Sandilands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandilands Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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	12% 10% 4 9% 6% 5% 5% 4 6% 7% 33%	105817
	Night	11% 11% 4 10% 6% 5% 5% 4 6% 7% 32%	102580
Transparency	Day	49% 6% 14% 23% 8%	105558
	Night	51% 6% 14% 21% 8%	102486
Seeing	Day	18% 16% 35% 21% 8% 2	16346
	Night	17% 12% 34% 26% 10% 1	26316

By Month

Percentage Cloud Cover NW (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	6% 15% 7% 15% 7% 7% 6% 5% 6% 4 22%	2219
	11th to 20th	Day	4 9% 6% 16% 11% 10% 12% 7% 7% 6% 12%	2199
	21st to end	Day	7% 21% 10% 15% 7% 4 5% 4 5% 6% 15%	2702
February	1st to 10th	Day	7% 12% 7% 11% 8% 7% 8% 5% 8% 7% 20%	1700
	11th to 20th	Day	7% 11% 5% 10% 8% 8% 7% 5% 6% 8% 26%	1664
	21st to end	Day	6% 12% 4 14% 9% 8% 7% 4 6% 5% 26%	1433
March	1st to 10th	Day	11% 12% 3 10% 11% 5% 5% 4 5% 6% 28%	3281
	11th to 20th	Day	6% 9% 4 15% 9% 6% 5% 4 5% 8% 29%	3478
	21st to end	Day	10% 16% 6% 10% 5% 4 4 3 4 6% 31%	3814
April	1st to 10th	Day	12% 13% 4 9% 5% 4 4 4 5% 6% 34%	2293
	11th to 20th	Day	14% 11% 3 9% 5% 5% 4 3 6% 7% 33%	2442
	21st to end	Day	16% 10% 4 7% 5% 5% 5% 4 6% 6% 31%	2470
May	1st to 10th	Day	17% 10% 3 7% 5% 4 5% 4 6% 6% 34%	4339
	11th to 20th	Day	20% 9% 2 7% 6% 4 5% 4 6% 7% 31%	4484
	21st to end	Day	21% 11% 3 7% 4 4 4 3 4 5% 34%	5173
June	1st to 10th	Day	15% 7% 3 6% 6% 5% 5% 4 6% 8% 34%	2850
	11th to 20th	Day	16% 7% 3 7% 5% 5% 6% 4 6% 8% 32%	2850
	21st to end	Day	14% 7% 2 7% 7% 6% 6% 5% 7% 9% 31%	2850
July	1st to 10th	Day	9% 5% 2 7% 6% 5% 6% 5% 7% 10% 38%	4750
	11th to 20th	Day	9% 4 2 5% 3 4 4 4 6% 9% 48%	4750
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 8% 44%	5150
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	4190
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 8% 42%	4041
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 9% 40%	4532
September	1st to 10th	Day	12% 5% 2 6% 5% 4 4 4 5% 7% 45%	2333
	11th to 20th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 36%	2131
	21st to end	Day	18% 9% 3 6% 4 3 4 3 5% 7% 39%	2082
October	1st to 10th	Day	13% 9% 3 6% 4 4 4 4 5% 8% 40%	3160
	11th to 20th	Day	12% 12% 4 8% 4 3 4 2 5% 8% 38%	3140
	21st to end	Day	15% 13% 3 8% 5% 5% 5% 3 6% 7% 29%	3083
November	1st to 10th	Day	14% 18% 5% 11% 5% 4 3 4 4 5% 25%	1567
	11th to 20th	Day	12% 20% 5% 15% 7% 5% 3 3 4 5% 23%	1600
	21st to end	Day	13% 15% 7% 14% 10% 6% 4 3 4 5% 19%	1560
December	1st to 10th	Day	8% 14% 6% 17% 8% 6% 7% 4 7% 6% 15%	1386
	11th to 20th	Day	8% 16% 7% 15% 8% 5% 5% 3 6% 7% 20%	1679
	21st to end	Day	11% 14% 5% 14% 6% 6% 5% 4 8% 7% 21%	2442
January	1st to 10th	Night	5% 13% 7% 16% 9% 10% 6% 6% 6% 6% 18%	4737
	11th to 20th	Night	5% 13% 5% 15% 12% 13% 9% 6% 7% 5% 10%	4709
	21st to end	Night	9% 20% 8% 14% 7% 6% 6% 4 5% 5% 17%	4961
February	1st to 10th	Night	7% 13% 3 11% 9% 6% 6% 5% 7% 7% 24%	2479
