Digby
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Digby clear sky chart from 2005-11-06 to 2026-04-02.

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	19% 12% 4 10% 5% 4 4 3 4 6% 29%	131433
	Night	19% 15% 5% 12% 6% 4 4 3 4 5% 23%	130452
Transparency	Day	57% 5% 11% 17% 9% 1	134981
	Night	63% 6% 10% 15% 7% 1	134990
Seeing	Day	31% 20% 30% 14% 4 1	18474
	Night	30% 19% 29% 18% 4	26409

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% 24% 10% 18% 12% 5% 3 2 2 3 9%	4093
	11th to 20th	Day	18% 21% 10% 23% 6% 4 4 2 2 2 7%	4922
	21st to end	Day	14% 14% 8% 23% 15% 7% 3 2 2 2 9%	5781
February	1st to 10th	Day	17% 20% 8% 18% 9% 6% 4 3 3 4 10%	1890
	11th to 20th	Day	16% 16% 6% 18% 11% 4 4 2 3 4 17%	1843
	21st to end	Day	12% 16% 8% 11% 9% 5% 4 3 4 5% 24%	1662
March	1st to 10th	Day	22% 17% 6% 13% 9% 4 3 2 3 4 18%	4938
	11th to 20th	Day	16% 14% 5% 11% 4 4 4 3 5% 6% 29%	5090
	21st to end	Day	27% 15% 6% 9% 5% 3 4 3 4 4 20%	5587
April	1st to 10th	Day	23% 16% 3 6% 4 4 4 3 4 5% 29%	2321
	11th to 20th	Day	20% 11% 3 6% 3 3 3 2 5% 7% 38%	2554
	21st to end	Day	20% 12% 3 6% 4 3 4 3 5% 6% 35%	2600
May	1st to 10th	Day	23% 14% 4 9% 5% 5% 4 3 5% 5% 23%	5886
	11th to 20th	Day	18% 10% 3 7% 5% 5% 5% 4 5% 7% 32%	6171
	21st to end	Day	17% 10% 3 7% 5% 4 4 3 5% 7% 34%	6948
June	1st to 10th	Day	22% 9% 2 6% 4 4 4 3 5% 7% 33%	2918
	11th to 20th	Day	20% 7% 2 5% 4 3 4 3 5% 8% 38%	3000
	21st to end	Day	22% 9% 3 7% 5% 4 4 4 5% 5% 32%	3000
July	1st to 10th	Day	21% 9% 3 6% 4 4 4 4 6% 8% 32%	6820
	11th to 20th	Day	21% 9% 2 6% 3 3 4 3 5% 7% 38%	6586
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 9% 42%	6425
August	1st to 10th	Day	22% 10% 3 7% 4 4 4 3 4 6% 33%	4235
	11th to 20th	Day	13% 7% 3 7% 4 4 4 3 5% 7% 42%	4175
	21st to end	Day	10% 5% 2 5% 3 3 4 3 4 7% 54%	4616
September	1st to 10th	Day	17% 6% 2 5% 3 3 3 3 4 6% 49%	2400
	11th to 20th	Day	15% 6% 2 6% 3 3 4 2 4 7% 48%	2200
	21st to end	Day	17% 8% 2 5% 4 3 3 3 3 6% 45%	2200
October	1st to 10th	Day	18% 8% 2 6% 4 3 4 4 4 6% 41%	2011
	11th to 20th	Day	20% 11% 3 7% 4 3 4 3 5% 6% 34%	2000
	21st to end	Day	22% 12% 3 8% 4 4 5% 3 4 5% 31%	2146
November	1st to 10th	Day	21% 12% 4 8% 5% 4 4 3 4 7% 29%	1783
	11th to 20th	Day	20% 21% 5% 9% 4 4 3 3 5% 6% 21%	1680
	21st to end	Day	20% 20% 6% 12% 6% 4 4 3 4 5% 17%	1680
December	1st to 10th	Day	20% 21% 10% 18% 4 4 3 2 3 4 11%	2690
	11th to 20th	Day	22% 17% 5% 16% 5% 5% 3 2 3 4 16%	3029
	21st to end	Day	19% 18% 9% 17% 7% 3 3 2 3 4 15%	3553
January	1st to 10th	Night	14% 23% 9% 23% 9% 4 3 2 2 2 9%	8831
	11th to 20th	Night	21% 23% 9% 19% 5% 3 2 2 2 3 9%	7945
	21st to end	Night	12% 16% 7% 24% 14% 7% 4 2 3 3 9%	8400
February	1st to 10th	Night	19% 18% 7% 19% 8% 5% 4 3 3 3 10%	2730
	11th to 20th	Night	17% 17% 6% 17% 11% 5% 4 3 3 4 14%	2663
