Predicting Weather

Using Temperature and Dew Point

If dew point is the temperature at which dew, or liquid water, forms, the closer the temperature is to the dew point, the more likely you are to have rain or unsettled conditions because the water vapor in the air is close to its condensation point.

The further apart the dew point is to the temperature, it is further from the condensation point and therefore the less likely it is to have rain and the more likely it is to be fair weather.


Using Relative Humidity

Since relative humidity is a measure of how much moisture is in the air, the higher the humidity, the more moisture is in the air and the more likely it is to have unsettled conditions. A measurement of 71% or higher is considered high relative humidity.

A low relative humidity, or 70% or lower, means less moisture is in the air and is a good indicator of fair weather.


Using Air Pressure

The uneven heating and cooling of the Earth’s surfaces produces pressure differences in the atmosphere, leading to local and global atmospheric movement. When air is slowly descending over a very large area, the air pressure is high. As this air descends towards the ground it warms. This warming effect disrupts the formation of clouds and is the reason that high pressure areas generally produce good weather conditions.

When air is slowly rising over a very large area, the pressure is low. As the air rises, it cools and clouds form from the water vapor in the air. This cooling effect, which causes water vapor to condense into droplets, is characteristic of low pressure areas and generally produces bad weather.

Air pressure can be measured using a barometer. A measurement of 30.00 inches or higher is considered high pressure and indicates fair weather. A measurement of 29.99 inches or lower is considered low pressure and indicates unsettled conditions.

You can also use the change in air pressure to predict weather. If the air pressure in rising, it indicates clearing or fair weather. If the air pressure is falling, it indicates unsettled conditions.


Using Fronts

A front is a place where air masses meet. All fronts are low pressure systems and bring unsettled conditions.

A warm front is created when a maritime tropical air mass moves into an area. This warm air mass is less dense and therefore moves more slowly than a cold front as it overtakes the denser air. The warm air rises vertically. Generally, a warm front brings light, steady rain and showers. Warm fronts are typically slow moving (several days).

A cold front is created when continental polar or maritime polar air masses move into an area. In a cold front, cold, dense air overtakes warm, less dense air. Cold fronts usually bring storms, move fast (2-5 hours), and produce cooler air following the storm.

A stationary front forms when a warm and cold front meet, but do not overtake each other, creating lingering rain.


Using Winds

Ocean currents, global winds, and storm systems, redistribute heat energy on Earth’s surface and therefore affect weather and long-term climatic patterns of a region. Wind direction can be useful in weather predication.

If the winds come from the following direction, you can expect...
N- cold, dry weather
NE- cold, moist weather
E- moist weather
SE- warm, moist weather
S- warm, moist weather
SW- warm, dry weather
W- dry weather
NW- cold, dry weather


Using Cloud Types

Basic cloud types can help you predict weather. Cumulus clouds indicate fair weather. Cirrus clouds predict a change is weather in the next 24 to 48 hours. Stratus clouds indicate unsettled conditions.