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 2026-07-31.

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% 5% 4 3 5% 6% 32%	138798
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 30%	134196
Transparency	Day	52% 4 10% 21% 11% 1	142340
	Night	53% 5% 11% 20% 11% 1	138741
Seeing	Day	24% 17% 32% 18% 7% 2	16574
	Night	22% 14% 31% 23% 9% 1	25753

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% 15% 4 13% 7% 5% 5% 3 6% 5% 23%	4661
	11th to 20th	Day	10% 23% 11% 16% 7% 6% 4 3 3 3 13%	4650
	21st to end	Day	11% 16% 8% 18% 7% 7% 4 3 4 4 17%	5753
February	1st to 10th	Day	13% 16% 5% 15% 7% 5% 5% 3 5% 5% 22%	1890
	11th to 20th	Day	13% 16% 6% 13% 6% 5% 5% 4 4 5% 25%	1843
	21st to end	Day	17% 17% 5% 11% 6% 5% 4 3 4 5% 24%	1646
March	1st to 10th	Day	18% 15% 4 11% 5% 4 4 2 4 6% 27%	4858
	11th to 20th	Day	12% 15% 5% 10% 5% 5% 5% 4 5% 7% 27%	5090
	21st to end	Day	19% 13% 4 7% 4 3 3 3 3 4 38%	5697
April	1st to 10th	Day	21% 14% 4 8% 5% 4 4 3 4 5% 29%	2520
	11th to 20th	Day	22% 12% 3 6% 4 4 3 2 5% 4 35%	2494
	21st to end	Day	22% 12% 4 7% 5% 3 4 2 3 5% 33%	2725
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 5% 6% 32%	8053
	11th to 20th	Day	18% 8% 3 7% 4 5% 5% 4 5% 7% 34%	8221
	21st to end	Day	20% 8% 2 6% 4 4 4 3 5% 7% 36%	9060
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 5% 8% 31%	2940
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 7% 8% 35%	2940
	21st to end	Day	17% 7% 3 6% 5% 5% 4 4 6% 8% 34%	2940
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 9% 36%	6500
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 44%	6500
	21st to end	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 45%	7066
August	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 4 6% 7% 34%	4236
	11th to 20th	Day	17% 6% 3 8% 4 5% 5% 4 7% 8% 33%	4174
	21st to end	Day	11% 6% 3 6% 5% 4 5% 5% 7% 9% 40%	4391
September	1st to 10th	Day	15% 7% 2 6% 5% 4 4 3 6% 8% 40%	2400
	11th to 20th	Day	12% 6% 1 6% 5% 3 4 3 5% 7% 47%	2400
	21st to end	Day	18% 6% 2 7% 5% 5% 5% 3 6% 7% 36%	2088
October	1st to 10th	Day	20% 9% 3 8% 4 3 4 3 4 7% 35%	2000
	11th to 20th	Day	23% 11% 3 8% 5% 4 4 3 5% 6% 27%	2000
	21st to end	Day	27% 12% 3 8% 6% 4 3 2 4 6% 24%	2150
November	1st to 10th	Day	17% 10% 4 8% 5% 4 5% 4 6% 5% 32%	1758
	11th to 20th	Day	16% 15% 5% 9% 6% 4 4 4 5% 6% 27%	1680
	21st to end	Day	21% 20% 6% 10% 5% 4 5% 3 3 4 19%	1680
December	1st to 10th	Day	17% 25% 6% 11% 6% 5% 3 3 3 5% 15%	2680
	11th to 20th	Day	17% 21% 7% 13% 5% 4 3 2 4 4 21%	3066
	21st to end	Day	22% 21% 6% 12% 4 3 3 2 3 5% 21%	4048
January	1st to 10th	Night	14% 16% 8% 15% 7% 5% 4 3 4 5% 19%	8234
	11th to 20th	Night	10% 19% 8% 17% 9% 5% 5% 3 4 4 14%	8235
	21st to end	Night	11% 14% 7% 15% 8% 5% 5% 3 5% 5% 22%	8424
February	1st to 10th	Night	12% 17% 5% 11% 6% 5% 5% 4 5% 7% 23%	2730
	11th to 20th	Night	13% 15% 6% 11% 6% 5% 5% 3 4 5% 27%	2663
