What is the formula to convert Fahrenheit to Celsius and vice-versa?

F = (9/5 * C) + 32
C = 5/9 (F - 32)
Temperature Converter

Is there a formula to convert the barometric preasure given in inches of Hg to Millibars and vice versa?

1 inch of Mercury (Hg), as measured by a Mercury Barometer is equal to 33.864 millibars, as measured by an Aneroid Barometer.

Is there a reasonably simple formula that is used to calculate wind chill?

The following equation can be used if the wind speed in mph (V) and the temperature in deg. Fahrenheit (T) are known (SQRT = square root):
TWC=91.4-((91.4-T)*(.478+(.301*SQRT(V))-.02*V))
(NOTE: AT WIND SPEEDS OF 4 MPH OR LESS, TWC IS THE SAME AS THE ACTUAL AIR TEMPERATURE).
A table is also available to help determine the wind chill factor.

How does one calculate the heat index, given the relative humidity and the temperature?

If you know the relative humidity and the air temperature (in deg. Fahrenheit), you can calculate the heat index using the following formula:
Heat Index= -42.379 + 2.04901523(T) + 10.14333127(RH) - 0.22475541(T)(RH) - 6.83783x(10 to the -3rd power)x(T to the 2nd power) - 5.481717x(10 to the -2nd power)x(RH to the 2nd power) + 1.22874x(10 to the -3rd power)x(T to the 2nd power)(RH) + 8.5282x(10 to the -4th power)x(T)x(RH to the 2nd power) - 1.99x(10 to the -6th power)x(T to the 2nd power)x(RH to the 2nd power). where RH is Relative Humidity and T is temperature in deg. Fahrenheit.
A table is also available to help determine the heat index.

What is the dew point?

The "official" meaning of dew point is the temperature to which air must be cooled at constant pressure in order for it to become saturated.
The Dew point cannot be more than the air temperature, and is usually much lower. At the earth's surface, the pressure varies only slightly, so the dew point is a good indicator of the moisture content of the air, as well as a good indicator of the level of human discomfort in warm, humid weather. For example, most people begin to feel uncomfortable when the dew point rises above 20 deg C/70 deg F. In contrast to the dew point, the relative humidity reading, which used to be presented in weathercasts so often, depends as much upon the temperature of the air as it does upon its moisture content. On a sunny day, the relative humidity may drop as much as 50% from morning to afternoon just because of the rise in air temperature, but the dew point would rise along with the temperature, giving a more accurate view of the discomfort level.

If one knows the temperature and relative humidity (often given in weather forecasts), how can one calculate the dew point (often not given in weather reports)?

The formula is as follows:
Td = (-430.22 + 237.7 * ln(E)) / (-ln(E) + 19.08)
Where Td is the dew point temperature in degrees Celsius, and E is the actual vapor pressure of the air.
To start with, you should translate the current air temperature from Fahrenheit to Celsius, using the above noted formula. You then must determine the saturation vapor pressure (Es) as shown on this table. Using that number, then determine the actual vapor pressure E with the following formula:
E = (RH * Es) / 100
Plug the value of E into the formula (note: ln(E) means take the natural log of E) and the result will be the dew point temperature in degrees Celsius. Use the above noted formula to convert back to Farenheit.

What is the formula to convert dew point and ambient temperature to relative humidity?

The formula is as follows:
RH = (E/Es) * 100
Where E is the actual vapor pressure, and Es is the saturation vapor pressure of the air.
To start with, you should translate the current air temperature T and dew point Td from Fahrenheit to Celsius, using the above noted formula. You then must determine the saturation vapor pressure using the following formula:
Es = 6.11 * (10.0 to the power of (7.5*T/(237.7+T)))
Then determine the actual vapor pressure with the following formula:
E = 6.11 * (10.0 to the power of (7.5*Td/(237.7+Td)))
Plug the values of E and Es into the formula and the result will be the relative humidity.

What is an ice fog and what conditions are necessary for such an event to take place? Are they common?

An ice fog is a type of fog composed of suspended particles of ice. It occurs at very low temperatures and usually in clear, calm weather in high latitudes. Ice fog is rare at temperatures warmer than -30 deg. Celsius or -20 deg. Fahrenheit, and increases in frequency with decreasing temperature until it is almost always present at air temperatures of -45 deg. Celsius or -50 deg. Fahrenheit in the vicinity of a source of water vapor. Such sources include open water such as streams or oceans, herds of animals, volcanoes, and products of combustion for heating or propulsion.

I have a question regarding fronts and troughs. If a trough is an elongated area of low pressure, then what is a front? Because a front has a narrow transition zone represented by the blue line or red line...but where the cold or warm front lies, is that the area of lowest pressure?

You are correct in your definition of a trough, it is an elongated area of relatively low atmospheric pressure. The axis of the trough is called the trough line, and is depicted on frontal analyses as a dashed white line. A front is defined as the transition zone between two air masses of different density. Since the temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperatures. Many other features distinguish a front, such as a change in wind direction, moisture discontinuity, certain characteristic clouds, and precipitation forms. A cold front, represented by a blue line with triangles on it, has warm air ahead of it, and colder air behind it. A warm front, represented by a red line with half circles, has the opposite, with the colder air in front of it, and the warmer air behind it. The pips (triangles or half circles) are placed on the side of the front towards which it is moving.

