this site outlines atmospheric effects daily.
http://home.cogeco.ca/~dxinfo/tropo.html
going to be a good day in new york tomorrow.
Below is the long answer.
Tropospheric propagation
The service area from a TV or FM radio transmitter extends to just beyond the optical horizon, at which point signals start to rapidly reduce in strength. Viewers living in such a "deep fringe" reception area will notice that during certain conditions weak signals normally masked by noise, increase in signal strength to allow quality reception. Such conditions are related to the current state of the troposphere.
Tropospheric propagated signals travel in the part of the atmosphere adjacent to the surface and extending to some 25,000 feet. Such signals are thus directly affected by weather conditions extending over some hundreds of miles. During very settled, warm, anti-cyclonic weather (i.e., high pressure), usually weak snowy TV signals from distant transmitters improve in signal strength.
Another symptom during such conditions may be interference to the local transmitter, resulting in co-channel interference, which may be in the form of horizontal lines or an extra floating picture. A settled high-pressure system gives the classic conditions for enhanced tropospheric propagation, in particular favouring signals which travel along the prevailing isobar pattern rather than across it. Such weather conditions can occur at any time, but generally the summer and autumn months are the best periods. In certain favourable locations, enhanced tropospheric propagation may enable reception of UHF TV signals up to 1,000 miles or more.
The observable characteristics of such high-pressure systems are usually clear, cloudless days with little or no wind. At sunset the upper air cools, as does the surface temperature, but at different rates. This produces a boundary or temperature gradient, which allows an inversion level to form - a similar effect occurs at sunrise. The inversion is capable of allowing VHF and UHF signal propagation well beyond the normal radio horizon distance.
The inversion effectively reduces sky wave radiation from a transmitter - normally VHF and UHF signals travel on into space when they reach the horizon, the refractive index of the ionosphere preventing signal return. With temperature inversion, however, the signal is to a large extent refracted over the horizon rather than continuing along a direct path into outer space.
Fog also produces good tropospheric results, again due to inversion effects. Fog occurs during high pressure weather, and if such conditions result in a large belt of fog with clear sky above, there will be heating of the upper fog level and thus an inversion. This situation often arises towards night fall, continues overnight and clears with the sunrise over a period of around 4-5 hours.
Tropospheric ducting
Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anti-cyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (though some people have received "tropo", as it is usually abbreviated, from beyond 1000 miles), while with tropospheric bending, stable signals with good signal strength from 500+ miles away are not uncommon when the refractive index of the atmosphere is fairly high.
Tropospheric ducting of UHF television signals is relatively common during the summer and autumn months, and is the result of change in the refractive index of the atmosphere at the boundary between air masses of different temperatures and humidities. Using an analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rare upper air, imparting a downward curve to the wave travel.
Ducting can occur on a very large scale when a large mass of cold air is overrun by warm air. This is termed a temperature inversion, and the boundary between the two air masses may extend for 1,000 miles (1,800 km) or more along a stationary weather front.
Temperature inversions occur most frequently along coastal areas bordering large bodies of water. This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper layers.
Even though tropospheric ducting has been occasionally observed down to 40 MHz, the signal levels are usually very weak. Higher frequencies above 90 MHz are generally more favourably propagated.
High mountainous areas and undulating terrain between the transmitter and receiver can form an effective barrier to tropospheric signals. Ideally, a relatively flat land path between the transmitter and receiver is ideal for tropospheric ducting. Sea paths also tend to produce superior results.
In certain parts of the world, notably the Mediterranean Sea and the Persian Gulf, tropospheric ducting conditions can become established for many months of the year to the extent that viewers receive regular quality reception of television signals over distances up to around 1,000 miles. Such conditions are normally optimum during very hot settled summer weather.
Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, and Bahrain and Pakistan, has produced VHF/UHF reception ranging from 1,000 to 3,000 miles (4,500 km).
Tropospheric signals exhibit a slow cycle of fading and will occasionally produce signals sufficiently strong for noise-free stereo reception on FM or noise-free TV pictures, sometimes in full color.
Virtually all long distance reception of digital television occurs by tropospheric ducting (due to most, but not all, DTV stations broadcasting in the UHF band).