	11th to 20th	Night	6% 10% 4 10% 6% 7% 7% 5% 7% 6% 31%	2403
	21st to end	Night	7% 10% 5% 13% 8% 6% 6% 5% 6% 7% 28%	2036
March	1st to 10th	Night	10% 11% 4 11% 8% 5% 6% 3 5% 6% 30%	3577
	11th to 20th	Night	4 10% 5% 10% 7% 5% 4 4 6% 9% 36%	3480
	21st to end	Night	12% 15% 5% 10% 5% 5% 4 4 5% 6% 30%	3817
April	1st to 10th	Night	12% 10% 3 6% 4 4 5% 4 6% 7% 38%	1882
	11th to 20th	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 36%	1689
	21st to end	Night	12% 10% 3 8% 5% 4 5% 3 5% 7% 36%	1710
May	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 6% 6% 36%	2631
	11th to 20th	Night	17% 9% 2 6% 4 3 4 3 5% 7% 38%	2420
	21st to end	Night	19% 11% 2 6% 4 3 3 3 5% 5% 39%	2414
June	1st to 10th	Night	12% 7% 3 7% 4 4 4 4 7% 10% 40%	1330
	11th to 20th	Night	14% 6% 3 6% 5% 4 4 4 6% 7% 42%	1330
	21st to end	Night	13% 6% 2 4 4 4 4 5% 7% 8% 43%	1330
July	1st to 10th	Night	8% 3 1 4 3 3 5% 4 7% 10% 52%	2220
	11th to 20th	Night	5% 4 1 4 3 3 4 4 6% 11% 55%	2220
	21st to end	Night	6% 3 2 4 4 4 5% 4 7% 10% 51%	2414
August	1st to 10th	Night	5% 4 2 7% 4 5% 6% 5% 7% 9% 46%	2746
	11th to 20th	Night	11% 5% 1 5% 3 4 4 3 6% 9% 50%	2798
	21st to end	Night	10% 7% 2 6% 3 5% 4 4 6% 8% 45%	3135
September	1st to 10th	Night	12% 5% 2 5% 3 4 3 2 5% 7% 52%	1846
	11th to 20th	Night	14% 6% 3 6% 4 3 4 4 5% 8% 40%	2047
	21st to end	Night	19% 7% 2 5% 4 3 4 3 5% 6% 41%	2090
October	1st to 10th	Night	15% 8% 2 6% 4 4 4 3 4 6% 44%	3788
	11th to 20th	Night	13% 11% 3 7% 4 4 5% 3 5% 7% 38%	3779
	21st to end	Night	13% 13% 4 9% 4 4 5% 3 5% 6% 34%	4474
November	1st to 10th	Night	14% 18% 5% 9% 6% 5% 5% 3 5% 5% 27%	2731
	11th to 20th	Night	12% 18% 5% 14% 7% 5% 4 3 3 5% 23%	2800
	21st to end	Night	12% 16% 6% 15% 10% 6% 5% 3 4 5% 18%	2837
December	1st to 10th	Night	8% 14% 5% 17% 9% 9% 6% 5% 5% 5% 16%	2977
	11th to 20th	Night	9% 16% 6% 15% 9% 6% 6% 5% 6% 6% 18%	3518
	21st to end	Night	10% 13% 5% 15% 9% 7% 6% 4 5% 5% 21%	5225

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% 7% 11% 16% 4	2219
	11th to 20th	Day	68% 6% 12% 11% 3	2220
	21st to end	Day	69% 4 10% 12% 5%	2702
February	1st to 10th	Day	58% 7% 12% 14% 8% 1	1700
	11th to 20th	Day	54% 5% 12% 20% 9%	1673
	21st to end	Day	58% 7% 10% 15% 10% 1	1433
March	1st to 10th	Day	56% 5% 13% 19% 6%	3281
	11th to 20th	Day	51% 8% 13% 19% 7%	3480
	21st to end	Day	54% 5% 10% 22% 8%	3814
April	1st to 10th	Day	50% 5% 10% 23% 12% 1	2293
	11th to 20th	Day	51% 4 11% 21% 12% 1	2442
	21st to end	Day	52% 6% 12% 20% 8% 1	2470
May	1st to 10th	Day	47% 6% 12% 23% 11% 1	4339
	11th to 20th	Day	51% 7% 10% 20% 12% 1	4488
	21st to end	Day	52% 6% 9% 23% 10% 1	5173
June	1st to 10th	Day	46% 8% 14% 20% 11% 1	2850
	11th to 20th	Day	47% 8% 14% 21% 10%	2850
	21st to end	Day	48% 9% 16% 21% 7%	2850
July	1st to 10th	Day	37% 10% 20% 27% 6%	4676
	11th to 20th	Day	30% 7% 21% 34% 8%	4669
