Crossfield
Clear Sky Chart
History

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

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	9% 9% 3 8% 5% 5% 5% 4 7% 8% 37%	105692
	Night	7% 9% 3 9% 6% 5% 5% 4 6% 8% 37%	102053
Transparency	Day	42% 7% 16% 23% 10% 2	105437
	Night	42% 7% 15% 22% 11% 2	101954
Seeing	Day	13% 9% 29% 30% 15% 4	15400
	Night	11% 7% 32% 34% 14% 2	24422

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	3 9% 6% 11% 5% 5% 5% 5% 6% 9% 35%	2219
	11th to 20th	Day	3 9% 3 12% 7% 5% 7% 6% 8% 8% 33%	2213
	21st to end	Day	5% 8% 3 8% 5% 6% 6% 5% 7% 9% 39%	2794
February	1st to 10th	Day	5% 14% 5% 10% 7% 7% 5% 4 7% 8% 29%	1520
	11th to 20th	Day	6% 11% 5% 11% 8% 6% 5% 5% 8% 7% 28%	1643
	21st to end	Day	3 12% 5% 11% 8% 5% 6% 6% 7% 7% 30%	1522
March	1st to 10th	Day	2 12% 5% 12% 7% 7% 8% 4 6% 7% 30%	3138
	11th to 20th	Day	5% 11% 2 6% 5% 4 4 4 6% 9% 43%	3447
	21st to end	Day	8% 23% 6% 12% 4 4 4 3 5% 6% 24%	3886
April	1st to 10th	Day	8% 16% 4 9% 7% 5% 5% 5% 7% 8% 26%	2334
	11th to 20th	Day	12% 15% 4 9% 6% 5% 5% 4 5% 6% 28%	2440
	21st to end	Day	14% 11% 4 8% 5% 5% 5% 5% 5% 9% 30%	2470
May	1st to 10th	Day	19% 13% 3 8% 5% 5% 5% 4 5% 6% 28%	4377
	11th to 20th	Day	12% 9% 3 9% 6% 6% 6% 5% 7% 8% 30%	4694
	21st to end	Day	19% 10% 3 10% 7% 6% 6% 4 7% 6% 22%	5161
June	1st to 10th	Day	18% 8% 2 7% 6% 6% 6% 5% 7% 8% 28%	2850
	11th to 20th	Day	20% 10% 3 7% 6% 6% 7% 5% 7% 6% 24%	2850
	21st to end	Day	9% 5% 3 7% 5% 5% 7% 6% 7% 10% 37%	2850
July	1st to 10th	Day	8% 4 2 5% 5% 5% 5% 5% 7% 9% 46%	4740
	11th to 20th	Day	8% 4 1 4 3 4 4 5% 7% 10% 48%	4740
	21st to end	Day	5% 4 1 5% 4 4 5% 4 7% 9% 52%	5049
August	1st to 10th	Day	9% 6% 2 6% 4 5% 6% 5% 7% 8% 41%	4401
	11th to 20th	Day	8% 5% 2 5% 5% 5% 6% 4 8% 11% 42%	4091
	21st to end	Day	10% 5% 2 5% 4 3 4 3 6% 8% 50%	4521
September	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 5% 7% 48%	2364
	11th to 20th	Day	15% 8% 2 5% 4 3 4 4 5% 7% 43%	2090
	21st to end	Day	9% 9% 2 7% 4 5% 5% 4 6% 7% 43%	2090
October	1st to 10th	Day	8% 5% 3 7% 4 4 5% 5% 8% 10% 42%	3344
	11th to 20th	Day	6% 7% 2 7% 5% 5% 5% 5% 7% 10% 41%	2904
	21st to end	Day	7% 7% 3 8% 6% 5% 5% 3 5% 7% 44%	3101
November	1st to 10th	Day	7% 16% 5% 10% 5% 4 3 4 6% 6% 33%	1627
	11th to 20th	Day	6% 13% 3 8% 6% 5% 5% 4 6% 7% 38%	1438
	21st to end	Day	5% 14% 4 7% 5% 5% 6% 4 7% 11% 34%	1330
December	1st to 10th	Day	6% 10% 5% 12% 7% 6% 5% 4 7% 7% 32%	1330
	11th to 20th	Day	4 10% 4 12% 7% 7% 4 5% 7% 8% 30%	1682
	21st to end	Day	2 9% 4 8% 6% 4 5% 5% 6% 8% 43%	2442
January	1st to 10th	Night	3 13% 5% 13% 8% 6% 6% 5% 7% 8% 27%	4726
	11th to 20th	Night	3 8% 4 13% 6% 6% 6% 6% 7% 8% 34%	4699
	21st to end	Night	3 10% 4 9% 6% 5% 5% 5% 7% 8% 37%	4865
February	1st to 10th	Night	5% 11% 4 12% 8% 8% 5% 4 5% 8% 31%	2660
	11th to 20th	Night	5% 12% 5% 11% 7% 6% 5% 5% 6% 8% 30%	2423
