Sheldrake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheldrake clear sky chart from 2005-11-06 to 2025-02-21.

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	18% 10% 3 7% 5% 4 4 4 5% 7% 33%	108225
	Night	17% 12% 4 8% 5% 4 4 3 5% 6% 31%	110505
Transparency	Day	50% 7% 14% 20% 8% 1	111669
	Night	53% 6% 12% 19% 9% 1	115231
Seeing	Day	24% 8% 25% 29% 12% 2	16768
	Night	25% 8% 25% 28% 12% 2	24432

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% 19% 5% 10% 5% 3 2 2 4 6% 29%	3157
	11th to 20th	Day	11% 18% 5% 12% 6% 4 4 3 5% 7% 24%	3134
	21st to end	Day	4 7% 3 8% 5% 4 6% 5% 7% 10% 40%	3702
February	1st to 10th	Day	8% 13% 5% 10% 6% 6% 5% 3 5% 7% 34%	1800
	11th to 20th	Day	10% 17% 3 9% 6% 5% 5% 4 5% 6% 31%	1753
	21st to end	Day	8% 14% 5% 10% 5% 5% 5% 5% 6% 8% 31%	1413
March	1st to 10th	Day	10% 11% 4 10% 6% 5% 5% 4 6% 6% 33%	3280
	11th to 20th	Day	20% 19% 4 12% 5% 4 4 3 4 4 22%	3480
	21st to end	Day	21% 17% 5% 7% 4 3 3 3 4 4 29%	3816
April	1st to 10th	Day	20% 13% 4 8% 5% 4 5% 3 5% 5% 27%	2311
	11th to 20th	Day	15% 10% 3 7% 5% 4 4 4 5% 6% 38%	2442
	21st to end	Day	24% 10% 3 6% 4 3 3 3 5% 6% 33%	2470
May	1st to 10th	Day	27% 10% 3 8% 5% 4 4 4 5% 6% 25%	4425
	11th to 20th	Day	17% 8% 3 8% 5% 4 4 4 5% 7% 35%	4483
	21st to end	Day	29% 10% 3 7% 4 4 4 3 5% 6% 26%	5185
June	1st to 10th	Day	25% 7% 3 7% 5% 3 4 4 5% 6% 32%	2850
	11th to 20th	Day	22% 6% 2 6% 4 4 5% 3 6% 7% 34%	2850
	21st to end	Day	25% 8% 2 6% 4 4 4 4 6% 7% 31%	2850
July	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 7% 37%	4760
	11th to 20th	Day	13% 8% 3 7% 4 5% 5% 4 7% 9% 34%	4760
	21st to end	Day	15% 5% 2 5% 4 3 4 4 6% 8% 44%	5141
August	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 4 7% 9% 39%	4282
	11th to 20th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 6% 26%	4045
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 5% 7% 42%	4532
September	1st to 10th	Day	20% 6% 2 6% 5% 5% 5% 4 5% 6% 36%	2290
	11th to 20th	Day	25% 6% 2 4 4 3 3 3 5% 6% 39%	2187
	21st to end	Day	22% 7% 1 5% 4 3 4 4 5% 7% 39%	2047
October	1st to 10th	Day	19% 8% 3 7% 4 4 4 3 6% 6% 38%	1900
	11th to 20th	Day	25% 7% 3 7% 4 4 4 4 6% 6% 31%	1900
	21st to end	Day	26% 9% 3 8% 4 3 3 3 5% 5% 32%	1861
November	1st to 10th	Day	25% 10% 2 6% 4 3 5% 3 5% 6% 31%	1565
	11th to 20th	Day	23% 15% 3 7% 5% 4 3 3 5% 5% 28%	1600
	21st to end	Day	22% 15% 5% 7% 6% 4 3 3 5% 4 26%	1580
December	1st to 10th	Day	12% 15% 6% 10% 5% 5% 5% 4 6% 6% 26%	2280
	11th to 20th	Day	22% 10% 4 9% 5% 4 4 3 5% 7% 27%	2618
	21st to end	Day	13% 13% 5% 7% 4 4 3 3 5% 6% 36%	3476
January	1st to 10th	Night	16% 21% 5% 10% 5% 4 4 3 4 5% 23%	6808
	11th to 20th	Night	12% 15% 6% 12% 5% 5% 5% 4 5% 8% 24%	6771
	21st to end	Night	4 9% 4 10% 4 4 4 3 5% 8% 44%	7250
February	1st to 10th	Night	8% 13% 6% 9% 6% 5% 4 3 6% 7% 33%	2600
	11th to 20th	Night	12% 16% 5% 9% 5% 4 5% 3 4 5% 32%	2533