	21st to end	Night	13% 16% 5% 9% 6% 4 4 4 6% 7% 27%	2160
March	1st to 10th	Night	18% 14% 4 11% 5% 4 3 3 4 4 30%	5187
	11th to 20th	Night	10% 13% 5% 8% 5% 5% 5% 4 7% 8% 28%	5089
	21st to end	Night	19% 10% 4 9% 5% 4 4 3 5% 6% 32%	5477
April	1st to 10th	Night	20% 13% 3 7% 5% 4 4 3 5% 6% 31%	2100
	11th to 20th	Night	18% 10% 3 8% 4 4 4 2 5% 6% 37%	2075
	21st to end	Night	18% 9% 4 7% 5% 4 5% 4 5% 7% 33%	1911
May	1st to 10th	Night	22% 8% 2 6% 5% 4 4 4 6% 7% 33%	4845
	11th to 20th	Night	16% 8% 3 5% 4 4 4 3 5% 7% 41%	4639
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 7% 39%	5114
June	1st to 10th	Night	16% 7% 3 7% 4 4 4 5% 6% 8% 36%	1680
	11th to 20th	Night	12% 6% 2 6% 4 5% 5% 3 5% 8% 43%	1680
	21st to end	Night	15% 7% 3 7% 5% 4 5% 4 6% 7% 37%	1680
July	1st to 10th	Night	11% 6% 2 7% 6% 6% 6% 4 6% 9% 38%	3680
	11th to 20th	Night	13% 5% 2 6% 4 3 4 3 5% 6% 50%	3680
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 8% 51%	4006
August	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 6% 7% 46%	2910
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 5% 6% 44%	2875
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 6% 8% 45%	3472
September	1st to 10th	Night	11% 5% 2 6% 4 3 4 4 6% 8% 47%	2000
	11th to 20th	Night	12% 4 2 4 3 4 5% 4 5% 7% 52%	2000
	21st to end	Night	19% 7% 2 6% 4 4 4 3 5% 5% 41%	2306
October	1st to 10th	Night	18% 7% 3 6% 4 5% 4 3 5% 6% 39%	2400
	11th to 20th	Night	21% 11% 3 8% 5% 5% 4 3 4 5% 31%	2400
	21st to end	Night	22% 12% 3 7% 4 5% 4 4 5% 6% 27%	2690
November	1st to 10th	Night	18% 11% 3 7% 5% 5% 4 3 5% 5% 34%	2756
	11th to 20th	Night	17% 14% 4 9% 6% 3 4 4 5% 6% 29%	2940
	21st to end	Night	19% 17% 5% 9% 5% 5% 5% 4 4 6% 21%	2940
December	1st to 10th	Night	19% 20% 7% 10% 5% 5% 4 4 5% 5% 16%	4720
	11th to 20th	Night	18% 19% 7% 13% 5% 4 3 3 3 4 20%	5348
	21st to end	Night	24% 19% 5% 12% 5% 4 3 3 3 4 17%	7150

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	62% 3 9% 17% 7% 1	4662
	11th to 20th	Day	76% 3 7% 7% 7% 1	4680
	21st to end	Day	71% 3 7% 11% 8% 1	5753
February	1st to 10th	Day	64% 5% 9% 14% 7% 1	1890
	11th to 20th	Day	61% 4 8% 14% 10% 1	1852
	21st to end	Day	63% 4 9% 13% 10% 1	1646
March	1st to 10th	Day	60% 3 10% 11% 14% 2	4861
	11th to 20th	Day	56% 4 14% 16% 9% 2	5090
	21st to end	Day	51% 2 6% 21% 16% 3	5697
April	1st to 10th	Day	58% 4 7% 18% 12% 3	2520
	11th to 20th	Day	52% 3 7% 21% 15% 1	2494
	21st to end	Day	55% 4 7% 18% 13% 2	2725
May	1st to 10th	Day	52% 4 8% 21% 11% 3	8063
	11th to 20th	Day	49% 6% 10% 20% 16% 1	8225
	21st to end	Day	48% 4 10% 23% 13% 2	9061
June	1st to 10th	Day	52% 5% 9% 19% 14% 1	2940
	11th to 20th	Day	44% 6% 12% 24% 12% 2	2940
	21st to end	Day	46% 6% 13% 22% 12% 1	2940
July	1st to 10th	Day	44% 5% 13% 25% 12% 1	6430
	11th to 20th	Day	37% 5% 12% 29% 17%	6425
	21st to end	Day	35% 6% 13% 35% 12%	6868
