% 14% 26% 8%	4083
	21st to end	Day	40% 5% 12% 32% 10% 1	4257
September	1st to 10th	Day	42% 7% 11% 27% 12% 1	2280
	11th to 20th	Day	36% 5% 13% 30% 16% 1	2280
	21st to end	Day	46% 6% 13% 24% 11%	1984
October	1st to 10th	Day	46% 5% 10% 29% 9% 1	3160
	11th to 20th	Day	49% 4 14% 21% 12% 1	3140
	21st to end	Day	58% 4 12% 18% 7%	3345
November	1st to 10th	Day	51% 5% 14% 21% 7% 1	1673
	11th to 20th	Day	56% 4 13% 16% 9% 1	1600
	21st to end	Day	68% 5% 10% 10% 6% 1	1600
December	1st to 10th	Day	73% 4 12% 8% 3	2600
	11th to 20th	Day	68% 5% 7% 13% 7%	2986
	21st to end	Day	68% 3 6% 14% 8%	3960
January	1st to 10th	Night	66% 3 9% 15% 8%	6349
	11th to 20th	Night	70% 5% 9% 10% 6% 1	6360
	21st to end	Night	68% 4 8% 8% 12% 1	6513
February	1st to 10th	Night	60% 5% 11% 16% 7% 1	2600
	11th to 20th	Night	58% 4 9% 19% 9% 1	2535
	21st to end	Night	55% 5% 10% 20% 9% 1	2060
March	1st to 10th	Night	57% 4 8% 19% 11% 2	3641
	11th to 20th	Night	56% 5% 12% 18% 7% 1	3590
	21st to end	Night	45% 5% 9% 24% 15% 2	3809
April	1st to 10th	Night	53% 3 8% 21% 12% 3	1900
	11th to 20th	Night	48% 5% 7% 23% 15% 1	1875
	21st to end	Night	51% 6% 9% 17% 15% 3	1729
May	1st to 10th	Night	48% 5% 11% 20% 14% 2	2621
	11th to 20th	Night	46% 7% 12% 24% 10% 2	2496
	21st to end	Night	39% 5% 13% 23% 19% 2	2739
June	1st to 10th	Night	46% 7% 12% 22% 13% 1	1520
	11th to 20th	Night	37% 6% 13% 25% 18% 2	1520
	21st to end	Night	44% 5% 15% 22% 12% 2	1520
July	1st to 10th	Night	40% 6% 14% 23% 16% 2	2480
	11th to 20th	Night	38% 5% 12% 33% 13%	2472
	21st to end	Night	33% 7% 16% 30% 14%	2651
August	1st to 10th	Night	37% 6% 14% 27% 15% 2	2810
	11th to 20th	Night	40% 6% 16% 26% 10% 1	2812
	21st to end	Night	36% 6% 13% 32% 11% 2	3366
September	1st to 10th	Night	35% 7% 15% 28% 14% 1	1900
	11th to 20th	Night	32% 5% 15% 33% 14% 1	1900
	21st to end	Night	42% 6% 12% 28% 11%	2190
October	1st to 10th	Night	38% 5% 10% 28% 19%	3800
	11th to 20th	Night	47% 4 15% 24% 10%	3779
	21st to end	Night	51% 6% 15% 21% 7%	4203
November	1st to 10th	Night	50% 5% 13% 23% 9%	2622
	11th to 20th	Night	55% 5% 12% 18% 9% 1	2800
	21st to end	Night	63% 6% 12% 15% 4	2800
December	1st to 10th	Night	69% 5% 12% 12% 2	4580
	11th to 20th	Night	69% 4 8% 11% 7%	5208
	21st to end	Night	72% 3 6% 11% 8%	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	23% 26% 34% 14% 4 1	200
	11th to 20th	Day	27% 19% 35% 17% 2 1	200
	21st to end	Day	28% 21% 35% 13% 3	324
February	1st to 10th	Day	30% 28% 31% 9% 2	300
	11th to 20th	Day	32% 28% 33% 6% 1	300
	21st to end	Day	35% 20% 35% 9% 1	334
March	1st to 10th	Day	25% 22% 36% 14% 3	500
	11th to 20th	Day	27% 23% 32% 14% 4 1	500
	21st to end	Day	32% 20% 28% 14% 5% 1	530
April	1st to 10th	Day	29% 19% 34% 13% 5%	460
	11th to 20th	Day	35% 22% 22% 12% 8% 2	452
	21st to end	Day	30% 19% 31% 13% 4 2	460
