Weather Hints

[GordMay]

Weather Wisdom - Some Simple Rules of Thumb

A series of weather maps will enable simple deductions to be made about the future movement of pressure systems using the rule of persistence. This assumes that future movetnents and changes in intensity of pressure systems may be estimated from past development. Obviously, the shorter the interval between weather maps the better the estimate that can be made of future developnents, but in any case forecasts for a greater period than 6 hours ahead will not be reliable.

Even if only one map is available useful deductions can still be made. For example:

A ~ From the Weather Map:

1. Frontal depressions tend to move in families, each depression following its predecessor but in a slightly lower latitude.

2. A depression with a warm sector tends to move with the wind parallel to the isobars in the warm sector at about three-quarters of the wind speed.

3. Depressions tend to move with the wind around large, well-established anticyclones.

4. An occluded depression tends to move slowly and irregularly.

5. If the depression has a large warm sector it has a tendency to deepen.

6. As a depression occludes the deepening processes decrease.

7. A non-frontal depression tends to move with the strongest wind circulating around it.

8. A front which is crossed by isobars which are close together will probably be fast moving.

9. A front which is parallel to the isobars will be slow-moving.

10. Warm fronts move at about half the speed of the wind at the front.

11. Small anticyclones usually move faster than large ones.

Even without a weather map the observer who notes carefully the weather changes taking place can form (for a period of about 6 hours ahead) a useful estimate of the weather to come. The following points should be of assistance:

B ~ From Local Observations:

1. A falling barometer is an indication of bad weather to come.

2. A rising barometer does not necessarily indicate good weather to come. In unsettled conditions a rapid rise can quickly be followed by a rapid fall. In general, if the barometer rises and stays high for at least 12 hours, 24 hours of settled weather may be expected. If it stays high for 24 hours, several days of settled weather may be expected.

3. If cirrus cloud approaches from the west, and at the same time the barometer is falling, and the wind is backing (N.H.) then bad weather may be expected.

4. Fast-moving, high clouds often indicate that bad weather is to follow.

5. If after the passage of a cold front the barometer falls and the wind backs (N.H.) further bad weather may be expected.

C ~ Glossary of Some Weather Terms:

Air Mass:
A large body of air with the uniform temperature and humidity of its source region.

Anticyclone:
A large area of high atmospheric pressure, characterized by outward-spiraling winds. A high.
Atmospheric Pressure: The force exerted by the atmosphere's weight on a surface of unit area.

Climate:
The long-term average weather of a region.

Cyclone:
A large area of low atmospheric pressure, characterized by inward-spiraling winds. A low.

Front:
The surface between two different air masses.

High:
The center of an area with high atmospheric pressure.

Humidity:
The amount of water vapor in the air.

Isobar:
A line through points of equal atmospheric pressure.

Low:
The center of an area with low atmospheric pressure.

Polar Front:
The surface between polar and tropical air masses, along which cyclonic disturbances form.

Synoptic Chart:
A map depicting the weather in an area at a given moment.

Wave Cyclone:
A storm or low-pressure center that moves along a front.

E. & O. E.
Gord May

PS:
Watch for an article on reading weather maps in the November-December issue of “Good Old Boat” magazine. http://www.goodoldboat.com/

[GordMay]

BAROMETRIC PRESSURE

Atmospheric pressure at the Earth's surface is one of the keys to weather, which is one reason weather maps feature H's and L's, representing areas of high and low air pressure. High and low are pressure are important because they affect the weather.

As the name says, a "high" is an area where they air's pressure is higher than the pressure of the surrounding air. A "low' is where it's lower. Meteorologists don't have any particular number that divides high from low pressure; it's the relative differences that count.

The pressure is high at the surface where air is slowly descending - much to slowly to feel. And, this is going on over a large area, maybe a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is generally - but not quite always - associated with good weather.

The air that descends in high pressure areas, has to get to high altitudes in some way, and that way is by rising in areas where the pressure at the surface is low.

As air rises it cools. As the air cools, the humidity in it begins to condense into tiny drops of water, or if it's cold enough, into tiny ice crystals. If there is enough water, or ice, rain or snow begin to fall. This is why low pressure is associated with bad weather.

The air descending in high pressure flows out in a clockwise spiral in the Northern Hemisphere. Air flowing into an area of low pressure to rise, makes a counterclockwise spiral on the way in.

In general, falling air pressure means that clouds and precipitation are likely. Rising air pressure signals that clear weather is likely.

Using winds and a barometer to make forecasts
from the National Weather Service
Here are some general statements of wind-barometer indications that are generally applicable to all parts of the country (USA):

* "When the wind sets in form points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest.

* "When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.

* "As a rule, winds from the east quadrants and falling barometric pressure indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather, but again there are exceptions and in some parts of the country these rules do not apply."

The following table generally summarizes wind and barometer indications in the United States. The amateur forecaster should modify the table in accordance with his or her own observations. The following show the wind direction, the barometer reduced to sea level and the character of the weather indicated:
Source: NWS

* SW to NW, 30.10 to 30.20 and steady - Fair with slight temperature change for 1 to 2 days.