Is there a difference between the "normal" daily high (and low) temperature and the "average" daily high (and low) temperature?

The average is simply the arithmetic mean of a range of temperatures. The normal is the average value of a meteorological element over any fixed period of years that is recognized as standard for the country and element concerned. Recommended international usage is to recalculate the normals at the end of every decade, using the previous 30 years. This practice is used to take account of the slow changes in climate and to add more recently established stations to the network with observed normals. Depending on the time frame used to establish the "average" temperature, it could be the same as the "normal", or vary quite dramatically.

I saw your weather site on the WWW and was wondering if you could help me find the cities with the most days of rain ( Most frequent rainfall vs rain volume)?

Cherrapunji, India has long been regarded as one of the "wettest place on earth". They receive an average of 425 inches of rain a year most of which falls from April to October. They average 106 inches in June alone. The northeastern coasts (such as Hilo) and windward slopes of the mountains in the Hawaiian Islands receive heavy rain. Some of the northeast facing mountain slopes average over 400 inches. In these areas, the heavy rain occurs in every month of the year with some locations having more than 260 rain days per year.

Could you give me some information about the "pineapple connection."

The pineapple connection (also called the pineapple express) refers to the situation when a deep upper level trough off the west coast causes a long feed of tropical moisture to extend from the vicinity of Hawaii to somewhere along the west coast. This tropical moisture produces heavy rains and at the higher elevations snows over the western mountains. Because the air is of tropical origin, snow levels tend to be higher than usual. The heavy rains and high snow levels often lead to flooding.

How does a cut off low form?

A cut off low is a an upper level low pressure system that has been seperated, or "cut off", from the normal west to east wind flow, and lies south of this flow. These start as deep troughs in the flow that seperates the cold air and the warm air masses. The warm air moves northward around the southern tip of the trough, eventually pinching off the area of low pressure. Such lows can stay in place for several days, drifting slowly eastward with its associated cloudiness and precipitation.

Does the old saying about red skies at night or morning actually have any scientific basis?

Red sky in morning, sailor take warning -
Red sky at night, sailor's delight

This is a good rule of thumb throughout much of the United States, as the weather patterns generally move from west to east across this continent. The red sky described above is the sky directly overhead, rather than the red at the horizon usually associated with sunrise/sunset. The red color is a result of the sunlight reflecting off the clouds. In order to have the red sky in the morning, the eastern horizon must be clear, and clouds will be moving in from the west - the direction from which weather, including storms, comes. A red sky at night would require a clear western horizon with clouds overhead moving east - which would indicate the stormy weather is moving away while the clear skies move in from the west.

I do not understand jetstreams. The weather maps make it look lika a narrow column of hi speed wind? If so, how does it sustain itself without the surrounding air slowing it down? How wide is a jet stream?

In the upper atmosphere, there are narrow bands of high speed winds, greater than 57 mph, called jet streams, which move from west to east. These are created when strong temperature differences cause large pressure differences in the upper levels of the atmosphere. The jet stream has northern and southern components to its movement as well, and these curves in the flow follow the boundary between warm and cold air. Due to the greater temperature contrasts in the winter, the jet stream tends to move faster in this season, and is located further south. There is no predefined width for a jet stream, as it can vary depending on the surrounding conditions.

Why is the sky blue?

The colors in the sky, as well as the colors of the clouds in the sky, are all caused by light scattering, or being sent off in different directions, by various molecules in the atmosphere, or by the water and ice crystals in clouds.

Light, which appears "white" to us, is actually made up of all the colors of the rainbow, each with its own unique wavelength. The purple and blue colors in the spectrum have the shortest wavelengths, with the oranges and reds having the longest. Light also always follows a straight path until forced otherwise by something. Air molecules are an example of such a "something" - they are just the right size to separate and redirect the short wavelength of blue along new paths.

So, as the "white" light enters the atmosphere from the sun, the blue light waves are scattered by air molecules all over the sky, making it the dominant color light you see no matter what direction you look in the sky.

Clouds are made up of more than just air molecules, of course, and the water and ice crystals found within clouds are capable of scattering all wavelengths, which combine to create the color "white". Dark clouds are simply those thick enough to allow little light through, or those in the shadow of other clouds or of the top of their own cloud.

Please explain the weather conditions required to produce fog. What temp, dew point and/or relative humidity is required? I am most interested in the fog that forms at Nantucket, Mass.

Fog is a cloud on the ground. The most common kinds of fog form when humid air is cooled to its dew point, causing the water vapor to begin condensing into tiny drops. The type of fog you refer to above is often called Sea Smoke, or Steam Fog. This occurs when cold air blows over much warmer water. Water molecules evaporate into the air from the body of water over which the air moves, increasing the dew point enough to match the temperature. This causes the condensation into tiny water droplets, thereby forming fog.

What's the chance that solar radiation interacting with the earths magnetosphere is a cause of the el nino/la nina phenomena? Is there a correlation between years of high solar activity and the occurences of the el nino?

A good question.

If there is a connection it isn't an obvious one. El Ninos, including strong El Ninos have happened with both quiet and active suns. This year for example in what may be the strongest El Nino on record, the sun is just coming out of the minimum phase.

The solar activity in conjunction with other factors like the QBO has been shown to correlate with weather features thus indicating the solar activity has an effect on our atmosphere.