	21st to end	Night	13% 16% 6% 14% 7% 5% 4 3 5% 6% 20%	2141
March	1st to 10th	Night	21% 18% 7% 13% 9% 4 3 2 3 3 16%	5121
	11th to 20th	Night	17% 16% 5% 11% 5% 5% 4 3 4 6% 24%	5089
	21st to end	Night	28% 13% 5% 8% 4 4 4 2 3 4 25%	5587
April	1st to 10th	Night	24% 15% 3 8% 5% 3 3 3 4 5% 27%	2101
	11th to 20th	Night	22% 11% 3 6% 4 4 4 3 6% 7% 30%	1793
	21st to end	Night	22% 12% 3 7% 4 5% 3 3 5% 6% 31%	1800
May	1st to 10th	Night	24% 17% 4 8% 5% 4 4 3 3 4 24%	4054
	11th to 20th	Night	21% 10% 3 7% 5% 4 4 4 6% 6% 31%	3733
	21st to end	Night	18% 12% 3 7% 5% 4 5% 4 5% 7% 30%	3948
June	1st to 10th	Night	24% 9% 2 6% 5% 4 4 3 6% 8% 29%	1497
	11th to 20th	Night	19% 8% 3 6% 4 3 4 4 6% 9% 35%	1400
	21st to end	Night	20% 9% 4 10% 5% 5% 5% 4 5% 6% 28%	1400
July	1st to 10th	Night	23% 10% 3 8% 5% 4 5% 4 6% 8% 25%	3160
	11th to 20th	Night	21% 8% 3 6% 5% 4 4 4 5% 8% 33%	3312
	21st to end	Night	15% 6% 2 7% 4 4 4 5% 6% 8% 39%	4428
August	1st to 10th	Night	24% 10% 3 6% 4 4 3 3 5% 6% 31%	2915
	11th to 20th	Night	17% 8% 2 7% 5% 5% 5% 4 6% 9% 31%	2872
	21st to end	Night	11% 7% 2 6% 4 4 4 4 6% 7% 46%	3281
September	1st to 10th	Night	16% 5% 2 4 5% 4 4 4 6% 7% 44%	2000
	11th to 20th	Night	16% 7% 2 6% 3 3 3 3 6% 7% 44%	2195
	21st to end	Night	18% 7% 2 7% 5% 4 5% 3 5% 6% 39%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 5% 7% 40%	2387
	11th to 20th	Night	17% 10% 3 8% 4 4 4 3 5% 6% 36%	2400
	21st to end	Night	25% 13% 4 7% 4 3 3 3 4 5% 30%	2690
November	1st to 10th	Night	22% 13% 3 8% 5% 4 3 3 5% 5% 30%	2735
	11th to 20th	Night	20% 22% 5% 8% 5% 4 4 3 4 6% 20%	2940
	21st to end	Night	21% 19% 6% 10% 5% 3 3 3 4 5% 20%	2940
December	1st to 10th	Night	17% 22% 8% 18% 6% 4 4 3 4 4 11%	4720
	11th to 20th	Night	17% 18% 7% 19% 6% 4 3 2 3 4 17%	5377
	21st to end	Night	21% 20% 7% 16% 6% 3 2 2 2 4 18%	7667

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	83% 3 9% 4 1	4094
	11th to 20th	Day	85% 6% 4 4 1	4954
	21st to end	Day	80% 7% 6% 4 2	5781
February	1st to 10th	Day	79% 6% 7% 5% 3	1890
	11th to 20th	Day	73% 5% 8% 9% 4	1851
	21st to end	Day	63% 6% 10% 14% 6% 1	1662
March	1st to 10th	Day	73% 4 7% 11% 5% 1	4942
	11th to 20th	Day	56% 5% 14% 18% 6% 1	5090
	21st to end	Day	69% 3 5% 13% 8% 1	5587
April	1st to 10th	Day	59% 5% 8% 15% 12% 1	2321
	11th to 20th	Day	48% 5% 10% 23% 12% 2	2554
	21st to end	Day	51% 4 10% 20% 13% 2	2600
May	1st to 10th	Day	61% 6% 11% 16% 6%	5897
	11th to 20th	Day	51% 7% 13% 19% 8% 2	6175
	21st to end	Day	49% 7% 14% 17% 13% 1	6948
June	1st to 10th	Day	52% 6% 10% 21% 11%	2918
	11th to 20th	Day	46% 5% 12% 22% 13% 2	3000
	21st to end	Day	53% 7% 10% 19% 10% 1	3000
July	1st to 10th	Day	50% 5% 14% 20% 10% 1	6745
	11th to 20th	Day	46% 5% 13% 25% 11% 1	6505
	21st to end	Day	39% 6% 17% 24% 13%	6242
August	1st to 10th	Day	53% 6% 10% 20% 11%	4222