August	1st to 10th	Day	48% 5% 13% 23% 11%	4222
	11th to 20th	Day	46% 6% 13% 26% 8% 1	4213
	21st to end	Day	39% 5% 13% 33% 10% 1	4391
September	1st to 10th	Day	41% 6% 11% 28% 12% 1	2400
	11th to 20th	Day	35% 4 13% 31% 15% 1	2400
	21st to end	Day	46% 6% 13% 24% 11%	2088
October	1st to 10th	Day	45% 5% 10% 29% 10% 1	3260
	11th to 20th	Day	49% 4 13% 21% 12% 1	3240
	21st to end	Day	58% 4 12% 19% 7%	3452
November	1st to 10th	Day	53% 5% 13% 21% 7% 1	1758
	11th to 20th	Day	57% 4 13% 16% 9% 1	1680
	21st to end	Day	68% 5% 10% 11% 7% 1	1680
December	1st to 10th	Day	73% 4 12% 8% 3	2680
	11th to 20th	Day	68% 5% 8% 13% 7%	3066
	21st to end	Day	68% 3 6% 15% 8%	4048
January	1st to 10th	Night	68% 3 8% 14% 7%	8234
	11th to 20th	Night	72% 4 7% 9% 7% 1	8280
	21st to end	Night	64% 3 8% 11% 13% 1	8424
February	1st to 10th	Night	60% 5% 11% 16% 7% 1	2730
	11th to 20th	Night	59% 4 9% 19% 8% 1	2665
	21st to end	Night	56% 5% 10% 19% 9% 1	2160
March	1st to 10th	Night	58% 3 7% 16% 12% 5%	5197
	11th to 20th	Night	51% 6% 16% 18% 8% 2	5090
	21st to end	Night	52% 4 8% 20% 14% 2	5477
April	1st to 10th	Night	54% 3 8% 20% 12% 2	2100
	11th to 20th	Night	49% 5% 7% 23% 14% 1	2075
	21st to end	Night	50% 6% 8% 19% 15% 3	1911
May	1st to 10th	Night	50% 4 9% 21% 12% 3	4851
	11th to 20th	Night	43% 6% 12% 24% 15% 2	4640
	21st to end	Night	44% 4 11% 23% 17% 1	5115
June	1st to 10th	Night	45% 7% 11% 22% 15% 1	1680
	11th to 20th	Night	38% 6% 12% 25% 17% 2	1680
	21st to end	Night	44% 5% 15% 23% 12% 2	1680
July	1st to 10th	Night	42% 5% 15% 22% 15% 1	3640
	11th to 20th	Night	36% 5% 12% 33% 15%	3632
	21st to end	Night	32% 6% 13% 35% 14%	3889
August	1st to 10th	Night	37% 5% 14% 27% 15% 2	2900
	11th to 20th	Night	40% 6% 16% 27% 10% 1	2902
	21st to end	Night	36% 6% 13% 32% 11% 2	3472
September	1st to 10th	Night	34% 7% 15% 29% 14% 1	2000
	11th to 20th	Night	31% 5% 15% 35% 14% 1	2000
	21st to end	Night	43% 6% 11% 27% 12%	2306
October	1st to 10th	Night	37% 5% 10% 29% 19%	3920
	11th to 20th	Night	46% 4 14% 24% 10%	3899
	21st to end	Night	50% 6% 14% 22% 8%	4338
November	1st to 10th	Night	51% 5% 13% 23% 9%	2756
	11th to 20th	Night	55% 5% 12% 18% 9% 1	2940
	21st to end	Night	63% 6% 12% 15% 4	2940
December	1st to 10th	Night	69% 4 12% 12% 3	4720
	11th to 20th	Night	68% 3 8% 12% 8%	5348
	21st to end	Night	72% 3 6% 11% 8%	7150

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	24% 25% 35% 13% 4	220
	11th to 20th	Day	29% 19% 34% 16% 2	220
	21st to end	Day	27% 22% 34% 15% 3	357
February	1st to 10th	Day	29% 27% 31% 10% 2 1	330
	11th to 20th	Day	31% 27% 35% 6% 1	330
	21st to end	Day	34% 20% 36% 9% 1	362
March	1st to 10th	Day	25% 22% 37% 14% 3	540
	11th to 20th	Day	30% 22% 31% 13% 4 1	540
	21st to end	Day	33% 21% 28% 13% 4 1	594
April	1st to 10th	Day	31% 21% 32% 12% 5%	540
	11th to 20th	Day	36% 21% 22% 13% 7% 1	532
	21st to end	Day	31% 19% 32% 14% 4 1	540
