Author’s Note

One thing that is often overlooked by preppers is the ability to read the skies. There may come a day where you no longer have access to that convenient weather app or Accuweather forecasts. Understanding the basics of meteorology can help you understand what the weather forecast may be for the near future. We first need to understand some of the common terms and phenomenon before we can use that knowledge to predict the weather. There are several aspects of weather that I will get into:

  • Clouds
  • Wind
  • Pressure and Fronts
  • Weather Instruments
  • Forecasting from Observations

Please note that I am not a meteorologist and not an expert of weather. As a little kid, I wanted to grow up and be a weatherman, but that changed shortly before I entered my undergraduate studies. Still, I have always been interested in the subject.

I am drawing very heavily on Weather: A Golden Guide. I have two versions of the book – a 1965 print that was discarded from the local library, and a 1987 version. Each of these copies have been in my possession for 30 years, and are well worn. The good news is that there is a newer version from 2001! It can be found on Amazon here, and it costs $6.95 new as of this article. I highly recommend picking up a copy. Are there more detailed books out there? Sure! But I have found none that are as good as this tiny little field book, and the price point makes it well worth it.

 

Pressures

There are two types of pressures: high and low. The highs, called anticyclones, generally bring fair weather. High pressure rotates clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere with light winds and temperatures (either warm or cold) that change very little. The lows, called cyclones, generally bring unsettled weather. These rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere with stronger winds and either cold (could be warm going to cold) or, in the case of tropical systems, very warm. You will often hear meteorologists talk about pressure in terms of “millibars”, and these are represented on weather maps as “isobars”. As a reference, standard sea-level air pressure is 1,013 millibars.

There is a general rule of thumb you can use to locate where the high pressure and the low pressure are located. First, stand with your back to the wind. Then, turn yourself 45 degrees to the right. Your back is now to the wind at the upper levels of the system. To your right lies the high pressure and the left, the low pressure.

As mentioned in a previous post, we use a barometer to measure the pressure. This will get touched on more in a later post.

 

Cold Front

Cold fronts, as illustrated above, occur when cold air pushes under warm air, causing it to rise and create clouds. The average speed of a cold front is about 20 miles per hour and moves faster in the winter than the summer. Cold fronts are almost always oriented northeast to southwest.

If the warm air is stable, you get precipitation of steady rain right at the edge of the front. If the warm air is unstable, you get cumulonimbus clouds and thunderstorms, and the precipitation generally falls after the edge of the front has passed. In very fast moving cold fronts, you may get squall lines of violent thunderstorms.

Weather Sequence

Wikipedia has an excellent entry on cold fronts, so I will republish their sequence below (original article here).

Weather phenomenon Prior to the passing of the front While the front is passing After the passing of the front
Temperature Warm Cooling suddenly Steadily cooling
Atmospheric pressure Decreasing steadily Lowest, then sudden increase Increasing steadily
Winds
  • Southwest to southeast (northern hemisphere)
  • Northwest to northeast (southern hemisphere)
Gusty; shifting
  • North to west, usually northwest (northern hemisphere)
  • South to west, usually southwest (southern hemisphere)
Precipitation/conditions* Light patchy rain can be produced by stratocumulus or stratus in the warm sector. In summer, sometimes thunderstorms if a preceding squall line is present. In winter snow squalls or showers may occur. Prolonged rain (nimbostratus) or thunderstorms (cumulonimbus): depends on conditions. Showers, then clearing
Clouds* Often preceded by cirrus, cirrostratus then altostratus like a warm front (but usually with smaller amounts of these clouds). Areas of cirrocumulus and altocumulus within cirrostratus and altostratus more commonly seen than at a warm front. Larger cumulus clouds under the higher cloud types than at a warm front, where stratocumulus and cumulus humilis usually occur. Some of these cumulus clouds may produce showers ahead of the front. Cumulonimbus and cumulus congestus producing frequent showers, with a sheet of upper altostratus, through which the sun can sometimes be seen. Less commonly nimbostratus occurs with continuous rain. Patchy altocumulus or stratocumulus and higher cirrus clouds along with fast moving stratus fractus then eventually scattered cumulus and sometimes cumulonimbus.
Visibility* Fair to poor in haze Poor, but improving Good, except in showers
Dew Point High, steady Sudden drop Falling

 

Warm Front

Warm fronts occur when warm air advances and replaces the colder air in front of it. The cold air forms a wedge because the friction with the ground causes the cold air to “drag”. In the Northern Hemisphere, they typically occur on the east side of low pressures. The average speed is about 15 mph. The weather associated with warm fronts can stretch for hundreds of miles and the associated cloud types can be seen up to 48 hours in advance.

If the warm air is stable, you get lower level clouds such as stratus, and the precipitation is steady rain. If the warm air is unstable, you get cumulonimbus clouds and thunderstorms, causing the precipitation to be spotty and alternate between downpours and drizzle.

Weather Sequence

Again, Wikipedia has an excellent entry on warm fronts, so I  default to them and will republish their sequence below (original article here).

Weather phenomenon Prior to the passing of the front While the front is passing After the passing of the front
Temperature Cool Warming suddenly Warmer, then leveling off
Atmospheric pressure Decreasing steadily Leveling off Slight rise followed by a decrease
Winds
  • South to southeast (backing) (northern hemisphere)
  • North to northeast (veering) (southern hemisphere)
Variable
  • South to southwest (veering) (northern hemisphere)
  • North to northwest (backing) (southern hemisphere)
Precipitation Usually none, but in summer or warm temperatures, cumulus congestus may continue to exist under cirrostratus and altostratus creating light to moderate showers. Persistent rain, usually moderate with some lighter periods and some heavier bursts. In winter, snow may turn to rain.[4] Light drizzle, gradually ceasing.
Clouds Cirrus, cirrostratus, altostratus, nimbostratus, then stratus (pilots use the acronym CCANS). Other clouds can also often be seen, including cirrocumulus amongst the approaching cirrus, altocumulus with or instead of altostratus (particularly if the front is weak), and occasionally cumulonimbus along with or instead of nimbostratus in summer. Additionally, stratocumulus often appears underneath the main altostratus deck and stratus fractus typically forms in precipitation falling from the thick nimbostratus layer. Often in warm temperatures, rain bearing cumulus congestus clouds can appear under the cirrostratus, and more rarely altocumulus castellanus clouds if convection is sufficient. In cold humid conditions, low airmass stratus or fog may obscure the main frontal clouds. Nimbostratus, sometimes cumulonimbus Clearing with scattered stratus and stratocumulus. If the warm front is part of a depression, there is often a sheet of altostratus (often broken in places to altocumulus) above this which thickens when the cold front approaches.
Visibility Poor Poor, but improving Sunny
Dew point Steady rise Steady Rise, then steady

 

Other Types of Front

Stationary fronts move very little if at all and resemble the weather caused by warm fronts. This means that rain, if present, may fall for several days. Weak fronts are almost never noticed as the weather will not change much, if at all. Occluded fronts occur when a cold front following a warm front overtakes it (because, as we see above, they move faster). The warm air mass is lifted off the ground, causing either the cold front or the warm front to go aloft.

 

The symbols used on weather maps for the different fronts can be found below, courtesy of NOAA.

 

 

Conclusion

We will see how these fronts play into our observational forecasts in a later piece of the series. For now, pat yourself on the back – you are beginning to have the skill set to talk about weather and understand the concepts that the weathermen mention in their reports!

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