May	1st to 10th	Day	26% 17% 31% 17% 5% 3	463
	11th to 20th	Day	22% 16% 35% 19% 6% 3	487
	21st to end	Day	18% 19% 31% 20% 9% 3	529
June	1st to 10th	Day	24% 13% 34% 21% 7% 1	490
	11th to 20th	Day	16% 16% 35% 21% 10% 2	490
	21st to end	Day	22% 13% 34% 19% 9% 2	490
July	1st to 10th	Day	13% 9% 34% 28% 13% 2	485
	11th to 20th	Day	13% 13% 33% 24% 13% 4	484
	21st to end	Day	13% 11% 35% 26% 12% 3	532
August	1st to 10th	Day	15% 8% 34% 27% 11% 5%	486
	11th to 20th	Day	15% 8% 26% 32% 13% 7%	479
	21st to end	Day	15% 11% 31% 24% 13% 6%	539
September	1st to 10th	Day	16% 12% 33% 19% 13% 6%	490
	11th to 20th	Day	11% 12% 27% 20% 19% 11%	490
	21st to end	Day	19% 15% 25% 24% 12% 5%	490
October	1st to 10th	Day	27% 16% 24% 17% 13% 3	490
	11th to 20th	Day	26% 23% 30% 14% 6% 1	490
	21st to end	Day	33% 19% 30% 15% 3	514
November	1st to 10th	Day	28% 18% 33% 16% 6%	307
	11th to 20th	Day	31% 23% 31% 10% 5% 1	200
	21st to end	Day	31% 20% 36% 10% 4 1	200
December	1st to 10th	Day	31% 20% 33% 15% 2	200
	11th to 20th	Day	35% 25% 28% 10% 2 1	200
	21st to end	Day	35% 20% 27% 13% 4 1	220
January	1st to 10th	Night	26% 21% 34% 15% 3	800
	11th to 20th	Night	28% 18% 30% 19% 5%	800
	21st to end	Night	21% 16% 34% 22% 6% 1	880
February	1st to 10th	Night	23% 21% 36% 19% 1	800
	11th to 20th	Night	23% 19% 37% 17% 3	783
	21st to end	Night	23% 21% 34% 18% 3	660
March	1st to 10th	Night	25% 19% 35% 19% 3	800
	11th to 20th	Night	23% 19% 34% 19% 4 1	800
	21st to end	Night	21% 16% 34% 21% 7% 1	863
April	1st to 10th	Night	26% 15% 34% 21% 5%	760
	11th to 20th	Night	24% 19% 30% 18% 8% 1	751
	21st to end	Night	24% 12% 31% 23% 10%	589
May	1st to 10th	Night	22% 11% 28% 27% 11% 1	442
	11th to 20th	Night	16% 13% 32% 30% 9%	374
	21st to end	Night	14% 9% 36% 29% 12% 1	411
June	1st to 10th	Night	18% 8% 34% 31% 9%	380
	11th to 20th	Night	14% 12% 29% 33% 12% 1	380
	21st to end	Night	19% 8% 28% 34% 10%	380
July	1st to 10th	Night	6% 7% 31% 40% 14% 1	370
	11th to 20th	Night	11% 6% 30% 36% 16% 1	368
	21st to end	Night	10% 4 30% 41% 14% 1	416
August	1st to 10th	Night	12% 5% 23% 35% 24% 2	562
	11th to 20th	Night	12% 5% 23% 35% 23% 3	564
	21st to end	Night	13% 7% 31% 28% 19% 2	765
September	1st to 10th	Night	13% 7% 32% 29% 16% 3	760
	11th to 20th	Night	13% 6% 26% 28% 22% 5%	760
	21st to end	Night	22% 11% 21% 26% 17% 3	760
October	1st to 10th	Night	22% 12% 29% 22% 14% 2	760
	11th to 20th	Night	28% 15% 29% 20% 8% 1	760
	21st to end	Night	29% 16% 30% 19% 5%	836
November	1st to 10th	Night	24% 14% 30% 24% 7%	780
	11th to 20th	Night	25% 18% 33% 17% 7% 1	800
	21st to end	Night	31% 16% 28% 19% 5% 1	800
December	1st to 10th	Night	31% 19% 31% 15% 4	800
	11th to 20th	Night	27% 19% 35% 16% 3 1	800
	21st to end	Night	33% 15% 33% 16% 3	880

Page generated at 2025-03-24 09:04:28. Version ac07

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