	21st to end	Night	10% 15% 4 9% 6% 5% 4 3 5% 6% 34%	2042
March	1st to 10th	Night	9% 12% 4 8% 6% 5% 5% 4 6% 7% 35%	3558
	11th to 20th	Night	19% 22% 6% 10% 5% 4 3 3 4 4 20%	3479
	21st to end	Night	24% 17% 4 8% 5% 4 3 3 3 4 26%	3816
April	1st to 10th	Night	22% 12% 3 8% 5% 5% 4 3 5% 6% 27%	1866
	11th to 20th	Night	20% 12% 3 6% 4 3 3 3 4 6% 35%	1688
	21st to end	Night	25% 10% 3 6% 3 4 4 3 4 5% 33%	1710
May	1st to 10th	Night	29% 11% 4 8% 4 5% 4 2 4 6% 23%	2525
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 4 6% 5% 37%	2451
	21st to end	Night	26% 11% 4 8% 5% 4 5% 4 5% 5% 24%	2401
June	1st to 10th	Night	27% 7% 2 5% 5% 3 4 3 5% 6% 33%	1330
	11th to 20th	Night	22% 7% 3 6% 4 4 4 4 6% 8% 31%	1330
	21st to end	Night	25% 8% 2 7% 6% 4 5% 3 5% 6% 28%	1330
July	1st to 10th	Night	19% 7% 3 6% 4 4 4 4 5% 7% 37%	2210
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 4 7% 7% 32%	2210
	21st to end	Night	14% 6% 3 5% 4 4 4 4 6% 9% 41%	2402
August	1st to 10th	Night	14% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	2627
	11th to 20th	Night	20% 8% 3 8% 5% 5% 4 3 6% 8% 30%	2782
	21st to end	Night	17% 6% 2 8% 4 5% 4 3 5% 6% 39%	3124
September	1st to 10th	Night	23% 6% 2 6% 3 4 4 3 4 6% 37%	1890
	11th to 20th	Night	23% 6% 2 6% 4 3 3 4 4 6% 39%	1990
	21st to end	Night	21% 8% 2 6% 4 3 3 3 5% 6% 39%	2133
October	1st to 10th	Night	22% 8% 3 7% 5% 4 4 3 5% 6% 34%	2280
	11th to 20th	Night	29% 8% 3 7% 4 4 4 3 4 6% 29%	2280
	21st to end	Night	25% 10% 3 7% 4 4 3 3 4 6% 31%	2735
November	1st to 10th	Night	25% 11% 3 6% 4 4 4 3 5% 6% 30%	2733
	11th to 20th	Night	24% 14% 4 7% 5% 4 4 3 4 5% 27%	2800
	21st to end	Night	24% 16% 4 7% 4 4 3 3 4 4 25%	2818
December	1st to 10th	Night	14% 14% 5% 10% 6% 5% 5% 3 4 5% 28%	4910
	11th to 20th	Night	21% 11% 4 9% 4 4 4 3 5% 5% 29%	5591
	21st to end	Night	13% 14% 4 8% 5% 4 4 3 5% 7% 33%	7502

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	60% 5% 7% 16% 12% 1	3158
	11th to 20th	Day	59% 9% 13% 16% 3	3160
	21st to end	Day	38% 11% 14% 17% 17% 3	3702
February	1st to 10th	Day	51% 9% 9% 17% 12% 2	1800
	11th to 20th	Day	54% 6% 11% 18% 11%	1761
	21st to end	Day	49% 7% 12% 21% 10% 1	1413
March	1st to 10th	Day	51% 6% 12% 19% 11% 1	3284
	11th to 20th	Day	65% 4 11% 11% 7% 2	3480
	21st to end	Day	59% 4 10% 17% 7% 3	3816
April	1st to 10th	Day	57% 4 11% 15% 12% 2	2311
	11th to 20th	Day	46% 4 12% 24% 12% 2	2442
	21st to end	Day	51% 4 13% 19% 11% 1	2470
May	1st to 10th	Day	56% 6% 13% 18% 6% 1	4440
	11th to 20th	Day	48% 6% 14% 23% 9% 1	4487
	21st to end	Day	57% 5% 13% 18% 6% 1	5185
June	1st to 10th	Day	52% 6% 11% 20% 9% 2	2850
	11th to 20th	Day	47% 7% 15% 20% 11% 1	2850
	21st to end	Day	52% 8% 11% 21% 8%	2850
July	1st to 10th	Day	43% 8% 14% 27% 7%	4685
	11th to 20th	Day	44% 8% 19% 23% 6%	4679
	21st to end	Day	36% 7% 19% 29% 9%	4977
