Dewpoint Barometric Pressure Pressure Trend

Heat Index Wind Chill Precipitation Rate Mixing Ratio

Dewpoint

Dewpoint is the temperature to which air must be cooled for saturation (100% relative humidity) to occur. The dewpoint is an important measurement used to predict the formation of dew, frost, and fog. If dewpoint and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dewpoint is also a good indicator of the air’s actual water vapor content, unlike relative humidity, which takes the air’s temperature into account. High dewpoint indicates high vapor content; low dewpoint indicates low vapor content. In addition a high dewpoint indicates a better chance of rain and severe thunderstorms. You can even use dewpoint to predict the minimum overnight temperature. Provided no fronts or other weather pattern changes are expected overnight , the afternoon’s dewpoint gives you an idea of what minimum temperature to expect overnight, since the air is not likely to get colder than the dewpoint anytime during the night.

Barometric Pressure

The air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure. This, in turn, means that atmospheric pressure changes with altitude. For example, atmospheric pressure is greater at sea-level than on a mountaintop. To compensate for this difference in pressure at different elevations, and to facilitate comparison between locations with different altitudes, meteorologists adjust atmospheric pressure so that it reflects what the pressure would be if measured at sea-level. This adjusted pressure is known as barometric pressure.

Barometric pressure changes with local weather conditions, making barometric pressure an important and useful weather forecasting tool. High pressure zones are generally associated with fair weather, while low pressure zones are generally associated with poor weather. For forecasting purposes, the absolute barometric pressure value is generally less important than the change in barometric pressure. In general, rising pressure indicates improving weather conditions, while falling pressure indicates deteriorating weather conditions.

Pressure Trend

shows the direction of change (higher, lower, steady) of the barometric pressure over the last three hours.

· "Rising Rapidly" is indicated if the pressure increases > 2 mb (0.06")
· "Rising Slowly" is indicated if the pressure increases >1 mb but < 2 mb (> 0.02" but < 0.06")
· "Steady" is indicated if the pressure changes < 1 mb (< 0.02")
· "Falling Slowly" is indicated if the pressure falls > 1 mb but < 2 mb (> 0.02" but < 0.06")
· "Falling Rapidly" is indicated when the pressure decreases > 2 mb (>0.06")

Heat Index

The Heat Index uses the temperature and the relative humidity to determine how hot the air actually "feels." When humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates rapidly to cool the body. However, when humidity is high (i.e., the air is saturated with water vapor) the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly. Heat Index displays only when the actual temperature is above 70°F.

Wind Chill

Wind chill temperature is how cold people and animals feel when outside. Wind chill quantifies the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder. If the temperature is 0°F and the wind is blowing at 15 mph, the wind chill is -19°F. At this wind chill temperature, exposed skin can freeze in 30 minutes.

The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of time for the object to cool. The inanimate object cannot cool below the actual air temperature. For example, if the temperature outside is -5°F and the wind chill temperature is -31°F, then your car's radiator will not drop lower than -5°F. Wind chill is displayed when actual temperatures are below 70°F and will read the same as the actual temperature at temperatures above 50 as well as in very light wind conditions.

Note: 10-minute average wind speed is used to calculate wind chill

Precip Rate

The rain rate is calculated by measuring the time interval between each measured rainfall increment of .01". When there is rainfall within the archive period, the highest measured value is reported. When no rainfall occurs, the rain rate will slowly decay based on the elapse time since the last measured rainfall.

Mixing Ratio

Mixing ratio is expresses the ratio of water vapor to dry air for a given volume. Mixing ratio is usually expressed in terms of grams of water vapor per kilogram of dry air. The Mixing ratio is an absolute measure of the air's moisture content, unlike relative humidity which is a function of both moisture content and temperature.

Wet Bulb Temperature

The wet bulb temperature is a measure of the amount of moisture, in the form of invisible water vapor contained in the air. As the name implies it is measured by a standard thermometer whose bulb is covered by a muslin sleeve that has been moistened by pure water. This is the temperature air can be cooled to by evaporating water into it and therefore the wet bulb temperature provides a good estimate of how far temperatures will fall if it starts raining or snowing.

The principle of the wet bulb thermometer is as follows; water evaporates from the muslin cover passing into the air in the form of invisible water vapor. In so doing it absorbs heat from the thermometer bulb. The thermometer therefore indicates a lower temperature than that of the dry bulb thermometer. The difference between the readings of the dry and the wet thermometers is called the depression of the wet bulb.

If the air contains nearly all the moisture it can possibly hold, evaporation from the muslin will be slight and the depression of the wet bulb will be small. However, if the air is very dry, containing little moisture, evaporation will be quite rapid and the depression of the wet bulb will be quite large. In hot dry desert climates depressions of over 25°C have been observed, but at sea the depression is seldom more than 5°C. If the air contains all the moisture it can possibly hold, there is no evaporation from the muslin, and the dry and wet bulb thermometers will read the same. When this condition exists the air is said to be saturated.

Provided that the wet bulb is adequately moistened and given proper ventilation, its reading will always be equal to or less than that of the dry bulb when the air temperature is above freezing. Under certain conditions when the air temperature is below freezing and there is ice on the wet bulb, its reading may be slightly higher than the dry bulb. This is called a negative depression and usually occurs with fog or precipitation. Negative depressions are rare.

Storm Total Precipitation

The Storm Total continues counting rainfall until a 24 hour period passes without precipitation. Thus, the Storm Total can include several days worth of precipitation until a 24 hour dry interval occurs.