Understanding and Predicting the Weather

 
  Hort Snacks - July 2017
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 Atmospheric or Air Pressure | Weather Signs | Changes in Air Pressure | Cloud Formations | Other Signs

General Alberta Weather Rule of Thumb
"If you don't like the weather, wait 5 minutes"

In general terms, by definition, weather is the “symptom” of atmospheric change, as demonstrated by a range of different related factors or indicators. Some of these indicators include:

  • Air Pressure
  • Wind (speed or direction)
  • Humidity
  • Clouds
The study of meteorology (the science of the processes relating to the atmosphere) and the use of it to predict or forecast the weather is very complex, and represents a blend of experience, computer models and a great deal of historic and real-time data collection. Meteorologists present forecasts for the public to use, which show how weather systems are developing and what types of weather might be expected over the short, medium and long term. People can look at weather maps, specific weather station data (which shows a range of weather factors), or follow weather reports and forecasts.

As evidenced by the variability of daily, weekly, monthly and longer term forecasts, it is difficult for anyone to just “predict the weather”. However, by watching for trends in atmospheric changes and having a basic understanding of the different component weather indicators, it is possible for people to recognize potential weather events, predict weather changes (in the short term) and, in theory, prepare for those that might negatively affect their lifestyle.

The following content will discuss some of the basic weather indicators and some of the general weather that might be expected before, during or following particular weather signs. A number of general and/or traditional weather signs will also be discussed, with reference to the weather they “predict”.

Note: Producers should rely on professional weather forecasts for specific decisions and weather information. These weather signs represent general guidelines that may apply in some situations, but do not represent hard and fast rules for predicting weather events.

“The trouble with weather forecasting is that it's right too often for us to ignore it and wrong too often for us to rely on it. “ – Patrick Young

Atmospheric or Air Pressure

Air pressure affects weather as it changes in an area. Air pressure changes are caused by heating or cooling of the air in the atmosphere due to the effect of the sun’s radiation on the earth’s surface. Heating or cooling air rises or sinks, changing the pressure. Differences in air pressure between areas create pressure systems.

The Jet Streams are a narrow band of strong, high level (upper atmosphere) winds which move from west to east (in the Northern Hemisphere). These winds wave or bend in variable amounts, resulting in areas or systems of high and low pressure. These systems move west to east, which result in differences in the weather.

Low Pressure Systems are areas where atmospheric pressure is lower than that of the areas around it. Winds rotate in a counter clockwise rotation around a low pressure system. Low pressure systems (also known as Lows) are associated with unsettled weather.

In general, you will observe the following conditions in a LP system:
  • High winds
  • Warm air (relative to other areas)
  • Cloudy conditions
  • Precipitation
  • Less diurnal (day/night) variations in temperature
  • This is because the clouds block solar radiation during the day (results in less heating) and trap heat like a blanket at night (results in less cooling)
Low & high pressure systems and rotational direction of winds
Diagram by Robert Spencer

High Pressure Systems are areas where atmospheric pressure is higher than that of the areas around it. Winds rotate in a clockwise direction around a high pressure system. High pressure systems (a.k.a. Highs) are associated with clear skies and calm weather. High pressure systems generally have higher temperature extremes and greater diurnal variation, resulting in higher highs and lower lows.

As air pressure systems move, you will see the weather change. The greater the rate of change, the faster the system is moving. Many times, where the high pressure system originates affects what the approaching weather will be. If a HP system moves in from the south, you can expect warm and clear weather in summer. If a HP system moves in from the north, you can expect cold weather in winter.

Other signs that come with high pressure systems include:
  • Winds blow away from HP systems
  • Weather is usually drier in HP systems
Weather Signs

Since weather systems move, there is a continual replacement of weather systems. This allows you to predict how weather will develop based on what you observe in the present. Similarly, throughout history, there have been many rhymes, poems and tips and tricks handed down to help the population predict the weather. Here are some examples grouped by type.