	21st to end	Day	36% 7% 17% 32% 6%	4988
August	1st to 10th	Day	37% 7% 16% 29% 10%	4173
	11th to 20th	Day	38% 8% 21% 28% 6%	4080
	21st to end	Day	40% 5% 21% 27% 5%	4531
September	1st to 10th	Day	37% 7% 17% 30% 9% 1	2333
	11th to 20th	Day	45% 9% 18% 24% 5%	2131
	21st to end	Day	46% 5% 13% 30% 6%	2082
October	1st to 10th	Day	42% 5% 15% 29% 8%	3160
	11th to 20th	Day	45% 4 14% 23% 14%	3140
	21st to end	Day	54% 5% 15% 22% 4	3083
November	1st to 10th	Day	61% 6% 12% 15% 4 1	1567
	11th to 20th	Day	65% 5% 13% 13% 4	1600
	21st to end	Day	68% 5% 13% 11% 2	1560
December	1st to 10th	Day	66% 9% 10% 11% 3	1386
	11th to 20th	Day	63% 5% 14% 14% 3	1680
	21st to end	Day	60% 5% 9% 21% 5%	2442
January	1st to 10th	Night	66% 6% 13% 12% 3	4737
	11th to 20th	Night	72% 6% 7% 12% 2	4750
	21st to end	Night	68% 5% 12% 12% 3	4961
February	1st to 10th	Night	55% 6% 13% 16% 9% 1	2479
	11th to 20th	Night	49% 5% 16% 19% 10%	2404
	21st to end	Night	54% 6% 12% 18% 10%	2036
March	1st to 10th	Night	53% 6% 12% 17% 11% 1	3577
	11th to 20th	Night	44% 6% 19% 22% 9% 1	3480
	21st to end	Night	54% 6% 11% 20% 9% 1	3817
April	1st to 10th	Night	44% 6% 11% 22% 15% 2	1882
	11th to 20th	Night	49% 3 9% 20% 17% 2	1689
	21st to end	Night	47% 5% 10% 25% 12% 1	1710
May	1st to 10th	Night	46% 7% 10% 24% 13%	2631
	11th to 20th	Night	44% 7% 12% 25% 11% 2	2420
	21st to end	Night	46% 6% 9% 27% 12% 1	2414
June	1st to 10th	Night	40% 8% 18% 22% 12% 1	1330
	11th to 20th	Night	41% 8% 13% 24% 14% 1	1330
	21st to end	Night	37% 8% 20% 27% 8% 1	1330
July	1st to 10th	Night	26% 9% 24% 34% 7% 1	2185
	11th to 20th	Night	22% 7% 22% 40% 8%	2178
	21st to end	Night	27% 7% 22% 35% 8% 1	2337
August	1st to 10th	Night	32% 6% 16% 35% 10%	2739
	11th to 20th	Night	31% 6% 25% 33% 6%	2825
	21st to end	Night	36% 5% 21% 29% 8%	3135
September	1st to 10th	Night	33% 5% 16% 32% 13%	1846
	11th to 20th	Night	41% 6% 17% 26% 9% 1	2047
	21st to end	Night	43% 5% 13% 29% 9%	2090
October	1st to 10th	Night	41% 5% 11% 31% 11%	3788
	11th to 20th	Night	46% 5% 14% 28% 8%	3779
	21st to end	Night	51% 6% 13% 25% 6%	4474
November	1st to 10th	Night	59% 6% 14% 15% 6% 1	2731
	11th to 20th	Night	65% 7% 11% 13% 4 1	2800
	21st to end	Night	69% 6% 12% 10% 4	2837
December	1st to 10th	Night	68% 6% 13% 10% 3	2977
	11th to 20th	Night	65% 6% 13% 12% 4	3516
	21st to end	Night	64% 5% 10% 15% 5%	5225

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	13% 16% 39% 21% 9% 2	270
	11th to 20th	Day	14% 13% 40% 27% 6%	270
	21st to end	Day	15% 14% 42% 21% 4 3	297
February	1st to 10th	Day	16% 17% 40% 21% 5% 1	270
	11th to 20th	Day	18% 19% 38% 18% 7% 1	270
	21st to end	Day	16% 22% 40% 16% 5% 1	225
March	1st to 10th	Day	16% 19% 38% 17% 8% 2	270
	11th to 20th	Day	17% 20% 36% 19% 7% 1	270
	21st to end	Day	13% 22% 38% 22% 4 1	297
April	1st to 10th	Day	17% 17% 37% 21% 6% 1	473
	11th to 20th	Day	23% 20% 32% 18% 6% 1	640