	21st to end	Night	2 12% 5% 10% 7% 6% 5% 5% 6% 9% 34%	1928
March	1st to 10th	Night	3 10% 5% 15% 8% 7% 7% 5% 7% 7% 27%	3736
	11th to 20th	Night	7% 9% 2 5% 4 5% 5% 5% 7% 9% 42%	3531
	21st to end	Night	7% 20% 7% 15% 7% 4 5% 4 5% 6% 20%	3752
April	1st to 10th	Night	8% 14% 4 10% 6% 6% 5% 4 7% 8% 29%	1845
	11th to 20th	Night	11% 14% 4 9% 6% 6% 6% 4 5% 7% 29%	1691
	21st to end	Night	11% 10% 4 9% 6% 5% 5% 4 7% 7% 31%	1710
May	1st to 10th	Night	20% 13% 3 8% 5% 4 4 3 5% 5% 30%	2583
	11th to 20th	Night	9% 8% 2 7% 5% 6% 6% 5% 8% 10% 34%	2199
	21st to end	Night	17% 10% 4 9% 8% 6% 6% 5% 6% 6% 24%	2414
June	1st to 10th	Night	18% 7% 3 6% 6% 5% 5% 4 6% 9% 32%	1330
	11th to 20th	Night	18% 9% 3 8% 6% 5% 6% 6% 6% 6% 27%	1330
	21st to end	Night	11% 6% 2 5% 5% 4 6% 6% 6% 8% 42%	1330
July	1st to 10th	Night	7% 4 2 6% 5% 5% 6% 5% 7% 8% 43%	2220
	11th to 20th	Night	6% 2 1 4 3 7% 6% 5% 8% 11% 46%	2220
	21st to end	Night	4 3 3 6% 4 3 5% 4 6% 8% 54%	2500
August	1st to 10th	Night	11% 5% 3 5% 4 4 5% 4 6% 9% 44%	2529
	11th to 20th	Night	7% 5% 2 7% 5% 5% 5% 5% 7% 11% 42%	2746
	21st to end	Night	8% 5% 2 5% 3 3 5% 4 5% 8% 51%	3146
September	1st to 10th	Night	11% 7% 2 5% 4 3 3 3 5% 7% 49%	1814
	11th to 20th	Night	14% 7% 2 5% 3 3 4 4 7% 7% 45%	2090
	21st to end	Night	10% 8% 2 6% 3 4 4 4 6% 8% 45%	2090
October	1st to 10th	Night	8% 6% 2 6% 5% 4 4 4 5% 8% 48%	3631
	11th to 20th	Night	6% 7% 3 7% 5% 5% 5% 4 6% 8% 45%	4021
	21st to end	Night	5% 7% 3 7% 5% 5% 6% 4 7% 8% 42%	4457
November	1st to 10th	Night	8% 14% 4 9% 4 5% 5% 4 7% 7% 35%	2651
	11th to 20th	Night	4 12% 4 8% 6% 5% 5% 5% 7% 9% 36%	2762
	21st to end	Night	4 11% 4 8% 6% 6% 5% 5% 6% 8% 36%	2850
December	1st to 10th	Night	5% 10% 4 11% 7% 5% 6% 4 6% 7% 35%	2850
	11th to 20th	Night	2 10% 4 10% 8% 6% 6% 5% 7% 9% 33%	3510
	21st to end	Night	3 9% 3 8% 5% 5% 5% 4 6% 7% 44%	5214

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	43% 7% 13% 24% 11% 2	2219
	11th to 20th	Day	44% 7% 12% 15% 18% 4	2234
	21st to end	Day	40% 7% 15% 30% 8% 1	2794
February	1st to 10th	Day	51% 7% 16% 15% 9% 2	1520
	11th to 20th	Day	51% 8% 13% 17% 10% 2	1652
	21st to end	Day	48% 8% 13% 19% 10% 1	1522
March	1st to 10th	Day	49% 8% 16% 15% 11% 1	3138
	11th to 20th	Day	35% 7% 19% 25% 13% 1	3448
	21st to end	Day	60% 7% 12% 15% 6%	3886
April	1st to 10th	Day	53% 8% 16% 15% 7%	2334
	11th to 20th	Day	53% 7% 11% 19% 9% 1	2440
	21st to end	Day	49% 8% 15% 19% 7% 1	2470
May	1st to 10th	Day	55% 7% 12% 21% 5%	4377
	11th to 20th	Day	45% 10% 15% 21% 9%	4698
	21st to end	Day	56% 9% 14% 15% 5% 1	5162
June	1st to 10th	Day	50% 8% 16% 17% 7% 1	2850
	11th to 20th	Day	55% 9% 13% 15% 6% 2	2850
	21st to end	Day	39% 10% 19% 23% 8% 2	2850
July	1st to 10th	Day	33% 7% 16% 26% 16% 3	4666
	11th to 20th	Day	30% 7% 19% 34% 10% 1	4659
	21st to end	Day	27% 6% 13% 29% 20% 5%	4889
August	1st to 10th	Day	38% 7% 16% 32% 6% 1	4384