	11th to 20th	Day	41% 7% 16% 25% 10%	4214
	21st to end	Day	31% 5% 14% 28% 20% 1	4616
September	1st to 10th	Day	39% 4 11% 28% 16% 2	2400
	11th to 20th	Day	38% 5% 12% 26% 18% 2	2200
	21st to end	Day	42% 4 9% 25% 17% 2	2200
October	1st to 10th	Day	45% 6% 10% 27% 11% 1	3271
	11th to 20th	Day	46% 5% 9% 26% 13%	3240
	21st to end	Day	46% 4 12% 23% 13% 1	3448
November	1st to 10th	Day	58% 5% 12% 19% 7%	1783
	11th to 20th	Day	65% 5% 11% 13% 4 1	1680
	21st to end	Day	72% 5% 11% 11% 2	1680
December	1st to 10th	Day	79% 5% 6% 9% 1	2690
	11th to 20th	Day	73% 6% 9% 8% 4 1	3028
	21st to end	Day	74% 5% 9% 5% 6%	3553
January	1st to 10th	Night	84% 4 6% 5% 1	8831
	11th to 20th	Night	83% 4 6% 6% 1	7987
	21st to end	Night	80% 5% 6% 6% 2	8400
February	1st to 10th	Night	79% 5% 7% 6% 3	2730
	11th to 20th	Night	76% 4 8% 9% 3	2666
	21st to end	Night	64% 7% 10% 13% 5% 1	2141
March	1st to 10th	Night	73% 5% 6% 9% 6% 1	5130
	11th to 20th	Night	60% 7% 13% 15% 5%	5090
	21st to end	Night	64% 4 7% 16% 9% 1	5587
April	1st to 10th	Night	60% 5% 9% 14% 10% 2	2101
	11th to 20th	Night	53% 6% 9% 20% 11% 1	1793
	21st to end	Night	55% 5% 10% 17% 12% 1	1800
May	1st to 10th	Night	64% 6% 9% 15% 7%	4060
	11th to 20th	Night	52% 7% 11% 21% 6% 2	3734
	21st to end	Night	52% 8% 14% 19% 7% 1	3949
June	1st to 10th	Night	52% 7% 12% 21% 8% 1	1497
	11th to 20th	Night	45% 7% 14% 22% 10% 1	1400
	21st to end	Night	55% 8% 11% 15% 9% 2	1400
July	1st to 10th	Night	57% 7% 12% 15% 9%	3125
	11th to 20th	Night	48% 6% 15% 18% 12% 1	3270
	21st to end	Night	42% 7% 15% 23% 12% 1	4297
August	1st to 10th	Night	53% 6% 11% 23% 7%	2903
	11th to 20th	Night	47% 8% 17% 21% 7%	2899
	21st to end	Night	35% 6% 15% 27% 16% 1	3280
September	1st to 10th	Night	38% 6% 12% 28% 15% 2	2000
	11th to 20th	Night	39% 5% 10% 29% 16% 1	2195
	21st to end	Night	45% 6% 9% 23% 16% 1	2200
October	1st to 10th	Night	46% 5% 13% 23% 12% 1	3907
	11th to 20th	Night	43% 5% 13% 23% 16%	3899
	21st to end	Night	50% 3 9% 25% 12% 1	4338
November	1st to 10th	Night	56% 7% 12% 16% 8%	2735
	11th to 20th	Night	66% 6% 11% 12% 5%	2940
	21st to end	Night	67% 6% 14% 10% 2	2940
December	1st to 10th	Night	75% 7% 8% 8% 1	4720
	11th to 20th	Night	71% 6% 10% 11% 2	5379
	21st to end	Night	73% 5% 7% 11% 4	7667

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	43% 28% 21% 8%	220
	11th to 20th	Day	43% 26% 21% 9% 1	324
	21st to end	Day	32% 27% 32% 7% 1	473
February	1st to 10th	Day	36% 25% 29% 8% 2	430
	11th to 20th	Day	41% 27% 23% 7% 2	426
	21st to end	Day	34% 28% 25% 11% 1	410
March	1st to 10th	Day	36% 27% 31% 5% 1	540
	11th to 20th	Day	37% 27% 25% 8% 2 1	540
	21st to end	Day	39% 27% 22% 9% 2 1	594
April	1st to 10th	Day	44% 25% 23% 5% 2	504
	11th to 20th	Day	36% 25% 26% 10% 3	492
	21st to end	Day	33% 24% 27% 12% 4	500