* SW to NW, 30.10 to 30.20 and rising rapidly - Fair, followed within 2 days by rain.

* SW to NW, 30.20 and above and stationary - Continued fair, with no decided temperature change.

* SW to NW, 30.20 and above and falling slowly - Slowly rising temperature and fair for 2 days.

* S to SE, 30.10 to 30.20 and falling slowly - Rain within 24 hours.

* S to SE, 30.10 to 30.20 and falling rapidly - Wind increasing in force, with rain within 12 to 24 hours.

* SE to NE, 30.10 to 30.20 and falling slowly - Rain in 12 to 18 hours.

* SE to NE, 30.10 to 30.20 and falling rapidly - Increasing wind, and rain within 12 hours.

* E to NE, 30.10 and above and falling slowly - In summer, with light winds, rain may not fall for several days. In winter, rain within 24 hours.

* E to NE, 30.10 and above and falling rapidly - In summer, rain probably within 12 to 24 hours. In winter, rain or snow, with increasing winds, will often set in when the barometer begins to fall and the wind sets in from the NE.

* SE to NE, 30.00 or below and falling slowly - Rain will continue 1 to 2 days. SE to NE, 30.00 or below and falling rapidly - Rain, with high wind, followed, within 36 hours by clearing, and in winter by colder.

* S to SW, 30.00 or below and rising slowly - Clearing within a few hours, and fair for several days.

* S to E, 29.80 or below and falling rapidly - Severe storm imminent, followed within 24 hours, by clearing, and in winter by colder.

* E to N, 29.80 or below and falling rapidly - Severe northeast gale and heavy precipitation; in winter, heavy snow, followed by a cold wave.

* Going to W, 29.80 or below and rising rapidly - Clearing and colder.

E. & O. E.
Gord May

[GordMay]

Admiral Fitzroy’s Storm Glass:

Observing the liquid in the storm glass is supposed to indicate changes in the weather.
If the liquid in the glass is clear, the weather will be bright and clear.
If the liquid is cloudy, the weather will be cloudy as well, perhaps with precipitation.
If there are small dots in the liquid, humid or foggy weather can be expected.
A cloudy glass with small stars indicated thunderstorms.
If the liquid contains small stars on sunny winter days, then snow is coming.
If there are large flakes throughout the liquid, it will be overcast in temperate seasons, or snowy in the winter.
Crystals at the bottom indicate frost.
Threads near the top mean it will be windy.

Here are instructions for constructing a storm glass, attributed to a letter published in the June 1997 School Science Review.

Ingredients for Storm Glass
* 2.5 g potassium nitrate
* 2.5 g ammonium chloride
* 33 mL distilled water
* 40 mL ethanol
* 10 g camphor

Dissolve the potassium nitrate and ammonium chloride in the water; add the ethanol; add the camphor. Place in corked test tube.

The reader is advised to use proper care in handling the chemicals

The premise of the functioning of the storm glass is that temperature and pressure affect solubility, sometimes resulting in clear liquid; other times causing precipitants to form. The functioning of this type of storm glass is not fully understood. In similar barometers, the liquid level, generally brightly colored, moves up or down a tube in response to atmospheric pressure. Certainly temperature affects solubility, but sealed glasses are not exposed to the pressure changes that would account for much of the observed behavior. Some people have proposed that surface interactions between the glass wall of the barometer and the liquid contents account for the crystals. Explanations sometimes include effects of electricity or quantum tunneling across the glass.

Additional Reading:

ADMIRAL FITZROY'S RULES FOR FORETELLING THE WEATHER:
http://www.thebritishclockmaker.com/...arometers.html

UNUSUAL WEATHER INSTRUMENTS:
http://www.orgonelab.org/cgi-bin/sho...e=yweather.htm


BAROMETERS 101:

Water Barometer:
Rise and fall of the water level in the glass spout reflects changes in atmospheric pressure. The spout is open to the atmosphere.
NOTE: When pressure falls, the water level in the spout rises forecasting deteriorating weather conditions. A high water level indicates a low-pressure area, and a low level indicates high pressure.

Torricelli (Mercury) Barometer:
The Torricelli Barometer uses a long glass tube closed at one end, which is upside down in an open container holding liquid. The pressure of the air, bearing down on the liquid in the container forces it up the tube, and the measurement of the various lengths of the column of liquid expresses the changes in air pressure . In order to have a tube of manageable length, the heaviest of all liquids, mercury, is now later used.

Aneroid Barometer:
An aneroid is a flexible metal bellow that has been tightly sealed after having some air removed. Higher atmospheric pressures squeeze the metal bellows while lower pressures allow it to expand. The aneroid barometer is operated by a metal cell containing only a very small amount of air, or a series of such cells joined together. Increased air pressure causes the sides of the cell or cells to come closer together. One side is fixed to the base of the instrument while the other is connected by means of a system of levers and pulleys (which multiply the changes in capsule thickness) to a rotating pointer that moves over a scale on the face of the instrument.

E. & O. E.
Gord May