Simple tips for predicting the weather

July 29, 2015

Taking the weather into account is an essential­ part of any trip into the outdoors, no matter how short. Here are some tips to help you recognize the subtle weather signs and predict the weather.

Simple tips for predicting the weather

A brief introduction

All around you are signs that, if you know how to read them, can provide guidance almost as accurate as that indicated on a barometer or ­thermometer. Knowing how to read the signals given by the ­elements is part and parcel of good bushcraft.

  • Behind all the changes in the weather that occur day to day is air movement, masses of cold and warm air move across the earth from west to east at a rate of about 1000 kilometres (620 miles) a day, propelled into and around pools and eddies of high and low pressure.
  • Air blows clockwise away from the centre of a high-pressure area and anticlockwise towards the centre of a low-pressure area, where it escapes upwards, cooling as it rises and leaving behind the moisture it contains.
  • Thus a drop in air pressure (a falling barometer) generally indicates the arrival of a pocket of humid air, clouds and, often, rain or snow, particularly when the low-pressure area is at the front of an air mass.

An ancient art: detecting signs of change

There are many signs that indicate the approach of a low-pressure area: smoke hovers and turns downwards; birds tend to roost; clouds form at low altitudes; the rising humidity makes hair limp, causes distant objects to appear closer (because the usual evaporation haze is missing) and prevents the formation of morning dew.

  • These very real signs became obvious to primitive peoples many thousands of years ago and have become prominent among folklore's favourite foul-weather warnings.
  • The expression "red sky at night, sailors' delight, red sky at morning, sailors take warning" is similarly based on hard experience. The setting sun shines through tomorrow's air, 800 to 1000 kilometres (500 to 620 miles) westwards; the laws of light refraction are such that if that air is dry and cloudless, the sky will be red just after sunset.
  • The same laws decree that a red sky before sunrise means the air that has passed to the east is clearer and drier than where you are.

Look to the clouds

The surest weather indicators are clouds. There are three basic types: cirrus (wispy), ­stratus (layered) and cumulus (puffy), and each may presage a particular kind of storm.

  • Fluffy white cumulus clouds are formed by warm updrafts called thermals. They are common on clear days and generally mean more of the same, but they are also the stuff of which thunderstorms are made.
  • When a thermal is intensified by the moist updraft of a low-pressure area, the result is a billowing thunderhead (cumulonimbus), bringing strong winds, thunder, lightning and rain. The telltale step in this pattern is when fair weather cumulus clouds begin to puff upwards like the turrets of a castle.
  • Cirrus clouds are made of ice crystals, formed when warm air suddenly meets cooler air. Often they signify nothing more than a high-altitude wind pattern, but when they begin to form a thin, icy layer (cirrostratus) — causing the appearance of a halo around the sun or moon — it is probably the first warning of rain.
  • Cold air is heavier than warm, so the front of a cold-air mass hugs the ground as it moves, pushing warm air upwards like wood shavings before a chisel blade. Cold fronts give little warning: winds may change suddenly, often creating a squall. Cumulus clouds (cumulostratus) may accompany the front.

Don't let bad weather ruin your outdoor activities. Keep these tips in mind and learn how to accurately predict the weather.

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