	11th to 20th	Day	34% 6% 25% 27% 6% 1	4131
	21st to end	Day	31% 6% 19% 33% 10% 1	4520
September	1st to 10th	Day	35% 7% 17% 24% 13% 4	2364
	11th to 20th	Day	40% 7% 15% 26% 12% 1	2090
	21st to end	Day	38% 6% 16% 24% 13% 3	2090
October	1st to 10th	Day	35% 6% 21% 31% 5% 1	3344
	11th to 20th	Day	37% 5% 21% 23% 11% 3	2904
	21st to end	Day	41% 4 10% 33% 11% 1	3101
November	1st to 10th	Day	51% 7% 11% 20% 9% 2	1627
	11th to 20th	Day	44% 8% 15% 22% 10% 1	1438
	21st to end	Day	43% 8% 16% 22% 9% 2	1330
December	1st to 10th	Day	50% 7% 13% 16% 13% 3	1330
	11th to 20th	Day	46% 10% 17% 18% 8% 1	1684
	21st to end	Day	36% 7% 12% 21% 19% 6%	2442
January	1st to 10th	Night	50% 10% 12% 19% 8% 2	4726
	11th to 20th	Night	47% 7% 11% 17% 13% 6%	4740
	21st to end	Night	42% 8% 14% 23% 11% 1	4865
February	1st to 10th	Night	51% 7% 12% 17% 11% 2	2660
	11th to 20th	Night	49% 8% 13% 18% 11% 2	2424
	21st to end	Night	45% 6% 12% 20% 14% 2	1928
March	1st to 10th	Night	51% 7% 14% 19% 8% 1	3736
	11th to 20th	Night	34% 10% 20% 20% 14% 1	3531
	21st to end	Night	62% 7% 12% 13% 4 1	3752
April	1st to 10th	Night	49% 8% 14% 18% 11% 1	1845
	11th to 20th	Night	53% 6% 12% 19% 9% 1	1691
	21st to end	Night	50% 7% 14% 20% 7% 2	1710
May	1st to 10th	Night	54% 7% 12% 21% 6%	2583
	11th to 20th	Night	39% 11% 14% 26% 10%	2199
	21st to end	Night	53% 12% 14% 15% 4 1	2414
June	1st to 10th	Night	48% 7% 16% 18% 9% 1	1330
	11th to 20th	Night	52% 9% 14% 15% 7% 2	1330
	21st to end	Night	36% 9% 21% 25% 7% 1	1330
July	1st to 10th	Night	35% 8% 15% 22% 18% 2	2185
	11th to 20th	Night	27% 8% 20% 33% 7% 4	2178
	21st to end	Night	27% 7% 14% 31% 19% 3	2421
August	1st to 10th	Night	36% 7% 14% 33% 9%	2521
	11th to 20th	Night	34% 8% 25% 27% 6% 1	2772
	21st to end	Night	30% 6% 22% 31% 10% 1	3146
September	1st to 10th	Night	33% 8% 16% 25% 14% 5%	1814
	11th to 20th	Night	36% 8% 15% 25% 12% 4	2090
	21st to end	Night	35% 6% 18% 24% 14% 3	2090
October	1st to 10th	Night	34% 5% 17% 26% 16% 1	3631
	11th to 20th	Night	36% 5% 18% 25% 14% 2	4021
	21st to end	Night	38% 6% 11% 32% 13% 1	4457
November	1st to 10th	Night	46% 7% 13% 20% 11% 2	2651
	11th to 20th	Night	42% 7% 17% 19% 12% 1	2762
	21st to end	Night	42% 8% 17% 20% 10% 3	2850
December	1st to 10th	Night	46% 7% 13% 21% 9% 3	2850
	11th to 20th	Night	45% 10% 17% 21% 7% 1	3507
	21st to end	Night	36% 6% 12% 25% 14% 6%	5214

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	6% 7% 34% 31% 16% 8%	180
	11th to 20th	Day	9% 9% 38% 24% 17% 2	180
	21st to end	Day	8% 8% 31% 34% 16% 3	198
February	1st to 10th	Day	14% 12% 36% 23% 10% 6%	180
	11th to 20th	Day	10% 8% 35% 33% 11% 3	260
	21st to end	Day	7% 10% 31% 31% 17% 3	334
March	1st to 10th	Day	9% 12% 35% 30% 11% 3	460
	11th to 20th	Day	8% 12% 32% 33% 10% 4	460
	21st to end	Day	14% 13% 35% 23% 10% 5%	506
April	1st to 10th	Day	13% 15% 34% 28% 9% 1	460
	11th to 20th	Day	15% 12% 30% 29% 12% 3	452
