Weather

Weather: The present conditions of the atmosphere.

Page 13 ESRT

Atmospheric variables are the characteristics of the air that change frequently. They are measured and referenced in weather forecasts.

Some examples are listed below. (more details on each later)

1.    Temperature
2.    Pressure
3.    Wind Speed / Direction
4.    Humidity
5.    Dew Point

Weather Instruments

Main Weather Instruments

Anemometer:
Measures wind speed and direction

Barometer:
Measures atmospheric pressure

Sling Psychrometer:
Measures dew point
and relative humidity

Air Masses and Fronts

A large body of air with the same characteristics of temperature and moisture content is known as an air mass.

An air mass receives its characteristics from the location it forms, know as its source region.

Fronts represent the advancing edge of the air mass at ground level. Only low pressure systems have fronts, due to air masses converging at the low pressure.

There will be much more on pressure systems and fronts in a future lesson.

Air Mass Classification

Category 1: Moisture Content (land/sea)

The first letter in the symbol for classification is always lowercase, and is based on the following criteria:

- If it forms over land
- Dry / Arid

(m) Maritime

- If it forms over water
- Moist / Humid

combined with...

Key from page 13 of ESRT

Category 2: Temperature (Polar / Equatorial)

The second letter in the classification symbol is CAPITAL

Polar (P): Forms at high latitudes near poles (cold)
Tropical (T): Formed in low latitudes near equator (warm)
Arctic (A): Formed AT poles, ONLY FORM DURING THE WINTER (frigid / extreme cold)

Air Masses and New York State Weather:

The air mass and source region combination that typically cause stormy weather in NYS are listed below (and illustrated above):

cP from central Canada

mT from the Gulf of Mexico

The diagram on the left above shows the movement of air masses between latitudes. This will be the focus of an upcoming lesson, but the basics are that winds are driven by differences in air temperature, which leads to differences in density and pressure.

Review Games:

Frontal Boundary Precipitation

Fronts represent the ground level boundaries between two air masses that converge at the center of low pressure. Low pressure is created by warm air rising as colder, higher density air converges with it. This takes place along frontal boundaries and at the center of low pressure, which leads to these locations having a lot of cloud formation and precipitation.

Warm air masses that form over the Gulf of Mexico contain large amounts of water vapor. As the warm-moist maritime tropical (mT) air is forced upward, it will reach an altitude where the dew point temperature is reached, and moisture will be squeezed out of the air.

Use the animation below to see both warm and cold front interactions over time.

Synoptic Weather Maps

Station Models and Weather Maps

Station Models (like the one shown to the right) give all of the atmospheric variable data for a specific time and place. Each variable is recorded at the same location relative to the station models inner circle, with the exception of wind direction.

Key from page 13 of ESRT

Weather maps are generated based on analysis of station model data from several locations.

Isotherms:

Isotherms connect points of equal temperature.

They tend to run east to west across the country. This is because latitude is the largest factor in determining a locations temperature, and latitudes run east to west.

Their is usually a bend in the isotherms located in the central part of the country. This is due to differences in heating rates of land vs. water, and how far away from the oceans the region is.

Isobars:

Isobars are lines that connect points of equal barometric (air) pressure. They commonly form circles around the centers of high and low pressure that occur one after another.

Air pressure flows from areas of high to low concentration, so careful analysis of the barometric pressure values on isobars can indicate the direction wind flow.

The line spacing on a map shows how fast winds are moving, and the direction they are heading. The closer the isobars are to each other, the steeper the pressure gradient and the stronger the winds.

The diagram to the right illustrates the relationship between spacing and speed.

Frontal Boundaries on Weather Maps:

Front Symbol goes on the side of the line that the air mass is moving toward.

The cold front always moves faster than the warm front.

This allows cold fronts to catch up to the warm front, and essential zip the two fronts together like a zipper. The section where this occurs is called an occluded front.

Shaded areas represent possible areas of precipitation along the fronts.

End result is the cold and warm fronts join together in an occluded front, with the warm air on top