August	1st to 10th	Day	39% 10% 20% 26% 4	4268
	11th to 20th	Day	56% 8% 15% 17% 4	4084
	21st to end	Day	39% 7% 18% 27% 8%	4532
September	1st to 10th	Day	46% 8% 16% 22% 7%	2290
	11th to 20th	Day	46% 7% 17% 23% 7%	2187
	21st to end	Day	44% 7% 15% 25% 9%	2047
October	1st to 10th	Day	50% 5% 15% 24% 5% 1	3160
	11th to 20th	Day	48% 6% 17% 23% 6%	3140
	21st to end	Day	48% 4 16% 17% 14%	3042
November	1st to 10th	Day	53% 6% 14% 17% 9%	1565
	11th to 20th	Day	59% 7% 11% 16% 7%	1600
	21st to end	Day	61% 7% 10% 14% 7% 1	1580
December	1st to 10th	Day	58% 8% 10% 15% 7% 1	2280
	11th to 20th	Day	57% 8% 13% 17% 5%	2618
	21st to end	Day	49% 6% 12% 21% 9% 3	3476
January	1st to 10th	Night	64% 6% 7% 13% 8% 1	6808
	11th to 20th	Night	59% 9% 11% 16% 5% 1	6820
	21st to end	Night	39% 8% 14% 23% 15% 1	7250
February	1st to 10th	Night	51% 8% 9% 17% 13% 3	2600
	11th to 20th	Night	55% 5% 9% 17% 12% 1	2536
	21st to end	Night	52% 6% 8% 18% 14% 2	2042
March	1st to 10th	Night	48% 6% 12% 19% 14% 1	3567
	11th to 20th	Night	68% 4 6% 12% 8% 1	3480
	21st to end	Night	63% 4 8% 12% 9% 4	3816
April	1st to 10th	Night	58% 5% 9% 14% 13% 2	1866
	11th to 20th	Night	50% 4 8% 21% 14% 3	1688
	21st to end	Night	53% 4 11% 13% 18% 2	1710
May	1st to 10th	Night	60% 8% 9% 16% 5% 1	2530
	11th to 20th	Night	46% 7% 14% 24% 10% 1	2452
	21st to end	Night	60% 7% 10% 16% 7% 1	2402
June	1st to 10th	Night	51% 5% 12% 21% 11% 1	1330
	11th to 20th	Night	50% 7% 14% 18% 10% 1	1330
	21st to end	Night	57% 6% 10% 19% 8% 1	1330
July	1st to 10th	Night	46% 7% 14% 23% 9%	2175
	11th to 20th	Night	48% 8% 14% 23% 7% 1	2168
	21st to end	Night	38% 7% 16% 29% 10%	2325
August	1st to 10th	Night	46% 9% 15% 23% 6%	2617
	11th to 20th	Night	52% 8% 16% 20% 4	2809
	21st to end	Night	45% 6% 15% 24% 9%	3124
September	1st to 10th	Night	47% 6% 13% 25% 9% 1	1890
	11th to 20th	Night	46% 6% 16% 24% 7%	1990
	21st to end	Night	46% 5% 12% 24% 12%	2133
October	1st to 10th	Night	53% 4 12% 23% 7% 1	3800
	11th to 20th	Night	53% 4 15% 22% 7%	3779
	21st to end	Night	52% 5% 9% 24% 9% 1	4509
November	1st to 10th	Night	54% 8% 12% 16% 9% 1	2733
	11th to 20th	Night	60% 7% 11% 16% 6%	2800
	21st to end	Night	63% 6% 10% 13% 7%	2818
December	1st to 10th	Night	58% 9% 10% 15% 8%	4910
	11th to 20th	Night	57% 6% 11% 18% 7%	5592
	21st to end	Night	50% 7% 16% 19% 7% 2	7502

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	18% 15% 30% 23% 15%	200
	11th to 20th	Day	21% 12% 24% 27% 13% 4	200
	21st to end	Day	14% 9% 32% 29% 11% 4	235
February	1st to 10th	Day	16% 11% 36% 25% 11% 2	400
	11th to 20th	Day	15% 12% 30% 30% 10% 2	396
	21st to end	Day	16% 12% 30% 28% 11% 3	307
March	1st to 10th	Day	16% 11% 32% 30% 8% 2	445
	11th to 20th	Day	23% 11% 25% 28% 12% 2	460
	21st to end	Day	25% 14% 25% 26% 9% 1	506
April	1st to 10th	Day	27% 15% 30% 22% 6%	460
	11th to 20th	Day	19% 10% 29% 28% 12% 1	452