	21st to end	Day	19% 13% 28% 28% 10% 2	460
May	1st to 10th	Day	20% 13% 23% 26% 16% 3	515
	11th to 20th	Day	16% 7% 25% 30% 19% 3	675
	21st to end	Day	18% 7% 22% 29% 19% 5%	734
June	1st to 10th	Day	17% 11% 28% 29% 12% 3	680
	11th to 20th	Day	22% 10% 26% 28% 13% 2	680
	21st to end	Day	11% 5% 26% 33% 20% 5%	680
July	1st to 10th	Day	12% 6% 22% 35% 21% 4	671
	11th to 20th	Day	13% 6% 25% 31% 22% 3	668
	21st to end	Day	7% 5% 25% 37% 19% 7%	686
August	1st to 10th	Day	12% 6% 26% 31% 19% 6%	487
	11th to 20th	Day	10% 4 23% 38% 20% 5%	477
	21st to end	Day	12% 7% 24% 31% 19% 7%	539
September	1st to 10th	Day	14% 8% 30% 27% 16% 5%	490
	11th to 20th	Day	20% 5% 28% 28% 15% 6%	490
	21st to end	Day	14% 14% 30% 27% 11% 3	490
October	1st to 10th	Day	16% 10% 28% 29% 12% 5%	409
	11th to 20th	Day	14% 13% 33% 24% 12% 4	300
	21st to end	Day	15% 15% 33% 26% 8% 3	330
November	1st to 10th	Day	16% 9% 34% 28% 10% 3	300
	11th to 20th	Day	16% 15% 32% 27% 8% 2	231
	21st to end	Day	9% 11% 33% 24% 15% 7%	180
December	1st to 10th	Day	8% 9% 34% 32% 14% 3	180
	11th to 20th	Day	7% 10% 41% 28% 11% 3	180
	21st to end	Day	7% 6% 30% 35% 19% 3	198
January	1st to 10th	Night	8% 10% 37% 33% 11% 1	950
	11th to 20th	Night	9% 8% 39% 33% 10% 1	950
	21st to end	Night	10% 11% 36% 32% 9% 2	839
February	1st to 10th	Night	9% 11% 30% 32% 15% 3	760
	11th to 20th	Night	9% 6% 36% 35% 13% 1	744
	21st to end	Night	6% 8% 32% 31% 19% 5%	628
March	1st to 10th	Night	8% 6% 35% 35% 13% 3	760
	11th to 20th	Night	10% 6% 35% 36% 12% 2	760
	21st to end	Night	14% 6% 29% 35% 13% 3	836
April	1st to 10th	Night	12% 9% 30% 32% 15% 2	705
	11th to 20th	Night	17% 7% 32% 33% 11%	563
	21st to end	Night	16% 10% 28% 36% 9% 1	570
May	1st to 10th	Night	16% 10% 29% 32% 13%	462
	11th to 20th	Night	12% 6% 28% 37% 16%	374
	21st to end	Night	20% 4 24% 36% 16% 1	411
June	1st to 10th	Night	19% 7% 27% 34% 13% 1	380
	11th to 20th	Night	20% 6% 31% 29% 13% 1	380
	21st to end	Night	13% 3 22% 39% 21% 2	380
July	1st to 10th	Night	11% 5% 18% 42% 23% 1	370
	11th to 20th	Night	12% 1 25% 42% 19% 2	368
	21st to end	Night	5% 4 20% 43% 26% 2	416
August	1st to 10th	Night	12% 2 16% 40% 28% 2	378
	11th to 20th	Night	9% 3 19% 43% 24% 2	523
	21st to end	Night	9% 3 24% 41% 22% 1	627
September	1st to 10th	Night	14% 3 25% 38% 18% 2	665
	11th to 20th	Night	16% 4 21% 38% 19% 2	760
	21st to end	Night	14% 6% 32% 33% 14% 2	760
October	1st to 10th	Night	15% 5% 28% 34% 14% 3	760
	11th to 20th	Night	12% 5% 33% 35% 13% 2	760
	21st to end	Night	11% 9% 35% 34% 11% 1	836
November	1st to 10th	Night	15% 8% 37% 28% 11% 1	896
	11th to 20th	Night	10% 10% 43% 28% 8% 1	956
	21st to end	Night	7% 9% 41% 32% 8% 2	950
December	1st to 10th	Night	9% 10% 35% 31% 13% 2	950
	11th to 20th	Night	7% 8% 38% 36% 9% 1	950
	21st to end	Night	10% 6% 35% 36% 10% 2	1045

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