May	1st to 10th	Day	36% 22% 30% 9% 2	503
	11th to 20th	Day	29% 20% 30% 16% 4	655
	21st to end	Day	30% 19% 30% 17% 3 1	790
June	1st to 10th	Day	32% 19% 27% 18% 4	730
	11th to 20th	Day	25% 17% 34% 18% 5% 1	730
	21st to end	Day	30% 16% 33% 18% 3 1	730
July	1st to 10th	Day	23% 15% 38% 18% 6% 1	720
	11th to 20th	Day	23% 10% 38% 21% 8% 1	696
	21st to end	Day	21% 13% 42% 18% 5%	576
August	1st to 10th	Day	17% 14% 40% 23% 5% 1	526
	11th to 20th	Day	19% 12% 35% 23% 9% 1	519
	21st to end	Day	18% 15% 34% 23% 8% 2	583
September	1st to 10th	Day	18% 17% 38% 20% 6% 1	530
	11th to 20th	Day	23% 17% 31% 19% 7% 3	530
	21st to end	Day	22% 18% 32% 19% 7% 2	530
October	1st to 10th	Day	27% 16% 26% 20% 9% 2	425
	11th to 20th	Day	30% 18% 30% 16% 5% 2	420
	21st to end	Day	33% 22% 28% 12% 4 1	462
November	1st to 10th	Day	36% 20% 29% 11% 3 1	425
	11th to 20th	Day	43% 23% 20% 10% 3	430
	21st to end	Day	38% 28% 25% 6% 3	430
December	1st to 10th	Day	39% 26% 24% 10% 1	430
	11th to 20th	Day	47% 27% 20% 6% 1	409
	21st to end	Day	38% 29% 26% 6% 1	242
January	1st to 10th	Night	37% 26% 25% 11% 1	950
	11th to 20th	Night	42% 21% 24% 11% 3	845
	21st to end	Night	30% 29% 27% 12% 2	924
February	1st to 10th	Night	35% 26% 28% 10% 2	840
	11th to 20th	Night	38% 25% 26% 9% 2	823
	21st to end	Night	27% 28% 28% 14% 3	692
March	1st to 10th	Night	32% 25% 30% 12% 1	840
	11th to 20th	Night	32% 21% 26% 18% 3	840
	21st to end	Night	31% 24% 28% 13% 5%	924
April	1st to 10th	Night	39% 20% 27% 13% 2	694
	11th to 20th	Night	31% 22% 30% 16% 2	594
	21st to end	Night	30% 19% 29% 18% 5%	600
May	1st to 10th	Night	29% 17% 33% 18% 3 1	600
	11th to 20th	Night	28% 16% 30% 22% 3	592
	21st to end	Night	22% 15% 41% 18% 3 1	649
June	1st to 10th	Night	28% 15% 30% 23% 4	497
	11th to 20th	Night	22% 12% 33% 28% 5% 1	400
	21st to end	Night	22% 11% 39% 20% 7%	400
July	1st to 10th	Night	19% 8% 42% 26% 4	390
	11th to 20th	Night	20% 11% 35% 23% 10%	433
	21st to end	Night	19% 9% 37% 30% 4	656
August	1st to 10th	Night	19% 8% 34% 31% 7% 1	598
	11th to 20th	Night	22% 8% 31% 29% 9% 1	592
	21st to end	Night	16% 9% 36% 30% 9% 1	660
September	1st to 10th	Night	17% 11% 34% 29% 8% 1	600
	11th to 20th	Night	21% 10% 29% 28% 11% 1	795
	21st to end	Night	23% 12% 31% 23% 9% 1	800
October	1st to 10th	Night	23% 11% 28% 27% 11%	800
	11th to 20th	Night	23% 21% 29% 19% 7% 1	800
	21st to end	Night	36% 21% 22% 16% 4	880
November	1st to 10th	Night	35% 17% 27% 16% 5% 1	856
	11th to 20th	Night	35% 23% 23% 14% 5%	950
	21st to end	Night	36% 23% 24% 14% 3	950
December	1st to 10th	Night	40% 22% 24% 12% 1	950
	11th to 20th	Night	39% 28% 23% 8% 2	950
	21st to end	Night	39% 24% 23% 12% 2	1045

Page generated at 2026-03-30 03:18:44. Version ac07

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