	21st to end	Day	20% 20% 34% 19% 6% 1	650
May	1st to 10th	Day	23% 20% 32% 19% 5% 1	653
	11th to 20th	Day	23% 18% 30% 20% 8% 2	675
	21st to end	Day	23% 14% 33% 25% 5% 1	734
June	1st to 10th	Day	20% 16% 32% 20% 9% 3	680
	11th to 20th	Day	21% 17% 37% 18% 5% 2	680
	21st to end	Day	17% 14% 31% 26% 10% 3	680
July	1st to 10th	Day	11% 13% 35% 23% 13% 5%	671
	11th to 20th	Day	13% 13% 34% 24% 13% 3	668
	21st to end	Day	9% 12% 34% 25% 14% 6%	739
August	1st to 10th	Day	11% 11% 33% 28% 15% 3	676
	11th to 20th	Day	14% 9% 29% 29% 13% 6%	665
	21st to end	Day	16% 12% 34% 25% 12% 1	748
September	1st to 10th	Day	15% 14% 34% 26% 10% 1	543
	11th to 20th	Day	19% 16% 40% 19% 6%	341
	21st to end	Day	26% 16% 37% 18% 3	300
October	1st to 10th	Day	27% 14% 32% 17% 7% 2	300
	11th to 20th	Day	22% 18% 36% 20% 4	300
	21st to end	Day	29% 18% 31% 18% 3 1	330
November	1st to 10th	Day	24% 16% 35% 19% 5% 1	300
	11th to 20th	Day	24% 22% 37% 14% 3	300
	21st to end	Day	28% 22% 33% 15% 2	300
December	1st to 10th	Day	17% 16% 37% 22% 7% 1	294
	11th to 20th	Day	18% 19% 47% 14% 3	270
	21st to end	Day	16% 12% 42% 17% 10% 2	297
January	1st to 10th	Night	12% 16% 35% 25% 10% 1	950
	11th to 20th	Night	16% 12% 40% 24% 7% 1	950
	21st to end	Night	16% 14% 39% 25% 6% 1	1045
February	1st to 10th	Night	15% 12% 35% 30% 8%	950
	11th to 20th	Night	12% 13% 36% 27% 10% 2	929
	21st to end	Night	10% 16% 40% 24% 9% 1	780
March	1st to 10th	Night	16% 13% 33% 28% 9% 1	668
	11th to 20th	Night	11% 14% 44% 22% 8% 1	570
	21st to end	Night	20% 12% 29% 27% 11% 2	627
April	1st to 10th	Night	17% 11% 35% 28% 8% 1	570
	11th to 20th	Night	21% 11% 31% 26% 10% 1	563
	21st to end	Night	18% 11% 36% 28% 7%	570
May	1st to 10th	Night	20% 10% 38% 24% 7%	570
	11th to 20th	Night	20% 9% 34% 25% 10% 1	561
	21st to end	Night	21% 11% 36% 23% 8%	617
June	1st to 10th	Night	17% 9% 31% 29% 13%	570
	11th to 20th	Night	17% 9% 33% 28% 12% 1	570
	21st to end	Night	15% 7% 25% 35% 16% 1	570
July	1st to 10th	Night	7% 5% 33% 35% 19% 1	555
	11th to 20th	Night	8% 8% 30% 36% 17% 1	552
	21st to end	Night	9% 4 28% 38% 18% 2	624
August	1st to 10th	Night	10% 5% 29% 39% 16% 1	567
	11th to 20th	Night	12% 8% 24% 32% 21% 4	564
	21st to end	Night	16% 6% 28% 34% 15% 1	627
September	1st to 10th	Night	14% 7% 34% 31% 12% 2	570
	11th to 20th	Night	17% 10% 32% 29% 12%	570
	21st to end	Night	22% 10% 33% 25% 8% 1	570
October	1st to 10th	Night	23% 12% 36% 21% 7%	748
	11th to 20th	Night	24% 10% 36% 22% 8% 1	760
	21st to end	Night	27% 12% 32% 22% 6% 1	1016
November	1st to 10th	Night	21% 16% 35% 20% 8% 1	975
	11th to 20th	Night	22% 15% 35% 21% 6% 1	1000
	21st to end	Night	22% 19% 38% 17% 4 1	1000
December	1st to 10th	Night	15% 13% 38% 26% 7% 1	993
	11th to 20th	Night	19% 16% 39% 19% 5% 1	950
	21st to end	Night	20% 13% 36% 23% 8% 1	1045

Page generated at 2024-12-06 02:54:50. Version ac07

Copyright © 2024 Attilla Danko.