	21st to end	Day	28% 8% 23% 26% 12% 2	460
May	1st to 10th	Day	28% 4 25% 28% 13% 2	653
	11th to 20th	Day	25% 4 29% 26% 13% 3	674
	21st to end	Day	28% 7% 25% 30% 9% 1	735
June	1st to 10th	Day	27% 4 24% 30% 14% 1	680
	11th to 20th	Day	25% 4 21% 32% 15% 3	680
	21st to end	Day	26% 4 20% 32% 15% 3	680
July	1st to 10th	Day	20% 5% 21% 36% 16% 2	670
	11th to 20th	Day	21% 3 19% 37% 18% 1	668
	21st to end	Day	23% 4 17% 37% 16% 3	740
August	1st to 10th	Day	20% 2 17% 37% 20% 3	598
	11th to 20th	Day	21% 5% 18% 33% 21% 2	479
	21st to end	Day	21% 3 24% 30% 18% 5%	539
September	1st to 10th	Day	24% 8% 22% 31% 13% 1	490
	11th to 20th	Day	29% 6% 22% 30% 10% 3	490
	21st to end	Day	24% 7% 33% 24% 10% 3	467
October	1st to 10th	Day	23% 9% 31% 25% 9% 2	390
	11th to 20th	Day	28% 7% 31% 21% 11% 2	390
	21st to end	Day	27% 11% 23% 25% 11% 2	429
November	1st to 10th	Day	30% 10% 27% 26% 6% 1	395
	11th to 20th	Day	32% 11% 23% 26% 8% 2	400
	21st to end	Day	31% 14% 27% 23% 5% 1	380
December	1st to 10th	Day	19% 11% 24% 33% 10% 4	200
	11th to 20th	Day	19% 17% 33% 25% 7% 1	200
	21st to end	Day	22% 17% 27% 25% 8% 1	220
January	1st to 10th	Night	25% 10% 32% 25% 8% 1	900
	11th to 20th	Night	22% 13% 28% 26% 8% 2	900
	21st to end	Night	16% 10% 36% 28% 9%	943
February	1st to 10th	Night	14% 10% 37% 29% 10% 1	800
	11th to 20th	Night	20% 9% 32% 28% 9% 2	783
	21st to end	Night	16% 9% 31% 26% 15% 3	629
March	1st to 10th	Night	18% 10% 31% 29% 11% 1	760
	11th to 20th	Night	21% 9% 28% 31% 10% 1	760
	21st to end	Night	23% 11% 26% 29% 10% 2	836
April	1st to 10th	Night	28% 9% 27% 26% 10% 1	575
	11th to 20th	Night	25% 8% 27% 28% 11% 1	564
	21st to end	Night	28% 8% 20% 30% 12% 2	570
May	1st to 10th	Night	29% 4 18% 31% 16% 3	570
	11th to 20th	Night	22% 5% 22% 33% 15% 3	533
	21st to end	Night	30% 3 21% 31% 14% 1	411
June	1st to 10th	Night	27% 2 20% 35% 14% 1	380
	11th to 20th	Night	26% 3 14% 32% 23% 1	380
	21st to end	Night	28% 4 12% 36% 19% 2	380
July	1st to 10th	Night	20% 3 16% 35% 23% 3	370
	11th to 20th	Night	22% 2 11% 38% 26% 1	368
	21st to end	Night	23% 4 12% 34% 25% 3	416
August	1st to 10th	Night	21% 2 12% 32% 26% 6%	556
	11th to 20th	Night	25% 4 9% 35% 24% 3	562
	21st to end	Night	20% 3 16% 31% 26% 4	627
September	1st to 10th	Night	24% 3 23% 32% 16% 2	570
	11th to 20th	Night	24% 5% 20% 35% 13% 2	660
	21st to end	Night	23% 6% 23% 31% 16% 1	760
October	1st to 10th	Night	26% 7% 27% 28% 11% 1	760
	11th to 20th	Night	33% 5% 23% 25% 14% 1	760
	21st to end	Night	30% 9% 25% 26% 9% 1	879
November	1st to 10th	Night	30% 11% 30% 23% 6%	880
	11th to 20th	Night	32% 11% 27% 22% 7% 1	900
	21st to end	Night	34% 11% 26% 22% 7%	900
December	1st to 10th	Night	27% 11% 30% 23% 9% 1	900
	11th to 20th	Night	25% 14% 33% 23% 4 1	900
	21st to end	Night	25% 13% 27% 25% 8% 2	990

Page generated at 2025-02-18 03:26:28. Version ac07

Copyright © 2025 Attilla Danko.