Changes in Air pressure
  • The speed of changes in air pressure can indicate the duration of a particular weather system. For example, if air pressure increases rapidly you can expect calm and clear conditions to pass over quickly.
  • If a barometer rises (increases in air pressure), expect fair weather, decreased cloudiness, calmer winds and clear skies, all associated with a high pressure system.
  • Clear skies will result in no clouds and cool/cold conditions. The following adages would apply in this case:
“Cold is the night when the stars shine bright” or “When stars shine clear and bright, we will have a very cold night"

Clear skies / Bright stars
Diagram by Robert Spencer
  • There are a couple rhymes that predict the arrival of a high or low pressure system.
  • A red sky during sunset suggests a high pressure system is approaching, bring clear, dry air. A red sky at sunrise suggests a low pressure system is coming, bringing moisture.
“Evening red and morning gray, helps the traveler on his way.
Evening gray and morning red, brings down rain upon his head”

“Red sky at night, sailor’s delight; Red sky at morning, sailors take warning”

Red sky at morning / sunrise
Red sky at evening / sunset
Red sky at sunset / evening
Diagrams / Photo by Robert Spencer
  • A red, hazy or pale moon signals high pressure
  • Dropping air pressure will cause smoke to drop or stay close to the ground
“Chimney smoke descends, our nice weather ends”

Chimney smoke drops towards the ground / Smoke hovers close to the ground
"A rainbow afternoon, Good weather coming soon"

"A rainbow in the morning, is the shepherd's warning / A rainbow at night is the shepherd's delight"
Diagrams by Robert Spencer

Cloud Formations

Different types of clouds are associated with different types of weather. The way that clouds are moving or changing can signal a shift in the weather and the arrival of different weather systems.

Sky Watchers Guide to Cloud ID - Environment Canada

Multiple layers of clouds going in different directions suggests bad weather is on the way (probably hail)
Cirrocumulus (high, thin layers of clouds that look like waves on a beach) = continued good weather
Cumulus clouds – Fluffy white clouds with rounded tops, small and float slowly = continued good weather
...
Cumulus towers = possible shower later in the day
Thunderhead – cloud that is shaped like a mushroom = big thunderstorm
High layer of many tightly packed little clouds = rain is likely in 8-12 hours
(altocumulus clouds)
.
.
Altocumulus clouds (mackerel scales – scaly appearance)
Cirrus clouds (Mare’s tails – long streamers)

“Mare’s tails and mackerel scales, tall ships carry short sails”
“Mackerel scales and mare’s tails, sailors furl their sails”

If covers the whole sky = 80% chance of rain in next 24 hours; Bad weather in 36 hours
Altocumulus clouds (Mackerel scales)
Cirrus clouds (Mare’s tails – long streamers)
“Clouds with round tops and a flat base has rain upon its face”

“When clouds look like black smoke as wise man will put on his cloak”

“When smoke hovers close to the ground, there will be a weather change”

“When clouds appear like rocks and towns,
The Earth’s refreshed by frequent showers”
Diagrams / Photo by Robert Spencer
  • Early and developing cumulonimbus clouds = increased chance of severe weather
  • Cloud cover on a winter night = warmer weather
  • Fog in the fields = will be sunny
  • Slow / gradual wind direction change = weather will be fair
Other Signs

There are many adages or weather signs that may be used to indicate weather changes or predict how long current weather will last. Most relate to the arrival or duration of precipitation.

Some weather adages that relate to the sound of things (indicating for the most part changes in humidity or a decrease in pressure) include:
  • “When the chairs squeak, it’s of rain they speak”
  • “Catchy drawer and sticky door, coming rain will pour and pour”
  • “The squeak of snow will the temperature show”
Other adages relate to watching conditions during precipitation. Signs include:
  • “Rain before seven, fine before eleven”
  • “When leaves show their undersides, be very sure that rain betides”
  • “When night goes to bed with a fever, it will awake with a wet head”
  • "When the sun shines while raining, it will rain the same time tomorrow”
Greenish sky during rain = rain worsening
Deep blue sky during rain = showers will continue
Heavy dew = day will be nice
Light dew and no wind = expect rain

"When the dew is on the grass, Rain will never come to pass. When grass is dry at morning light, Look for rain before the night"
...
Circle around the moon = rain or snow soon;
Halo around the sun = rain in 10-12 hours
Caused by cirrostratus clouds = warm fronts and moisture
High flying birds = fair weather
Low flying or roosting birds - decreased air pressure = hurts their ears
Animals cluster together if bad weather approaching
Sea gull, sea gull, sit on the sand, it’s never good weather while you’re on the land”

“When sea-gulls fly to land, a storm is at hand”

“Summer fog for fair, a winter fog for rain.
A fact most everywhere, in valley or on plain”

“If bees stay at home, rain will soon come,
if they fly away, fine will be the day”

“When the ditch and pond offend the nose, then look out for rain and stormy blows”
Diagrams / Photo by Robert Spencer
 
 
 
 
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on June 29, 2017.