May	1st to 10th	Day	27% 18% 32% 16% 5% 3	543
	11th to 20th	Day	22% 17% 36% 17% 6% 3	567
	21st to end	Day	21% 19% 30% 19% 8% 2	617
June	1st to 10th	Day	24% 14% 34% 20% 7% 1	570
	11th to 20th	Day	16% 16% 37% 20% 9% 2	570
	21st to end	Day	21% 14% 35% 20% 9% 2	570
July	1st to 10th	Day	13% 10% 35% 28% 12% 2	565
	11th to 20th	Day	13% 13% 33% 24% 12% 4	564
	21st to end	Day	14% 12% 35% 25% 12% 3	616
August	1st to 10th	Day	14% 8% 33% 29% 12% 5%	526
	11th to 20th	Day	15% 7% 26% 32% 13% 6%	519
	21st to end	Day	15% 11% 32% 24% 13% 6%	583
September	1st to 10th	Day	16% 12% 31% 20% 14% 6%	530
	11th to 20th	Day	10% 12% 26% 21% 19% 12%	530
	21st to end	Day	20% 14% 27% 23% 12% 5%	530
October	1st to 10th	Day	25% 15% 24% 18% 13% 3	530
	11th to 20th	Day	25% 24% 31% 14% 5% 1	530
	21st to end	Day	32% 18% 30% 16% 3	555
November	1st to 10th	Day	29% 18% 32% 16% 5%	332
	11th to 20th	Day	30% 24% 31% 9% 5% 1	220
	21st to end	Day	33% 19% 35% 9% 3	220
December	1st to 10th	Day	30% 20% 33% 15% 1	220
	11th to 20th	Day	35% 24% 29% 9% 1 1	220
	21st to end	Day	37% 20% 26% 12% 4 1	242
January	1st to 10th	Night	26% 20% 35% 16% 3	840
	11th to 20th	Night	29% 18% 30% 18% 4	840
	21st to end	Night	21% 16% 34% 22% 6% 1	924
February	1st to 10th	Night	22% 21% 36% 20% 1	840
	11th to 20th	Night	23% 19% 37% 18% 3	823
	21st to end	Night	23% 22% 34% 18% 3	692
March	1st to 10th	Night	24% 19% 34% 19% 3	840
	11th to 20th	Night	24% 19% 34% 19% 4 1	840
	21st to end	Night	23% 16% 34% 20% 7% 1	924
April	1st to 10th	Night	26% 15% 34% 20% 5%	840
	11th to 20th	Night	24% 19% 30% 19% 7%	831
	21st to end	Night	24% 12% 31% 23% 10%	651
May	1st to 10th	Night	22% 12% 28% 27% 11% 1	488
	11th to 20th	Night	16% 13% 33% 30% 8%	414
	21st to end	Night	16% 9% 37% 28% 11% 1	455
June	1st to 10th	Night	18% 8% 34% 32% 8%	420
	11th to 20th	Night	15% 13% 29% 32% 10% 1	420
	21st to end	Night	18% 8% 29% 35% 9%	420
July	1st to 10th	Night	6% 7% 32% 41% 13% 1	410
	11th to 20th	Night	11% 7% 29% 37% 15% 1	408
	21st to end	Night	11% 5% 30% 40% 13% 1	458
August	1st to 10th	Night	11% 5% 22% 36% 24% 2	592
	11th to 20th	Night	12% 5% 23% 36% 22% 3	594
	21st to end	Night	13% 7% 31% 29% 19% 2	805
September	1st to 10th	Night	13% 7% 33% 29% 16% 3	800
	11th to 20th	Night	13% 5% 25% 30% 23% 4	800
	21st to end	Night	23% 11% 22% 26% 17% 3	800
October	1st to 10th	Night	21% 11% 29% 22% 15% 2	800
	11th to 20th	Night	27% 15% 30% 19% 8% 1	800
	21st to end	Night	29% 15% 30% 20% 5%	880
November	1st to 10th	Night	25% 15% 30% 24% 7%	820
	11th to 20th	Night	25% 18% 33% 16% 7% 1	840
	21st to end	Night	31% 17% 28% 19% 4	840
December	1st to 10th	Night	31% 19% 31% 15% 4	840
	11th to 20th	Night	27% 20% 35% 15% 3 1	840
	21st to end	Night	33% 15% 33% 16% 3	924

Page generated at 2026-07-28 03:25:20. Version ac07

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